# STklos Reference Manual4. Standard Procedures

Contents

### 4.1 Equivalence predicates

A predicate is a procedure that always returns a boolean value (`#t` or `#f`). An equivalence predicate is the computational analogue of a mathematical equivalence relation (it is symmetric, reflexive, and transitive). Of the equivalence predicates described in this section, `eq?` is the finest or most discriminating, and `equal?` is the coarsest. `Eqv?` is slightly less discriminating than `eq?`.

 `(eqv? obj1 obj2)` R5RS procedure

The `eqv?` procedure defines a useful equivalence relation on objects. Briefly, it returns `#t` if `obj1` and `obj2` should normally be regarded as the same object. This relation is left slightly open to interpretation, but the following partial specification of `eqv?` holds for all implementations of Scheme. The `eqv?` procedure returns `#t` if:
• `obj1` and `obj2` are both `#t` or both `#f`.
• `obj1` and `obj2` are both symbols and
 ```(string=? (symbol->string obj1) (symbol->string obj2)) ⇒ #t ```
Note: This assumes that neither `obj1` nor `obj2` is an "uninterned symbol".
• `obj1` and `obj2` are both keywords and
 ```(string=? (keyword->string obj1) (keyword->string obj2)) ⇒ #t ```
• `obj1` and `obj2` are both numbers, are numerically equal (see -), and are either both exact or both inexact.
• `obj1` and `obj2` are both characters and are the same character according to the `char=?` procedure (see char--).
• both `obj1` and `obj2` are the empty list.
• `obj1` and `obj2` are pairs, vectors, or strings that denote the same locations in the store.
• `obj1` and `obj2` are procedures whose location tags are equal.
Note: STklos extends R5RS `eqv?` to take into account the keyword type.
Here are some examples:
 ```(eqv? 'a 'a) ⇒ #t (eqv? 'a 'b) ⇒ #f (eqv? 2 2) ⇒ #t (eqv? :foo :foo) ⇒ #t (eqv? :foo :bar) ⇒ #f (eqv? '() '()) ⇒ #t (eqv? 100000000 100000000) ⇒ #t (eqv? (cons 1 2) (cons 1 2)) ⇒ #f (eqv? (lambda () 1) (lambda () 2)) ⇒ #f (eqv? #f 'nil) ⇒ #f (let ((p (lambda (x) x))) (eqv? p p)) ⇒ #t ```
The following examples illustrate cases in which the above rules do not fully specify the behavior of `eqv?`. All that can be said about such cases is that the value returned by eqv? must be a boolean.
 ```(eqv? "" "") ⇒ unspecified (eqv? '#() '#()) ⇒ unspecified (eqv? (lambda (x) x) (lambda (x) x)) ⇒ unspecified (eqv? (lambda (x) x) (lambda (y) y)) ⇒ unspecified ```
Note: In fact, the value returned by STklos depends of the way code is entered and can yield `#t` in some cases and `#f` in others.
See R5RS for more details on `eqv?`.

 `(eq? obj1 obj2)` R5RS procedure

`Eq?` is similar to `eqv?` except that in some cases it is capable of discerning distinctions finer than those detectable by `eqv?`.
`Eq?` and `eqv?` are guaranteed to have the same behavior on symbols, keywords, booleans, the empty list, pairs, procedures, and non-empty strings and vectors. `Eq?`'s behavior on numbers and characters is implementation-dependent, but it will always return either true or false, and will return true only when `eqv?` would also return true. `Eq?` may also behave differently from `eqv?` on empty vectors and empty strings.
Note: STklos extends R5RS `eq?` to take into account the keyword type.
Note: In STklos, comparison of character returns `#t` for identical characters and `#f` otherwise.
 ```(eq? 'a 'a) ⇒ #t (eq? '(a) '(a)) ⇒ unspecified (eq? (list 'a) (list 'a)) ⇒ #f (eq? "a" "a") ⇒ unspecified (eq? "" "") ⇒ unspecified (eq? :foo :foo) ⇒ #t (eq? :foo :bar) ⇒ #f (eq? '() '()) ⇒ #t (eq? 2 2) ⇒ unspecified (eq? #\A #\A) ⇒ #t (unspecified in R5RS) (eq? car car) ⇒ #t (let ((n (+ 2 3))) (eq? n n)) ⇒ #t (unspecified in R5RS) (let ((x '(a))) (eq? x x)) ⇒ #t (let ((x '#())) (eq? x x)) ⇒ #t (let ((p (lambda (x) x))) (eq? p p)) ⇒ #t (eq? :foo :foo) ⇒ #t (eq? :bar bar:) ⇒ #t (eq? :bar :foo) ⇒ #f ```

 `(equal? obj1 obj2)` R5RS procedure

`Equal?` recursively compares the contents of pairs, vectors, and strings, applying `eqv?` on other objects such as numbers and symbols. A rule of thumb is that objects are generally `equal?` if they print the same. `Equal?` may fail to terminate if its arguments are circular data structures.
 ```(equal? 'a 'a) ⇒ #t (equal? '(a) '(a)) ⇒ #t (equal? '(a (b) c) '(a (b) c)) ⇒ #t (equal? "abc" "abc") ⇒ #t (equal? 2 2) ⇒ #t (equal? (make-vector 5 'a) (make-vector 5 'a)) ⇒ #t ```

### 4.2 Numbers

R5RS description of numbers is quite long and will not be given here. STklos support the full number tower as described in R5RS; see this document for a complete description.

STklos extends the number syntax of R5RS with the following inexact numerical constants: `+inf.0` (infinity), `-inf.0` (negative infinity), `+nan.0` (not a number), and `-nan.0` (same as +nan.0).

 `(number? obj)` R5RS procedure
`(complex? obj)`
`(real? obj)`
`(rational? obj)`
`(integer? obj)`

These numerical type predicates can be applied to any kind of argument, including non-numbers. They return `#t` if the object is of the named type, and otherwise they return `#f`. In general, if a type predicate is true of a number then all higher type predicates are also true of that number. Consequently, if a type predicate is false of a number, then all lower type predicates are also false of that number. If `z` is an inexact complex number, then `(real? z)` is true if and only if `(zero? (imag-part z))` is true. If `x` is an inexact real number, then `(integer? x)` is true if and only if `(and (finite? x) (= x (round x)))`
 ``` (complex? 3+4i) ⇒ #t (complex? 3) ⇒ #t (real? 3) ⇒ #t (real? -2.5+0.0i) ⇒ #t (real? #e1e10) ⇒ #t (rational? 6/10) ⇒ #t (rational? 6/3) ⇒ #t (integer? 3+0i) ⇒ #t (integer? 3.0) ⇒ #t (integer? 3.2) ⇒ #f (integer? 8/4) ⇒ #t (integer? "no") ⇒ #f (complex? +inf.0) ⇒ #t (real? -inf.0) ⇒ #t (rational? +inf.0) ⇒ #f (integer? -inf.0) ⇒ #f ```

 `(exact? z)` R5RS procedure
`(inexact? z)`

These numerical predicates provide tests for the exactness of a quantity. For any Scheme number, precisely one of these predicates is true.

 `(bignum? x)` STklos procedure

This predicates returns `#t` if `x` is an integer number too large to be represented with a native integer.
 ```(bignum? (expt 2 300)) ⇒ `#t` (very likely) (bignum? 12) ⇒ `#f` (bignum? "no") ⇒ `#f` ```

 `(= z1 z2 z3 ...)` R5RS procedure
`(< x1 x2 x3 ...)`
`(> x1 x2 x3 ...)`
`(<= x1 x2 x3 ...)`
`(>= x1 x2 x3 ...)`

These procedures return `#t` if their arguments are (respectively): equal, monotonically increasing, monotonically decreasing, monotonically nondecreasing, or monotonically nonincreasing.
 ```(= +inf.0 +inf.0) ⇒ #t (= -inf.0 +inf.0) ⇒ #f (= -inf.0 -inf.0) ⇒ #t For any finite real number x: (< -inf.0 x +inf.0) ⇒ #t (> +inf.0 x -inf.0) ⇒ #t ```

 `(finite? z)` R5RS procedure
`(infinite? z)`
`(zero? z)`
`(positive? x)`
`(negative? x)`
`(odd? n)`
`(even? n)`

These numerical predicates test a number for a particular property, returning `#t` or `#f`.
 ```(positive? +inf.0) =⇒ #t (negative? -inf.0) =⇒ #t (finite? -inf.0) =⇒ #f (infinite? +inf.0) =⇒ #t ```

 `(max x1 x2 ...)` R5RS procedure
`(min x1 x2 ...)`

These procedures return the maximum or minimum of their arguments.
 ```(max 3 4) ⇒ 4 ; exact (max 3.9 4) ⇒ 4.0 ; inexact ```
For any real number x:
 ```(max +inf.0 x) ⇒ +inf.0 (min -inf.0 x) ⇒ -inf.0 ```
Note: If any argument is inexact, then the result will also be inexact

 `(+ z1 ...)` R5RS procedure
`(* z1 ...)`

These procedures return the sum or product of their arguments.
 ```(+ 3 4) ⇒ 7 (+ 3) ⇒ 3 (+) ⇒ 0 (+ +inf.0 +inf.0) ⇒ +inf.0 (+ +inf.0 -inf.0) ⇒ +nan.0 (* 4) ⇒ 4 (*) ⇒ 1 (* 5 +inf.0) ⇒ +inf.0 (* -5 +inf.0) ⇒ -inf.0 (* +inf.0 +inf.0) ⇒ +inf.0 (* +inf.0 -inf.0) ⇒ -inf.0 (* 0 +inf.0) ⇒ +nan.0 ```
Note: For any finite number z:
 ```(+ +inf.0 z) ⇒ +inf.0 (+ -inf.0 z) ⇒ -inf.0 ```

 `(- z)` R5RS procedure
`(- z1 z2)`
`(/ z)`
`(/ z1 z2 ...)`

With two or more arguments, these procedures return the difference or quotient of their arguments, associating to the left. With one argument, however, they return the additive or multiplicative inverse of their argument.
 ```(- 3 4) ⇒ -1 (- 3 4 5) ⇒ -6 (- 3) ⇒ -3 (- +inf.0 +inf.0) ⇒ +nan.0 (/ 3 4 5) ⇒ 3/20 (/ 3) ⇒ 1/3 (/ 0.0) ⇒ +inf.0 (/ 0) ⇒ error (division by 0) ```

 `(abs x)` R5RS procedure

`Abs` returns the absolute value of its argument.
 ```(abs -7) ⇒ 7 (abs -inf.0) ⇒ +inf.0 ```

 `(quotient n1 n2)` R5RS procedure
`(remainder n1 n2)`
`(modulo n1 n2)`

These procedures implement number-theoretic (integer) division. n2 should be non-zero. All three procedures return integers.

If `n1/n2` is an integer:
 ```(quotient n1 n2) ⇒ n1/n2 (remainder n1 n2) ⇒ 0 (modulo n1 n2) ⇒ 0 ```
If n1/n2 is not an integer:
 ```(quotient n1 n2) ⇒ nq (remainder n1 n2) ⇒ nr (modulo n1 n2) ⇒ nm ```
where `nq` is `n1/n2` rounded towards zero, 0 < abs(nr) < abs(n2), 0 < abs(nm) < abs(n2), `nr` and `nm` differ from n1 by a multiple of n2, `nr` has the same sign as n1, and `nm` has the same sign as n2.

From this we can conclude that for integers `n1` and `n2` with `n2` not equal to 0,
 ``` (= n1 (+ (* n2 (quotient n1 n2)) (remainder n1 n2))) ⇒ #t ```
provided all numbers involved in that computation are exact.
 ```(modulo 13 4) ⇒ 1 (remainder 13 4) ⇒ 1 (modulo -13 4) ⇒ 3 (remainder -13 4) ⇒ -1 (modulo 13 -4) ⇒ -3 (remainder 13 -4) ⇒ 1 (modulo -13 -4) ⇒ -1 (remainder -13 -4) ⇒ -1 (remainder -13 -4.0) ⇒ -1.0 ; inexact ```

 `(gcd n1 ...)` R5RS procedure
`(lcm n1 ...)`

These procedures return the greatest common divisor or least common multiple of their arguments. The result is always non-negative.
 ```(gcd 32 -36) ⇒ 4 (gcd) ⇒ 0 (lcm 32 -36) ⇒ 288 (lcm 32.0 -36) ⇒ 288.0 ; inexact (lcm) ⇒ 1 ```

 `(numerator q)` R5RS procedure
`(denominator q)`

These procedures return the numerator or denominator of their argument; the result is computed as if the argument was represented as a fraction in lowest terms. The denominator is always positive. The denominator of 0 is defined to be 1.
 ```(numerator (/ 6 4)) ⇒ 3 (denominator (/ 6 4)) ⇒ 2 (denominator (exact->inexact (/ 6 4))) ⇒ 2.0 ```

 `(floor x)` R5RS procedure
`(ceiling x)`
`(truncate x)`
`(round x)`

These procedures return integers. `Floor` returns the largest integer not larger than `x`. `Ceiling` returns the smallest integer not smaller than `x`. `Truncate` returns the integer closest to `x` whose absolute value is not larger than the absolute value of `x`. `Round` returns the closest integer to `x`, rounding to even when `x` is halfway between two integers.

Rationale: `Round` rounds to even for consistency with the default rounding mode specified by the IEEE floating point standard.

Note: If the argument to one of these procedures is inexact, then the result will also be inexact. If an exact value is needed, the result should be passed to the `inexact->exact` procedure.
 ```(floor -4.3) ⇒ -5.0 (ceiling -4.3) ⇒ -4.0 (truncate -4.3) ⇒ -4.0 (round -4.3) ⇒ -4.0 (floor 3.5) ⇒ 3.0 (ceiling 3.5) ⇒ 4.0 (truncate 3.5) ⇒ 3.0 (round 3.5) ⇒ 4.0 ; inexact (round 7/2) ⇒ 4 ; exact (round 7) ⇒ 7 ```

 `(rationalize x y)` R5RS procedure

Rationalize returns the simplest rational number differing from `x` by no more than `y`. A rational number `r1` is simpler than another rational number `r2` if `r1` = `p1/q1` and `r2` = `p2/q2` (in lowest terms) and abs(p1) <= abs(p2) and abs(q1) <= abs(q2). Thus `3/5` is simpler than `4/7`. Although not all rationals are comparable in this ordering (consider `2/7` and `3/5`) any interval contains a rational number that is simpler than every other rational number in that interval (the simpler `2/5` lies between `2/7` and `3/5`). Note that `0` = `0/1` is the simplest rational of all.
 ```(rationalize (inexact->exact .3) 1/10) ⇒ 1/3 ; exact (rationalize .3 1/10) ⇒ #i1/3 ; inexact ```

 `(exp z)` R5RS procedure
`(log z)`
`(sin z)`
`(cos z)`
`(tan z)`
`(asin z)`
`(acos z)`
`(atan z)`
`(atan y x)`

These procedures compute the usual transcendental functions. `Log` computes the natural logarithm of z (not the base ten logarithm). `Asin`, `acos`, and `atan` compute arcsine, arccosine, and arctangent, respectively. The two-argument variant of `atan` computes
 ```(angle (make-rectangular x y)) ```
When it is possible these procedures produce a real result from a real argument.

 `(sqrt z)` R5RS procedure

Returns the principal square root of `z`. The result will have either positive real part, or zero real part and non-negative imaginary part.

 `(expt z1 z2)` R5RS procedure

Returns `z1` raised to the power `z2`.

Note: `0z` is 1 if `z = 0` and `0` otherwise.

 `(make-rectangular x1 x2)` R5RS procedure
`(make-polar x3 x)`
`(real-part z)`
`(imag-part z)`
`(magnitude z)`
`(angle z)`

If x1, x2, x3, and x4 are real numbers and z is a complex number such that

`z = x1 + x2.i = x3 . ei.x4`

Then
 ```(make-rectangular x1 x2) ⇒ z (make-polar x3 x4) ⇒ z (real-part z) ⇒ x1 (imag-part z) ⇒ x2 (magnitude z) ⇒ abs(x3) (angle z) ⇒ xa ```
where `-π < xa <= π` with `xa = x4 + 2πn` for some integer n.
 ```(angle +inf.0) ⇒ 0.0 (angle -inf.0) ⇒ 3.14159265358979 ```

Note: `Magnitude` is the same as `abs` for a real argument.

 `(exact->inexact z)` R5RS procedure
`(inexact->exact z)`

`Exact->inexact` returns an inexact representation of z. The value returned is the inexact number that is numerically closest to the argument. `Inexact->exact` returns an exact representation of z. The value returned is the exact number that is numerically closest to the argument.

 `(number->string z)` R5RS procedure
`(number->string z radix)`

`Radix` must be an exact integer, either 2, 8, 10, or 16. If omitted, radix defaults to 10. The procedure `number->string` takes a number and a radix and returns as a string an external representation of the given number in the given radix such that
 ```(let ((number number) (radix radix)) (eqv? number (string->number (number->string number radix) radix))) ```
is true. It is an error if no possible result makes this expression true.

If `z` is inexact, the radix is 10, and the above expression can be satisfied by a result that contains a decimal point, then the result contains a decimal point and is expressed using the minimum number of digits (exclusive of exponent and trailing zeroes) needed to make the above expression true; otherwise the format of the result is unspecified.

The result returned by `number->string` never contains an explicit radix prefix.

Note: The error case can occur only when `z` is not a complex number or is a complex number with a non-rational real or imaginary part.

Rationale: If `z` is an inexact number represented using flonums, and the radix is 10, then the above expression is normally satisfied by a result containing a decimal point. The unspecified case allows for infinities, NaNs, and non-flonum representations.

 `(string->number string)` R5RS procedure
`(string->number string radix)`

Returns a number of the maximally precise representation expressed by the given `string`. `Radix` must be an exact integer, either 2, 8, 10, or 16. If supplied, `radix` is a default radix that may be overridden by an explicit radix prefix in `string` (e.g. `"#o177"`). If `radix` is not supplied, then the default radix is 10. If `string` is not a syntactically valid notation for a number, then `string->number` returns `#f`.
 ```(string->number "100") ⇒ 100 (string->number "100" 16) ⇒ 256 (string->number "1e2") ⇒ 100.0 (string->number "15##") ⇒ 1500.0 (string->number "+inf.0") ⇒ +inf.0 (string->number "-inf.0") ⇒ -inf.0 ```

 `(bit-and n1 n2 ...)` STklos procedure
`(bit-or n1 n2 ...)`
`(bit-xor n1 n2 ...)`
`(bit-not n)`
`(bit-shift n m)`

These procedures allow the manipulation of integers as bit fields. The integers can be of arbitrary length. `Bit-and`, `bit-or` and `bit-xor` respectively compute the bitwise and, inclusive and exclusive or. `bit-not` eturns the bitwise not of `n`. `bit-shift` returns the bitwise shift of `n`. The integer `n` is shifted left by `m` bits; If `m` is negative, `n` is shifted right by `-m` bits.
 ```(bit-or 5 3) ⇒ 7 (bit-xor 5 3) ⇒ 6 (bit-and 5 3) ⇒ 1 (bit-not 5) ⇒ -6 (bit-or 1 2 4 8) ⇒ 15 (bit-shift 5 3) ⇒ 40 (bit-shift 5 -1) ⇒ 2 ```

 `(random-integer n)` STklos procedure

Return an integer in the range [0, ..., `n`[. Subsequent results of this procedure appear to be independent uniformly distributed over the range [0, ..., `n`[. The argument `n` must be a positive integer, otherwise an error is signaled. This function is equivalent to the eponym function of SRFI-27 (see SRFI-27 (Source of random bits) definition for more details).

 `(random-real)` STklos procedure

Return a real number `r` such that `0 < r < 1`. Subsequent results of this procedure appear to be independent uniformly distributed. This function is equivalent to the eponym function of SRFI-27 (see SRFI-27 (Source of random bits) definition for more details).

 `(decode-float n)` STklos procedure

`decode-float` returns three exact integers: `significand`, `exponent` and `sign` (where `-1 <= sign <= 1`). The values returned by `decode-float` satisfy:
 ```n = (* sign significand (expt 2 exponent)) ```
Here is an example of `decode-float` usage.
 ```(receive l (decode-float -1.234) l) ⇒ (5557441940175192 -52 -1) (exact->inexact (* -1 5557441940175192 (expt 2 -52)) ⇒ -1.234 ```

#### 4.2.1 Fixnums

STklos defines small integers as fixnums. Operations on fixnums are generally faster than operations which accept general numbers. Fixnums operations, as described below, may produce results which are incorrect if some temporary computation falls outside the range of fixnum. These functions should be used only when speed really matters.

 `(fixnum? obj)` STklos procedure

Returns `#t` if obj is an exact integer within the fixnum range, `#f` otherwise.

 `(fixnum-width)` STklos procedure

Returns the number of bits used to represent a fixnum number

 `(least-fixnum)` STklos procedure
`(greatest-fixnum)`

These procedures return the minimum value and the maximum value of the fixnum range.

 `(fx+ fx1 fx2)` STklos procedure
`(fx- fx1 fx2)`
`(fx* fx1 fx2)`
`(fxdiv fx1 fx2)`
`(fxrem fx1 fx2)`
`(fxmod fx1 fx2)`
`(fx- fx)`

These procedures compute (respectively) the sum, the difference, the product, the quotient and the remainder and modulp of the fixnums `fx1` and `fx2`. The call of `fx-` with one parameter `fx` computes the opposite of `fx`.

 `(fx< fx1 fx2)` STklos procedure
`(fx<= fx1 fx2)`
`(fx> fx1 fx2)`
`(fx>= fx1 fx2)`
`(fx= fx1 fx2)`

These procedures compare the fixnums `fx1` and `fx2` and retun `#t` if the comparison is true and `#f` otherwise.

### 4.3 Booleans

Of all the standard Scheme values, only `#f` counts as false in conditional expressions. Except for `#f`, all standard Scheme values, including `#t`, pairs, the empty list, symbols, numbers, strings, vectors, and procedures, count as true.

Boolean constants evaluate to themselves, so they do not need to be quoted in programs.

 `(not obj)` R5RS procedure

Not returns `#t` if `obj` is false, and returns `#f` otherwise.
 ``` (not #t) ⇒ #f (not 3) ⇒ #f (not (list 3)) ⇒ #f (not #f) ⇒ #t (not '()) ⇒ #f (not (list)) ⇒ #f (not 'nil) ⇒ #f ```

 `(boolean? obj)` R5RS procedure

`Boolean?` returns `#t` if `obj` is either `#t` or `#f` and returns `#f` otherwise.
 ``` (boolean? #f) ⇒ #t (boolean? 0) ⇒ #f (boolean? '()) ⇒ #f ```

### 4.4 Pairs and lists

 `(pair? obj)` R5RS procedure

`Pair?` returns `#t` if `obj` is a pair, and otherwise returns `#f`.

 `(cons obj1 obj2)` R5RS procedure

Returns a newly allocated pair whose car is obj1 and whose cdr is obj2. The pair is guaranteed to be different (in the sense of eqv?) from every existing object.
 ``` (cons 'a '()) ⇒ (a) (cons '(a) '(b c d)) ⇒ ((a) b c d) (cons "a" '(b c)) ⇒ ("a" b c) (cons 'a 3) ⇒ (a . 3) (cons '(a b) 'c) ⇒ ((a b) . c) ```

 `(car pair)` R5RS procedure

Returns the contents of the car field of pair. Note that it is an error to take the `car` of the empty list.
 ``` (car '(a b c)) ⇒ a (car '((a) b c d)) ⇒ (a) (car '(1 . 2)) ⇒ 1 (car '()) ⇒ error ```

 `(cdr pair)` R5RS procedure

Returns the contents of the cdr field of pair. Note that it is an error to take the `cdr` of the empty list.
 ``` (cdr '((a) b c d)) ⇒ (b c d) (cdr '(1 . 2)) ⇒ 2 (cdr '()) ⇒ error ```

 `(set-car! pair obj)` R5RS procedure

Stores `obj` in the car field of `pair`. The value returned by `set-car!` is void.
 ``` (define (f) (list 'not-a-constant-list)) (define (g) '(constant-list)) (set-car! (f) 3) (set-car! (g) 3) ⇒ error ```

 `(set-cdr! pair obj)` R5RS procedure

Stores `obj` in the cdr field of `pair`. The value returned by `set-cdr!` is void.

 `(caar pair)` R5RS procedure
`(cadr pair)`
`...`
`(cdddar pair)`
`(cddddr pair)`

These procedures are compositions of `car` and `cdr`, where for example `caddr` could be defined by
 ``` (define caddr (lambda (x) (car (cdr (cdr x))))) ```
Arbitrary compositions, up to four deep, are provided. There are twenty-eight of these procedures in all.

 `(null? obj)` R5RS procedure

Returns `#t` if `obj` is the empty list, otherwise returns `#f`.

 `(pair-mutable? obj)` STklos procedure

Returns `#t` if `obj` is a mutable pair, otherwise returns `#f`.
 ```(pair-mutable? '(1 . 2)) ⇒ #f (pair-mutable? (cons 1 2)) ⇒ #t (pair-mutable? 12) ⇒ #f ```

 `(list? obj)` R5RS procedure

Returns `#t` if `obj` is a list, otherwise returns `#f`. By definition, all lists have finite length and are terminated by the empty list.
 ``` (list? '(a b c)) ⇒ #t (list? '()) ⇒ #t (list? '(a . b)) ⇒ #f (let ((x (list 'a))) (set-cdr! x x) (list? x)) ⇒ #f ```

 `(make-list k)` STklos procedure
`(make-list k fill)`

Returns a newly allocated list of k elements. If a second argument is given, then each element is initialized to fill . Otherwise the initial contents of each element is unspecified.

 `(list obj ...)` R5RS procedure

Returns a newly allocated list of its arguments.
 ``` (list 'a (+ 3 4) 'c) ⇒ (a 7 c) (list) ⇒ () ```

 `(list* obj ...)` STklos procedure

`list*` is like `list` except that the last argument to `list*` is used as the cdr of the last pair constructed.
 ``` (list* 1 2 3) ⇒ (1 2 . 3) (list* 1 2 3 '(4 5)) ⇒ (1 2 3 4 5) (list*) ⇒ () ```

 `(length list)` R5RS procedure

Returns the length of `list`.
 ``` (length '(a b c)) ⇒ 3 (length '(a (b) (c d e))) ⇒ 3 (length '()) ⇒ 0 ```

 `(append list ...)` R5RS procedure

Returns a list consisting of the elements of the first list followed by the elements of the other lists.
 ``` (append '(x) '(y)) ⇒ (x y) (append '(a) '(b c d)) ⇒ (a b c d) (append '(a (b)) '((c))) ⇒ (a (b) (c)) ```
The resulting list is always newly allocated, except that it shares structure with the last list argument. The last argument may actually be any object; an improper list results if the last argument is not a proper list.
 ``` (append '(a b) '(c . d)) ⇒ (a b c . d) (append '() 'a) ⇒ a ```

 `(append! list ...)` STklos procedure

Returns a list consisting of the elements of the first list followed by the elements of the other lists. Contrarily to `append`, the parameter lists (except the last one) are physically modified: their last pair is changed to the value of the next list in the `append!` formal parameter list.
 ```(let* ((l1 (list 1 2)) (l2 (list 3)) (l3 (list 4 5)) (l4 (append! l1 l2 l3))) (list l1 l2 l3)) ⇒ ((1 2 3 4 5) (3 4 5) (4 5)) ```
An error is signaled if one of the given lists is a constant list.

 `(reverse list)` R5RS procedure

Returns a newly allocated list consisting of the elements of `list` in reverse order.
 ``` (reverse '(a b c)) ⇒ (c b a) (reverse '(a (b c) d (e (f)))) ⇒ ((e (f)) d (b c) a) ```

 `(reverse! list)` STklos procedure

Returns a list consisting of the elements of `list` in reverse order. Contrarily to `reverse`, the returned value is not newly allocated but computed "in place".
 ```(let ((l '(a b c))) (list (reverse! l) l)) ⇒ ((c b a) (a)) (reverse! '(a constant list)) ⇒ error ```

 `(list-tail list k)` R5RS procedure

Returns the sublist of `list` obtained by omitting the first `k` elements. It is an error if list has fewer than `k` elements. List-tail could be defined by
 ``` (define list-tail (lambda (x k) (if (zero? k) x (list-tail (cdr x) (- k 1))))) ```

 `(last-pair list)` STklos procedure

Returns the last pair of `list`.
 ```(last-pair '(1 2 3)) ⇒ (3) (last-pair '(1 2 . 3)) ⇒ (2 . 3) ```

 `(list-ref list k)` R5RS procedure

Returns the `k`th element of `list`. (This is the same as the car of `(list-tail list k)`.) It is an error if list has fewer than `k` elements.
 ``` (list-ref '(a b c d) 2) ⇒ c (list-ref '(a b c d) (inexact->exact (round 1.8))) ⇒ c ```

 `(memq obj list)` R5RS procedure
`(memv obj list)`
`(member obj list)`

These procedures return the first sublist of list whose car is `obj`, where the sublists of list are the non-empty lists returned by `(list-tail list k)` for `k` less than the length of list. If `obj` does not occur in `list`, then `#f` (not the empty list) is returned. `Memq` uses `eq?` to compare obj with the elements of list, while `memv` uses `eqv?` and `member` uses `equal?`.
 ``` (memq 'a '(a b c)) ⇒ (a b c) (memq 'b '(a b c)) ⇒ (b c) (memq 'a '(b c d)) ⇒ #f (memq (list 'a) '(b (a) c)) ⇒ #f (member (list 'a) '(b (a) c)) ⇒ ((a) c) (memv 101 '(100 101 102)) ⇒ (101 102) ```

 `(assq obj alist)` R5RS procedure
`(assv obj alist)`
`(assoc obj alist)`

`Alist` (for "association list") must be a list of pairs. These procedures find the first pair in `alist` whose car field is `obj`, and returns that pair. If no pair in `alist` has `obj` as its car, then `#f` (not the empty list) is returned. `Assq` uses `eq?` to compare `obj` with the car fields of the pairs in `alist`, while `assv` uses `eqv?` and `assoc` uses `equal?`.
 ``` (define e '((a 1) (b 2) (c 3))) (assq 'a e) ⇒ (a 1) (assq 'b e) ⇒ (b 2) (assq 'd e) ⇒ #f (assq (list 'a) '(((a)) ((b)) ((c)))) ⇒ #f (assoc (list 'a) '(((a)) ((b)) ((c)))) ⇒ ((a)) (assv 5 '((2 3) (5 7) (11 13))) ⇒ (5 7) ```
Rationale: Although they are ordinarily used as predicates, `memq`, `memv`, `member`, `assq`, `assv`, and `assoc` do not have question marks in their names because they return useful values rather than just `#t` or #`f`.

 `(copy-tree obj)` STklos procedure

`Copy-tree` recursively copies trees of pairs. If `obj` is not a pair, it is returned; otherwise the result is a new pair whose `car` and `cdr` are obtained by calling `copy-tree` on the `car` and `cdr` of `obj`, respectively.

 `(filter pred list)` STklos procedure
`(filter! pred list)`

`Filter` returns all the elements of `list` that satisfy predicate `pred`. The `list` is not disordered: elements that appear in the result list occur in the same order as they occur in the argument list. `Filter!` does the same job than `filter` by physically modifying its `list` argument
 ```(filter even? '(0 7 8 8 43 -4)) ⇒ (0 8 8 -4) (let* ((l1 (list 0 7 8 8 43 -4)) (l2 (filter! even? l1))) (list l1 l2)) ⇒ ((0 8 8 -4) (0 8 8 -4)) ```
An error is signaled if `list` is a constant list.

 `(remove pred list)` STklos procedure

`Remove` returns `list` without the elements that satisfy predicate `pred`:

The list is not disordered -- elements that appear in the result list occur in the same order as they occur in the argument list. `Remove!` does the same job than `remove` by physically modifying its `list` argument
 ```(remove even? '(0 7 8 8 43 -4)) ⇒ (7 43) ```

 `(delete x list [=])` STklos procedure
`(delete! x list [=])`

`Delete` uses the comparison procedure `=`, which defaults to `equal?`, to find all elements of `list` that are equal to `x`, and deletes them from `list`. The dynamic order in which the various applications of `=` are made is not specified.

The list is not disordered -- elements that appear in the result list occur in the same order as they occur in the argument list.

The comparison procedure is used in this way: `(= x ei)`. That is, `x` is always the first argument, and a list element is always the second argument. The comparison procedure will be used to compare each element of list exactly once; the order in which it is applied to the various `ei` is not specified. Thus, one can reliably remove all the numbers greater than five from a list with
 ```(delete 5 list <) ```
`delete!` is the linear-update variant of `delete`. It is allowed, but not required, to alter the cons cells in its argument `list` to construct the result.

### 4.5 Symbols

The STklos reader can read symbols whose names contain special characters or letters in the non standard case. When a symbol is read, the parts enclosed in bars "|" will be entered verbatim into the symbol's name. The "|" characters are not part of the symbol; they only serve to delimit the sequence of characters that must be entered "as is". In order to maintain read-write invariance, symbols containing such sequences of special characters will be written between a pair of "|".

In addition, any character can be used within an identifier when specified via an inline hex escape . For example, the identifier H\x65;llo is the same as the identifier Hello, and, if the UTF-8 encoding is used, the identifier \x3BB; is the same as the identifier λ.

 ```'|a| ⇒ a (string->symbol "a") ⇒ |A| (symbol->string '|A|) ⇒ "A" '|a b| ⇒ |a b| 'a|B|c ⇒ |aBc| (write '|FoO|) -| |FoO| (display '|FoO|) -| FoO ```

 `(symbol? obj)` R5RS procedure

Returns `#t` if obj is a symbol, otherwise returns `#f`.
 ``` (symbol? 'foo) ⇒ #t (symbol? (car '(a b))) ⇒ #t (symbol? "bar") ⇒ #f (symbol? 'nil) ⇒ #t (symbol? '()) ⇒ #f (symbol? #f) ⇒ #f (symbol? :key) ⇒ #f ```

 `(symbol->string string)` R5RS procedure

Returns the name of `symbol` as a string. If the symbol was part of an object returned as the value of a literal expression or by a call to the `read` procedure, and its name contains alphabetic characters, then the string returned will contain characters in the implementation's preferred standard case -- STklos prefers lower case. If the symbol was returned by `string->symbol`, the case of characters in the string returned will be the same as the case in the string that was passed to `string->symbol`. It is an error to apply mutation procedures like `string-set!` to strings returned by this procedure.
 ``` (symbol->string 'flying-fish) ⇒ "flying-fish" (symbol->string 'Martin) ⇒ "martin" (symbol->string (string->symbol "Malvina")) ⇒ "Malvina" ```

 `(string->symbol string)` R5RS procedure

Returns the symbol whose name is `string`. This procedure can create symbols with names containing special characters or letters in the non-standard case, but it is usually a bad idea to create such symbols because in some implementations of Scheme they cannot be read as themselves.
 ``` (eq? 'mISSISSIppi 'mississippi) ⇒ #t (string->symbol "mISSISSIppi") ⇒ |mISSISSIppi| (eq? 'bitBlt (string->symbol "bitBlt")) ⇒ #f (eq? 'JollyWog (string->symbol (symbol->string 'JollyWog))) ⇒ #t (string=? "K. Harper, M.D." (symbol->string (string->symbol "K. Harper, M.D."))) ⇒ #t ```

 `(string->unterned-symbol string)` STklos procedure

Returns the symbol whose print name is made from the characters of `string`. This symbol is guaranteed to be unique (i.e. not `eq?` to any other symbol):
 ```(let ((ua (string->uninterned-symbol "a"))) (list (eq? 'a ua) (eqv? 'a ua) (eq? ua (string->uninterned-symbol "a")) (eqv? ua (string->uninterned-symbol "a")))) ⇒ (#f #t #f #t) ```

 `(gensym)` STklos procedure
`(gensym prefix)`

Creates a new symbol. The print name of the generated symbol consists of a prefix (which defaults to "G) followed by the decimal representation of a number. If "prefix| is specified, it must be either a string or a symbol.
 ```(gensym) ⇒ |G100| (gensym "foo-") ⇒ foo-101 (gensym 'foo-) ⇒ foo-102 ```

### 4.6 Characters

The following table gives the list of allowed character names with their ASCII eqivalent expressed in octal. Some chracaters have an alternate name which is also shown in this table.

namevaluealt. namenamevaluealt. name
nul000nullsoh001
stx002etx003
eot004enq005
ack006bel007alarm
bs010backspaceht011tab
nl012newlinevt013
np014pagecr015return
so016si017
dle020dc1021
dc2022dc3023
dc4024nak025
syn026etb027
can030em031
sub032esc033escape
fs034gs035
rs036us037
sp040spacedel177delete

STklos supports the complete Unicode character set, if UTF-8 encoding is used. Hereafter, are some examples of characters:

 ```#A ⇒ uppercase A #a ⇒ lowercase a #x41; ⇒ the U+0041 character (uppercase A) #x03BB; ⇒ λ ```

 `(char? obj)` R5RS procedure

Returns `#t` if `obj` is a character, otherwise returns `#f`.

 `(char=? char1 char2)` R5RS procedure
`(char<? char1 char2)`
`(char>? char1 char2)`
`(char<=? char1 char2)`
`(char>=? char1 char2)`

These procedures impose a total ordering on the set of characters. It is guaranteed that under this ordering:
• The upper case characters are in order.
• The lower case characters are in order.
• The digits are in order.
• Either all the digits precede all the upper case letters, or vice versa.
• Either all the digits precede all the lower case letters, or vice versa.

 `(char-ci=? char1 char2)` R5RS procedure
`(char-ci<? char1 char2)`
`(char-ci>? char1 char2)`
`(char-ci<=? char1 char2)`
`(char-ci>=? char1 char2)`

These procedures are similar to `char=?` et cetera, but they treat upper case and lower case letters as the same. For example, `(char-ci=? #A #a)` returns `#t`.

 `(char-alphabetic? char)` R5RS procedure
`(char-numeric? char)`
`(char-whitespace? char)`
`(char-upper-case? letter)`
`(char-lower-case? letter)`

These procedures return `#t` if their arguments are alphabetic, numeric, whitespace, upper case, or lower case characters, respectively, otherwise they return `#f`. The following remarks, which are specific to the ASCII character set, are intended only as a guide: The alphabetic characters are the 52 upper and lower case letters. The numeric characters are the ten decimal digits. The whitespace characters are space, tab, line feed, form feed, and carriage return.

 `(char->integer char)` R5RS procedure
`(integer->char n)`

Given a character, `char->integer` returns an exact integer representation of the character. Given an exact integer that is the image of a character under `char->integer`, `integer->char` returns that character. These procedures implement order-preserving isomorphisms between the set of characters under the `char<=?` ordering and some subset of the integers under the `<=` ordering. That is, if
 ``` (char<=? a b) ⇒ #t and (<= x y) ⇒ #t ```
and x and y are in the domain of `integer->char`, then
 ``` (<= (char->integer a) (char->integer b)) ⇒ #t (char<=? (integer->char x) (integer->char y)) ⇒ #t ```
`integer->char` accepts an exact number between 0 and #xD7FFF or between #xE000 and #x10FFFF, if UTF8 encoding is used. Otherwise it accepts a number between0 and #xFF.

 `(char-upcase char)` R5RS procedure
`(char-downcase char)`

These procedures return a character `char2` such that `(char-ci=? char char2)`. In addition, if char is alphabetic, then the result of `char-upcase` is upper case and the result of `char-downcase` is lower case.

 `(char-foldcase char)` STklos procedure

This procedure applies the Unicode simple case folding algorithm and returns the result. Note that language-sensitive folding is not used. If the argument is an uppercase letter, the result will be either a lowercase letter or the same as the argument if the lowercase letter does not exist.

### 4.7 Strings

STklos string constants allow the insertion of arbitrary characters by encoding them as escape sequences. An escape sequence is introduced by a backslash "\". The valid escape sequences are shown in the following table.

SequenceCharacter inserted
\aAlarm
\bBackspace
\e Escape
\n Newline
\t Horizontal Tab
\r Carriage Return
\" doublequote U+0022
\\ backslash U+005C
\0abc ASCII character with octal value abc
\x<hexa value>; ASCII character with given hexadecimal value
\<intraline whitespace><newline><intraline whitespace> None (permits to enter a string on several lines)
\<other> <other>

For instance, the string

 ```"ab\040\x20;c\nd e" ```

is the string consisting of the characters `#\a`, `#\b`, `#\space`, `#\space`, `#\c`, `#\newline`, `#\d` and `#\e`.

Notes:

• Using octal code is limited to characters in the range 0 to #xFF. It is then not convenient to enter Unicode characters. This form is deprecated should not be used anymore.
• A line ending which is preceded by <intraline whitespace> expands to nothing (along with any trailing <intraline whitespace>), and can be used to indent strings for improved legibility.

 `(string? obj)` R5RS procedure

Returns `#t` if `obj` is a string, otherwise returns `#f`.

 `(make-string k)` R5RS procedure
`(make-string k char)`

`Make-string` returns a newly allocated string of length `k`. If `char` is given, then all elements of the string are initialized to `char`, otherwise the contents of the string are unspecified.

 `(string char ...)` R5RS procedure

Returns a newly allocated string composed of the arguments.

 `(string-length string)` R5RS procedure

Returns the number of characters in the given `string`.

 `(string-ref string k)` R5RS procedure

`String-ref` returns character k of string using zero-origin indexing (`k` must be a valid index of string).

 `(string-set! string k char)` R5RS procedure

`String-set!` stores `char` in element `k` of `string` and returns void (`k` must be a valid index of `string`).
 ```(define (f) (make-string 3 #*)) (define (g) "***") (string-set! (f) 0 #?) ⇒ void (string-set! (g) 0 #?) ⇒ error (string-set! (symbol->string 'immutable) 0 #?) ⇒ error ```

 `(string=? string1 string2)` R5RS procedure
`(string-ci=? string1 string2)`

Returns `#t` if the two strings are the same length and contain the same characters in the same positions, otherwise returns `#f`. `String-ci=?` treats upper and lower case letters as though they were the same character, but `string=?` treats upper and lower case as distinct characters.

 `(string
`(string>? string1 string2)`
`(string<=? string1 string2)`
`(string>=? string1 string2)`
`(string-ci<? string1 string2)`
`(string-ci>? string1 string2)`
`(string-ci<=? string1 string2)`
`(string-ci>=? string1 string2)`

These procedures are the lexicographic extensions to strings of the corresponding orderings on characters. For example, `string<?` is the lexicographic ordering on strings induced by the ordering `char<?` on characters. If two strings differ in length but are the same up to the length of the shorter string, the shorter string is considered to be lexicographically less than the longer string.

 `(substring string start end)` R5RS procedure

`String` must be a string, and `start` and `end` must be exact integers satisfying
 ```0 <= start <= end <= (string-length string). ```
`Substring` returns a newly allocated string formed from the characters of `string` beginning with index `start` (inclusive) and ending with index `end` (exclusive).

 `(string-append string ...)` R5RS procedure

Returns a newly allocated string whose characters form the concatenation of the given strings.

 `(string->list string)` R5RS procedure
`(list->string list)`

`String->list` returns a newly allocated list of the characters that make up the given string. `List->string` returns a newly allocated string formed from the characters in the list `list`, which must be a list of characters. `String->list` and `list->string` are inverses so far as `equal?` is concerned.

 `(string-copy string)` R5RS procedure

Returns a newly allocated copy of the given `string`.

 `(string-split str)` STklos procedure
`(string-split str delimiters)`

parses `string` and returns a list of tokens ended by a character of the `delimiters` string. If `delimiters` is omitted, it defaults to a string containing a space, a tabulation and a newline characters.
 ```(string-split "/usr/local/bin" "/") ⇒ ("usr" "local" "bin") (string-split "once upon a time") ⇒ ("once" "upon" "a" "time") ```

 `(string-index str1 str2)` STklos procedure

Returns the (first) index where `str1` is a substring of `str2` if it exists; otherwise returns `#f`.
 ```(string-index "ca" "abracadabra") ⇒ 4 (string-index "ba" "abracadabra") ⇒ #f ```

 `(string-find? str1 str2)` STklos procedure

Returns `#t` if `str1` appears somewhere in `str2`; otherwise returns `#f`.

 `(string-fill! string char)` STklos procedure

Stores `char` in every element of the given `string` and returns void.

 `(string-blit! s1 s2 offset)` STklos procedure

This function places the characters of string `s2` in the string `s1` starting at position `offset`. The result of `string-blit!` may modify the string `s1`. Note that the characters of `s2` can be written after the end of `s1` (in which case a new string is allocated).
 ```(string-blit! (make-string 6 #X) "abc" 2) ⇒ "XXabcX" (string-blit! (make-string 10 #X) "abc" 5) ⇒ "XXXXXabcXX" (string-blit! (make-string 6 #X) "a" 10) ⇒ "XXXXXX0000a" ```

 `(string-mutable? obj)` STklos procedure

Returns `#t` if `obj` is a mutable string, otherwise returns `#f`.
 ```(string-mutable? "abc") ⇒ #f (string-mutable? (string-copy "abc")) ⇒ #t (string-mutable? (string #a #b #c)) ⇒ #t (string-mutable? 12) ⇒ #f ```

The following string primitives are compatible with SRFI-13 (String Library) and their documentation comes from the SRFI document.

Note: The string SRFI is supported by STklos. The function listed below just don't need to load the full SRFI to be used

 `(string-downcase str)` STklos procedure
`(string-downcase str start)`
`(string-downcase str start end)`

Returns a string in which the upper case letters of string `str` between the `start` and `end` indices have been replaced by their lower case equivalent. If `start` is omited, it defaults to 0. If `end` is omited, it defaults to the length of `str`.
 ```(string-downcase "Foo BAR") ⇒ "foo bar" (string-downcase "Foo BAR" 4) ⇒ "bar" (string-downcase "Foo BAR" 4 6) ⇒ "ba" ```

 `(string-downcase! str)` STklos procedure
`(string-downcase! str start)`
`(string-downcase! str start end)`

This is the in-place side-effecting variant of `string-downcase`.
 ```(string-downcase! (string-copy "Foo BAR") 4) ⇒ "Foo bar" (string-downcase! (string-copy "Foo BAR") 4 6) ⇒ "Foo baR" ```

 `(string-upcase str)` STklos procedure
`(string-upcase str start)`
`(string-upcase str start end)`

Returns a string in which the lower case letters of string `str` between the `start` and `end` indices have been replaced by their upper case equivalent. If `start` is omited, it defaults to 0. If `end` is omited, it defaults to the length of `str`.

 `(string-upcase! str)` STklos procedure
`(string-upcase! str start)`
`(string-upcase! str start end)`

This is the in-place side-effecting variant of `string-upcase`.

 `(string-titlecase str)` STklos procedure
`(string-titlecase str start)`
`(string-titlecase str start end)`

This function returns a string. For every character `c` in the selected range of `str`, if `c` is preceded by a cased character, it is downcased; otherwise it is titlecased. If `start` is omited, it defaults to 0. If `end` is omited, it defaults to the length of `str`. Note that if a `start` index is specified, then the character preceding `s`(start)` has no effect on the titlecase decision for character `s`(start)`.
 ```(string-titlecase "--capitalize tHIS sentence.") ⇒ "--Capitalize This Sentence." (string-titlecase "see Spot run. see Nix run.") ⇒ "See Spot Run. See Nix Run." (string-titlecase "3com makes routers.") ⇒ "3Com Makes Routers." (string-titlecase "greasy fried chicken" 2) ⇒ "Easy Fried Chicken" ```

 `(string-titlecase! str)` STklos procedure
`(string-titlecase! str start)`
`(string-titlecase! str start end)`

This is the in-place side-effecting variant of `string-titlecase`.

The functions string-foldcase and string-foldcase! described below are inspired from R6RS.

 `(string-foldcase str)` STklos procedure
`(string-foldcase str start)`
`(string-foldcase str start end)`

Returns a string in which the Unicode simple case-folding algorithm has been applied on `str` between the `start` and `end` indices. If `start` is omited, it defaults to 0. If `end` is omited, it defaults to the length of `str`.

 `(string-foldcase! str)` STklos procedure
`(string-foldcase! str start)`
`(string-foldcase! str start end)`

This is the in-place side-effecting variant of `string-foldcase`.

### 4.8 Vectors

Vectors are heterogenous structures whose elements are indexed by integers. A vector typically occupies less space than a list of the same length, and the average time required to access a randomly chosen element is typically less for the vector than for the list.

The length of a vector is the number of elements that it contains. This number is a non-negative integer that is fixed when the vector is created. The valid indexes of a vector are the exact non-negative integers less than the length of the vector. The first element in a vector is indexed by zero, and the last element is indexed by one less than the length of the vector.

Vectors are written using the notation `#(obj ...)`. For example, a vector of length 3 containing the number zero in element 0, the list `(2 2 2 2)` in element 1, and the string `"Anna"` in element 2 can be written as following:

 ```#(0 (2 2 2 2) "Anna") ```

Note: In STklos, vectors constants don't need to be quoted.

 `(vector? obj)` R5RS procedure

Returns `#t` if `obj` is a vector, otherwise returns `#f`.

 `(make-vector k)` R5RS procedure
`(make-vector k fill)`

Returns a newly allocated vector of `k` elements. If a second argument is given, then each element is initialized to `fill`. Otherwise the initial contents of each element is unspecified.

 `(vector obj ...)` R5RS procedure

Returns a newly allocated vector whose elements contain the given arguments. Analogous to `list`.
 ```(vector 'a 'b 'c) ⇒ #(a b c) ```

 `(vector-length vector)` R5RS procedure

Returns the number of elements in `vector` as an exact integer.

 `(vector-ref vector k)` R5RS procedure

`k` must be a valid index of `vector`. `Vector-ref` returns the contents of element `k` of vector.
 ```(vector-ref '#(1 1 2 3 5 8 13 21) 5) ⇒ 8 (vector-ref '#(1 1 2 3 5 8 13 21) (let ((i (round (* 2 (acos -1))))) (if (inexact? i) (inexact->exact i) i))) ⇒ 13 ```

 `(vector-set! vector k obj)` R5RS procedure

`k` must be a valid index of `vector`. `Vector-set!` stores `obj` in element `k` of `vector`. The value returned by `vector-set!` is void.
 ```(let ((vec (vector 0 '(2 2 2 2) "Anna"))) (vector-set! vec 1 '("Sue" "Sue")) vec) ⇒ #(0 ("Sue" "Sue") "Anna") (vector-set! '#(0 1 2) 1 "doe") ⇒ error ; constant vector ```

 `(vector->list vector)` R5RS procedure
`(list->vector list)`

`Vector->list` returns a newly allocated list of the objects contained in the elements of `vector`. `List->vector` returns a newly created vector initialized to the elements of the list `list`.
 ```(vector->list '#(dah dah didah)) ⇒ (dah dah didah) (list->vector '(dididit dah)) ⇒ #(dididit dah) ```

 `(vector->string string)` STklos procedure
`(string->vector vector)`

`Vector->string` returns a newly allocated string of the objects contained in the elements of `vector`, which must be characters allowed in a string. `String->vector` returns a newly created vector initialized to the elements of the string `string`.

 `(vector-fill! vector fill)` R5RS procedure

Stores `fill` in every element of `vector`. The value returned by `vector-fill!` is void.

 `(vector-copy v)` STklos procedure

Return a copy of vector `v`. Note that, if `v` is a constant vector, its copy is not constant.

 `(vector-resize v size)` STklos procedure
`(vector-resize v size fill)`

Returns a copy of v of the given `size`. If `size` is greater than the vector size of `v`, the contents of the newly allocated vector cells is set to the value of `fill`. If `fill` is omitted the content of the new cells is void.

 `(vector-mutable? obj)` STklos procedure

Returns `#t` if `obj` is a mutable vector, otherwise returns `#f`.
 ```(vector-mutable? '#(1 2 a b)) ⇒ #f (vector-mutable? (vector-copy '#(1 2))) ⇒ #t (vector-mutable? (vector 1 2 3)) ⇒ #t (vector-mutable? 12) ⇒ #f ```

 `(sort obj predicate)` STklos procedure

`Obj` must be a list or a vector. `Sort` returns a copy of `obj` sorted according to `predicate`. `Predicate` must be a procedure which takes two arguments and returns a true value if the first argument is strictly ``before'' the second.
 ```(sort '(1 2 -4 12 9 -1 2 3) <) ⇒ (-4 -1 1 2 2 3 9 12) (sort '#("one" "two" "three" "four") (lambda (x y) (> (string-length x) (string-length y)))) ⇒ '#("three" "four" "one" "two") ```

### 4.9 Structures

A structure type is a record data type composing a number of slots. A structure, an instance of a structure type, is a first-class value that contains a value for each field of the structure type.

Structures can be created with the `define-struct` high level syntax. However, STklos also offers some low-level functions to build and access the internals of a structure.

 `(define-struct ...)` STklos syntax

Defines a structure type whose name is `<name>`. Once a structure type is defined, the following symbols are bound:
• `<name>` denotes the structure type.
• `make-<name>` is a procedure which takes 0 to `n` parameters (if there are `n` slots defined). Each parameter is assigned to the corresponding field (in the definition order).
• `<name>?` is a predicate which returns `#t` when applied to an instance of the `<name>` structure type and `#f` otherwise.
• `<name>-<slot>` (one for each defined `<slot>`) to read the content of an instance of the `<name>` structure type. Writting the content of a slot can be done using a generalized `set!`.
 ```(define-struct point x y) (define p (make-point 1 2)) (point? p) ⇒ #t (point? 100) ⇒ #f (point-x p) ⇒ 1 (point-y p) ⇒ 2 (set! (point-x p) 10) (point-x p) ⇒ 10 ```

 `(make-struct-type name parent slots)` STklos procedure

This form which is more general than `define-struct` permits to define a new structure type whose name is `name`. Parent is the structure type from which is the new structure type is a subtype (or `#f` is the new structure-type has no super type). `Slots` is the list of the slot names which constitute the structure tpe.

When a structure type is s subtype of a previous type, its slots are added to the ones of the super type.

 `(struct-type? obj)` STklos procedure

Returns `#t` if `obj` is a structure type, otherwise returns `#f`.
 ```(let ((type (make-struct-type 'point #f '(x y)))) (struct-type? type)) ⇒ #t ```

 `(struct-type-slots structype)` STklos procedure

Returns the slots of the structure type `structype` as a list.
 ```(define point (make-struct-type 'point #f '(x y))) (define circle (make-struct-type 'circle point '(r))) (struct-type-slots point) ⇒ (x y) (struct-type-slots circle) ⇒ (x y r) ```

 `(struct-type-parent structype)` STklos procedure

Returns the super type of the structure type `structype`, if it exists or `#f` otherwise.

 `(struct-type-name structype)` STklos procedure

Returns the name associated to the structure type `structype`.

 `(struct-type-change-writer! structype proc)` STklos procedure

Change the default writer associated to structures of type `structype` to to the `proc` procedure. The `proc` procedure must accept 2 arguments (the structure to write and the port wher the structure must be written in that order). The value returned by `struct-type-change-writer!` is the old writer associated to `structype`. To restore the standard wtructure writer for `structype`, use the special value `#f`.
 ```(define point (make-struct-type 'point #f '(x y))) (struct-type-change-writer! point (lambda (s port) (let ((type (struct-type s))) (format port "{~A" (struct-type-name type)) ;; display the slots and their value (for-each (lambda (x) (format port " ~A=~S" x (struct-ref s x))) (struct-type-slots type)) (format port "}")))) (display (make-struct point 1 2)) -| {point x=1 y=2} ```

 `(make-struct structype expr ...)` STklos procedure

Returns a newly allocated instance of the structure type `structype`, whose slots are initialized to `expr` ... If fewer `expr` than the number of instances are given to `make-struct`, the remaining slots are inialized with the special void value.

 `(struct? obj)` STklos procedure

Returns `#t` if `obj` is a structure, otherwise returns `#f`.
 ```(let* ((type (make-struct-type 'point #f '(x y))) (inst (make-struct type 1 2))) (struct? inst)) ⇒ #t ```

 `(struct-type s)` STklos procedure

Returns the structure type of the `s` structure

 `(struct-ref s slot-name)` STklos procedure

Returns the value associated to slot `slot-name` of the `s` structure.
 ```(define point (make-struct-type 'point #f '(x y))) (define circle (make-struct-type 'circle point '(r))) (define p (make-struct point 1 2)) (define c (make-struct circle 10 20 30)) (struct-ref p 'y) ⇒ 2 (struct-ref c 'r) ⇒ 30 ```

 `(struct-set! s slot-name value)` STklos procedure

Stores value in the to slot `slot-name` of the `s` structure. The value returned by `struct-set!` is void.
 ```(define point (make-struct-type 'point #f '(x y))) (define p (make-struct point 1 2)) (struct-ref p 'x) ⇒ 1 (struct-set! p 'x 0) (struct-ref p 'x) ⇒ 0 ```

 `(struct-is-a? s structype)` STklos procedure

Return a boolean that indicates if the structure `s` is a of type `structype`. Note that if `s` is an instance of a subtype of S, it is considered also as an instance of type S.
 ```(define point (make-struct-type 'point #f '(x y))) (define circle (make-struct-type 'circle point '(r))) (define p (make-struct point 1 2)) (define c (make-struct circle 10 20 30)) (struct-is-a? p point) ⇒ #t (struct-is-a? c point) ⇒ #t (struct-is-a? p circle) ⇒ #f (struct-is-a? c circle) ⇒ #t ```

 `(struct->list s)` STklos procedure

Returns the content of structure `s` as an A-list whose keys are the slots of the structure type of `s`.
 ```(define point (make-struct-type 'point #f '(x y))) (define p (make-struct point 1 2)) (struct->list p) ⇒ ((x . 1) (y . 2)) ```

### 4.10 Bytevectors

Bytevectors represent blocks of binary data. They are fixed-length sequences of bytes, where a byte is an exact integer in the range `(0, 255). A bytevector is typically more space-efficient than a vector containing the same values.

The length of a bytevector is the number of elements that it contains. This number is a non-negative integer that is fixed when the bytevector is created. The valid indexes of a bytevector are the exact non-negative integers less than the length of the bytevector, starting at index zero as with vectors.

 `(bytevector? obj)` STklos procedure

Returns `!t` if `obj` is a bytevector and returns `!f` otherwise.

 `(make-bytevector)` STklos procedure

Returns a newly allocated bytevector of k bytes. The initial contents of each element is 0.

 `(bytevector-length bytevector)` STklos procedure

Returns the length of `bytevector` in bytes as an exact integer.

 `(bytevector-u8-ref bytevector k)` STklos procedure

Returns the byte at index `k` of `bytevector` as an exact integer in the range `(0..255).

 `(bytevector-u8-ref bytevector k u8)` STklos procedure

Stores `u8` in the byte at index `k` of `bytevector`. `u8` must be an exact integer in the range `(0..255). The value returned by `bytevector-u8-set!` is void.

 `(bytevector-copy bytevector)` STklos procedure

Returns a newly allocated bytevector containing the same bytes as `bytevector`.

 `(bytevector-copy! from to)` STklos procedure

Copy the bytes of bytevector `from` to bytevector `to`, which must not be shorter. The value returned by `bytevector-copy!` is void.

 `(bytevector-copy-partial bytevector start end)` STklos procedure

Returns a newly allocated bytevector containing the bytes in `bytevector` between `start` (inclusive) and `end` (exclusive).

 `(bytevector-copy-partial! from start end to at)` STklos procedure

Copy the bytes of `bytevector` from between `start` and `end` to bytevector `to`, starting at `at`. The order in which bytes are copied is unspecified, except that if the source and destination overlap, copying takes place as if the source is first copied into a temporary bytevector and then into the destination. The value returned by `partial-bytevector-copy!` is void.

### 4.11 Control features

 `(procedure? obj)` R5RS procedure

Returns `#t` if `obj` is a procedure, otherwise returns `#f`.
 ```(procedure? car) ⇒ #t (procedure? 'car) ⇒ #f (procedure? (lambda (x) (* x x))) ⇒ #t (procedure? '(lambda (x) (* x x))) ⇒ #f (call-with-current-continuation procedure?) ⇒ #t ```

 `(apply proc arg1 ... args)` R5RS procedure

`Proc` must be a procedure and `args` must be a list. Calls `proc` with the elements of the list
 ```(append (list arg1 ...) args) ```
as the actual arguments.
 ```(apply + (list 3 4)) ⇒ 7 (define compose (lambda (f g) (lambda args (f (apply g args))))) ((compose sqrt *) 12 75) ⇒ 30 ```

 `(map proc list1 list2 ...)` R5RS procedure

The `list`s must be lists, and `proc` must be a procedure taking as many arguments as there are lists and returning a single value. If more than one list is given, then they must all be the same length. `Map` applies `proc` element-wise to the elements of the `list`s and returns a list of the results, in order. The dynamic order in which proc is applied to the elements of the lists is unspecified.
 ```(map cadr '((a b) (d e) (g h))) ⇒ (b e h) (map (lambda (n) (expt n n)) '(1 2 3 4 5)) ⇒ (1 4 27 256 3125) (map + '(1 2 3) '(4 5 6)) ⇒ (5 7 9) (let ((count 0)) (map (lambda (ignored) (set! count (+ count 1)) count) '(a b))) ⇒ (1 2) or (2 1) ```

 `(string-map proc string1 string2 ...)` STklos procedure

The `strings` must be strings, and `proc` must be a procedure taking as many arguments as there are strings and returning a single value. If more than one string is given and not all strings have the same length, `string-map` terminates when the shortest list runs out. `String-map` applies `proc` element-wise to the elements of the strings and returns a string of the results, in order. The dynamic order in which proc is applied to the elements of the `strings` is unspecified.
 ```(string-map char-downcase "AbdEgH") ⇒ "abdegh" (string-map (lambda (c) (integer->char (+ 1 (char->integer c)))) "HAL") ⇒ "IBM" (string-map (lambda (c k) (if (eqv? k #u) (char-upcase c) (char-downcase c))) "studlycaps" "ululululul") ⇒ "StUdLyCaPs" ```

 `(vector-map proc vector1 vector2 ...)` STklos procedure

The `vectors` must be vectors, and `proc` must be a procedure taking as many arguments as there are vectors and returning a single value. If more than one vector is given and not all vectors have the same length, `vector-map` terminates when the shortest list runs out. `Vector-map` applies `proc` element-wise to the elements of the vectors and returns a vector of the results, in order. The dynamic order in which proc is applied to the elements of the `vectors` is unspecified.
 ```(vector-map cadr '#((a b) (d e) (g h))) ⇒ #(b e h) (vector-map (lambda (n) (expt n n)) '#(1 2 3 4 5)) ⇒ #(1 4 27 256 3125) (vector-map + '#(1 2 3) '#(4 5 6)) ⇒ #(5 7 9) (let ((count 0)) (vector-map (lambda (ignored) (set! count (+ count 1)) count) '#(a b))) ⇒ #(1 2) or #(2 1) ```

 `(for-each proc list1 list2 ...)` R5RS procedure

The arguments to `for-each` are like the arguments to `map`, but `for-each` calls proc for its side effects rather than for its values. Unlike `map`, `for-each` is guaranteed to call proc on the elements of the lists in order from the first element(s) to the last, and the value returned by `for-each` is void.
 ```(let ((v (make-vector 5))) (for-each (lambda (i) (vector-set! v i (* i i))) '(0 1 2 3 4)) v) ⇒ #(0 1 4 9 16) ```

 `(string-for-each proc string1 string2 ...)` STklos procedure

The arguments to `string-for-each` are like the arguments to `string-map`, but `string-for-each` calls `proc` for its side effects rather than for its values. Unlike `string-map`, `string-for-each` is guaranteed to call `proc` on the elements of the lists in order from the first element(s) to the last, and the value returned by `string-for-each` is unspecified. If more than one string is given and not all strings have the same length, `string-for-each` terminates when the shortest string runs out.
 ```(let ((v (list))) (string-for-each (lambda (c) (set! v (cons (char->integer c) v))) "abcde") v) ⇒ (101 100 99 98 97) ```

 `(vector-for-each proc vector1 vector2 ...)` STklos procedure

The arguments to `vector-for-each` are like the arguments to `vector-map`, but `vector-for-each` calls `proc` for its side effects rather than for its values. Unlike `vector-map`, `vector-for-each` is guaranteed to call `proc` on the elements of the lists in order from the first element(s) to the last, and the value returned by `vector-for-each` is unspecified. If more than one vector is given and not all vectors have the same length, `vector-for-each` terminates when the shortest vector runs out.
 ```(let ((v (make-vector 5))) (vector-for-each (lambda (i) (vector-set! v i (* i i))) '#(0 1 2 3 4)) v) ⇒ #(0 1 4 9 16) ```

 `(every pred list1 list2 ...)` STklos procedure

`every` applies the predicate `pred` across the lists, returning true if the predicate returns true on every application.

If there are n list arguments `list1` ... `listn`, then `pred` must be a procedure taking n arguments and returning a boolean result.

`every` applies pred to the first elements of the `listi` parameters. If this application returns false, every immediately returns `#f`. Otherwise, it iterates, applying `pred` to the second elements of the `listi` parameters, then the third, and so forth. The iteration stops when a false value is produced or one of the lists runs out of values. In the latter case, `every` returns the true value produced by its final application of pred. The application of pred to the last element of the lists is a tail call.

If one of the `listi` has no elements, `every` simply returns `#t`.

Like `any`, every's name does not end with a question mark -- this is to indicate that it does not return a simple boolean (`#t` or `#f`), but a general value.

 `(any pred list1 list2 ...)` STklos procedure

`any` applies the predicate across the lists, returning true if the predicate returns true on any application.

If there are n list arguments `list1` ... `listn`, then `pred` must be a procedure taking n arguments.

`any` applies `pred` to the first elements of the `listi` parameters. If this application returns a true value, `any` immediately returns that value. Otherwise, it iterates, applying `pred` to the second elements of the `listi` parameters, then the third, and so forth. The iteration stops when a true value is produced or one of the lists runs out of values; in the latter case, any returns `#f`. The application of `pred` to the last element of the lists is a tail call.

Like every, `any`'s name does not end with a question mark -- this is to indicate that it does not return a simple boolean (`#t` or `#f`), but a general value.
 ```(any integer? '(a 3 b 2.7)) ⇒ #t (any integer? '(a 3.1 b 2.7)) ⇒ #f (any < '(3 1 4 1 5) '(2 7 1 8 2)) ⇒ #t ```

 `(force promise)` R5RS procedure

Forces the value of `promise` (see delay). If no value has been computed for the promise, then a value is computed and returned. The value of the promise is cached (or "memoized") so that if it is forced a second time, the previously computed value is returned.
 ```(force (delay (+ 1 2))) ⇒ 3 (let ((p (delay (+ 1 2)))) (list (force p) (force p))) ⇒ (3 3) (define a-stream (letrec ((next (lambda (n) (cons n (delay (next (+ n 1))))))) (next 0))) (define head car) (define tail (lambda (stream) (force (cdr stream)))) (head (tail (tail a-stream))) ⇒ 2 ```
`Force` and `delay` are mainly intended for programs written in functional style. The following examples should not be considered to illustrate good programming style, but they illustrate the property that only one value is computed for a promise, no matter how many times it is forced.
 ```(define count 0) (define p (delay (begin (set! count (+ count 1)) (if (> count x) count (force p))))) (define x 5) p ⇒ a promise (force p) ⇒ 6 p ⇒ a promise, still (begin (set! x 10) (force p)) ⇒ 6 ```
Note: See R5RS for details on a posssible way to implement `force` and `delay`.

 `(call-with-current-continuation proc)` R5RS procedure
`(call/cc proc)`

`Proc` must be a procedure of one argument. The procedure `call-with-current-continuation` packages up the current continuation (see the rationale below) as an ``escape procedure'' and passes it as an argument to `proc`. The escape procedure is a Scheme procedure that, if it is later called, will abandon whatever continuation is in effect at that later time and will instead use the continuation that was in effect when the escape procedure was created. Calling the escape procedure may cause the invocation of before and after thunks installed using `dynamic-wind`.

The escape procedure accepts the same number of arguments as the continuation to the original call to `call-with-current-continuation`. Except for continuations created by the `call-with-values` procedure, all continuations take exactly one value.

The escape procedure that is passed to proc has unlimited extent just like any other procedure in Scheme. It may be stored in variables or data structures and may be called as many times as desired.

The following examples show only the most common ways in which `call-with-current-continuation` is used. If all real uses were as simple as these examples, there would be no need for a procedure with the power of `call-with-current-continuation`.
 ```(call-with-current-continuation (lambda (exit) (for-each (lambda (x) (if (negative? x) (exit x))) '(54 0 37 -3 245 19)) #t)) ⇒ -3 (define list-length (lambda (obj) (call-with-current-continuation (lambda (return) (letrec ((r (lambda (obj) (cond ((null? obj) 0) ((pair? obj) (+ (r (cdr obj)) 1)) (else (return #f)))))) (r obj)))))) (list-length '(1 2 3 4)) ⇒ 4 (list-length '(a b . c)) ⇒ #f ```
Rationale: A common use of `call-with-current-continuation` is for structured, non-local exits from loops or procedure bodies, but in fact `call-with-current-continuation` is extremely useful for implementing a wide variety of advanced control structures.

Whenever a Scheme expression is evaluated there is a continuation wanting the result of the expression. The continuation represents an entire (default) future for the computation. If the expression is evaluated at top level, for example, then the continuation might take the result, print it on the screen, prompt for the next input, evaluate it, and so on forever. Most of the time the continuation includes actions specified by user code, as in a continuation that will take the result, multiply it by the value stored in a local variable, add seven, and give the answer to the top level continuation to be printed. Normally these ubiquitous continuations are hidden behind the scenes and programmers do not think much about them. On rare occasions, however, a programmer may need to deal with continuations explicitly. `Call-with-current-continuation` allows Scheme programmers to do that by creating a procedure that acts just like the current continuation.

Note: `call/cc` is just another name for `call-with-current-continuation`.

 `(call/ec proc)` STklos procedure

`call/ec` is an short name for `call-with-escape-continuation`. `call/ec` calls `proc` with one parameter, which is the current escape continuation (a continuation which can only be used to abort a computation and hence cannot be "re-enterered".
 ```(list 1 (call/ec (lambda (return) (list 'a (return 'b) 'c))) 3) ⇒ (1 b 3) ```
`call/ec` is cheaper than the full call/cc. It is particularily useful when all the power of `call/cc` is not needded.

 `(values obj ...)` R5RS procedure

Delivers all of its arguments to its continuation. Note: R5RS imposes to use multiple values in the context of of a `call-with-values`. In STklos, if `values` is not used with `call-with-values`, only the first value is used (i.e. others values are ignored).

 `(call-with-values producer consumer)` R5RS procedure

Calls its producer argument with no values and a continuation that, when passed some values, calls the consumer procedure with those values as arguments. The continuation for the call to consumer is the continuation of the call to call-with-values.
 ```(call-with-values (lambda () (values 4 5)) (lambda (a b) b)) ⇒ 5 (call-with-values * -) ⇒ -1 ```

 `(receive )` STklos syntax

This form is defined in SRFI-8 (Receive: Binding to multiple values). It simplifies the usage of multiple values. Specifically, `<formals>` can have any of three forms:
• (`<variable1>` ... `<variablen>`):
The environment in which the receive-expression is evaluated is extended by binding `<variable1>`, ..., `<variablen>` to fresh locations.

The `<expression>` is evaluated, and its values are stored into those locations. (It is an error if `<expression>` does not have exactly n values.)
• `<variable>`:
The environment in which the receive-expression is evaluated is extended by binding `<variable>` to a fresh location. The `<expression>` is evaluated, its values are converted into a newly allocated list, and the list is stored in the location bound to `<variable>`.
• (`<variable1>` ... `<variablen>` . `<variablen + 1>`):
The environment in which the receive-expression is evaluated is extended by binding `<variable1>`, ..., `<variablen + 1>` to fresh locations. The `<expression>` is evaluated. Its first n values are stored into the locations bound to `<variable1>` ... `<variablen>`. Any remaining values are converted into a newly allocated list, which is stored into the location bound to `<variablen + 1>`. (It is an error if `<expression>` does not have at least n values.)
In any case, the expressions in `<body>` are evaluated sequentially in the extended environment. The results of the last expression in the body are the values of the receive-expression.
 ```(let ((n 123)) (receive (q r) (values (quotient n 10) (modulo n 10)) (cons q r))) ⇒ (12 . 3) ```

 `(dynamic-wind before thunk after)` R5RS procedure

Current version of `dynamic-wind` mimics the R5RS one. In particular, it does not yet interact with `call-with-current-continuation` as required by R5RS.

Calls `thunk` without arguments, returning the result(s) of this call. `Before` and `after` are called, also without arguments, as required by the following rules (note that in the absence of calls to continuations captured using `call-with-current-continuation` the three arguments are called once each, in order). `Before` is called whenever execution enters the dynamic extent of the call to `thunk` and `after` is called whenever it exits that dynamic extent. The dynamic extent of a procedure call is the period between when the call is initiated and when it returns. In Scheme, because of `call-with-current-continuation`, the dynamic extent of a call may not be a single, connected time period. It is defined as follows:
• The dynamic extent is entered when execution of the body of the called procedure begins.
• The dynamic extent is also entered when execution is not within the dynamic extent and a continuation is invoked that was captured (using `call-with-current-continuation`) during the dynamic extent.
• It is exited when the called procedure returns.
• It is also exited when execution is within the dynamic extent and a continuation is invoked that was captured while not within the dynamic extent.

If a second call to `dynamic-wind` occurs within the dynamic extent of the call to thunk and then a continuation is invoked in such a way that the afters from these two invocations of `dynamic-wind` are both to be called, then the after associated with the second (inner) call to `dynamic-wind` is called first. If a second call to `dynamic-wind` occurs within the dynamic extent of the call to `thunk` and then a continuation is invoked in such a way that the befores from these two invocations of `dynamic-wind` are both to be called, then the before associated with the first (outer) call to `dynamic-wind` is called first. If invoking a continuation requires calling the `before` from one call to `dynamic-wind` and the `after` from another, then the `after` is called first. The effect of using a captured continuation to enter or exit the dynamic extent of a call to `before` or `after` is undefined.
 ```(let ((path '()) (c #f)) (let ((add (lambda (s) (set! path (cons s path))))) (dynamic-wind (lambda () (add 'connect)) (lambda () (add (call-with-current-continuation (lambda (c0) (set! c c0) 'talk1)))) (lambda () (add 'disconnect))) (if (< (length path) 4) (c 'talk2) (reverse path)))) ⇒ (connect talk1 disconnect connect talk2 disconnect) ```

 `(eval expression environment)` R5RS procedure
`(eval expression)`

Evaluates expression in the specified environment and returns its value. `Expression` must be a valid Scheme expression represented as data. `Environment` may be a R5RS environment-specifier (`interaction-environment`, `scheme-report-environment` or `null-environment`) or a STklos module.
 ```(eval '(* 7 3) (scheme-report-environment 5)) ⇒ 21 (let ((f (eval '(lambda (f x) (f x x)) (null-environment 5)))) (f + 10)) ⇒ 20 (define-module A (define x 1)) (eval '(cons x x) (find-module 'A)) ⇒ (1 . 1) ```

 `(scheme-report-environment)` R5RS procedure
`(scheme-report-environment version)`

Returns a specifier for an environment that contains the bindings defined in the R5RS report.
Note: In STklos, `scheme-report-environment` function can be called without the version number (defaults to 5).

 `(null-environment)` R5RS procedure
`(null-environment version)`

Returns a specifier for an environment that is empty except for the (syntactic) bindings for all syntactic keywords defined in the R5RS report.
Note: In STklos, `null-environment` function can be called without the version number (defaults to 5).

 `(interaction-environment)` R5RS procedure

This procedure returns the environment in the expression are evaluated by default (the STklos module).

 `(eval-from-string str)` STklos procedure
`(eval-from-string str module)`

Read an expression from `str` and evaluates it with `eval`. If a `module` is passed, the evaluation takes place in the environment of this module. Otherwise, the evaluation takes place in the environment returned by `current-module`.
 ```(define x 10) (define-module M (define x 100)) (eval-from-string "(+ x x)") ⇒ 20 (eval-from-string "(+ x x)" (find-module 'M)) ⇒ 200 ```

### 4.12 Input and Output

R5RS states that ports represent input and output devices. However, it defines only ports which are attached to files. In STklos, ports can also be attached to strings, to a external command input or output, or even be virtual (i.e. the behavior of the port is given by the user).

• String ports are similar to file ports, except that characters are read from (or written to) a string rather than a file.
• External command input or output ports are implemented with Unix pipes and are called pipe ports. A pipe port is created by specifying the command to execute prefixed with the string `"| "` (that is a pipe bar followed by a space). Specification of a pipe port can occur everywhere a file name is needed.
• Virtual ports are created by supplying basic I/O functions at port creation time. These functions will be used to simulate low level accesses to a ``virtual device''. This kind of port is particularly convenient for reading or writing in a graphical window as if it was a file. Once a virtual port is created, it can be accessed as a normal port with the standard Scheme primitives.

#### 4.12.1 Ports

 `(call-with-input-file string proc)` R5RS procedure
`(call-with-output-file string proc)`

`String` should be a string naming a file, and `proc` should be a procedure that accepts one argument. For `call-with-input-file`, the file should already exist. These procedures call `proc` with one argument: the port obtained by opening the named file for input or output. If the file cannot be opened, an error is signaled. If `proc` returns, then the port is closed automatically and the value(s) yielded by the proc is(are) returned. If proc does not return, then the port will not be closed automatically.

Rationale: Because Scheme's escape procedures have unlimited extent, it is possible to escape from the current continuation but later to escape back in. If implementations were permitted to close the port on any escape from the current continuation, then it would be impossible to write portable code using both `call-with-current-continuation` and `call-with-input-file` or `call-with-output-file`.

 `(call-with-input-string string proc)` STklos procedure

behaves as `call-with-input-file` except that the port passed to `proc` is the sting port obtained from `port`.
 ```(call-with-input-string "123 456" (lambda (x) (let* ((n1 (read x)) (n2 (read x))) (cons n1 n2)))) ⇒ (123 . 456) ```

 `(call-with-output-string proc)` STklos procedure

`Proc` should be a procedure of one argument. `Call-with-output-string` calls `proc` with a freshly opened output string port. The result of this procedure is a string containing all the text that has been written on the string port.
 ```(call-with-output-string (lambda (x) (write 123 x) (display "Hello" x))) ⇒ "123Hello" ```

 `(input-port? obj)` R5RS procedure
`(output-port? obj)`

Returns `#t` if `obj` is an input port or output port respectively, otherwise returns #f.

 `(port? obj)` STklos procedure

Returns `#t` if `obj` is an input port or an output port, otherwise returns #f.

 `(input-string-port? obj)` STklos procedure
`(output-string-port? obj)`

Returns `#t` if `obj` is an input string port or output string port respectively, otherwise returns #f.

 `(input-file-port? obj)` STklos procedure
`(output-file-port? obj)`

Returns `#t` if `obj` is a file input port or a file output port respectively, otherwise returns #f.

 `(input-virtual-port? obj)` STklos procedure
`(output-virtual-port? obj)`

Returns `#t` if `obj` is a virtual input port or a virtual output port respectively, otherwise returns #f.

 `(interactive-port? port)` STklos procedure

Returns `#t` if `port` is connected to a terminal and `#f` otherwise.

 `(current-input-port obj)` R5RS procedure
`(current-output-port obj)`

Returns the current default input or output port.

 `(current-error-port obj)` STklos procedure

Returns the current default error port.

 `(with-input-from-file string thunk)` R5RS procedure
`(with-output-to-file string thunk)`

`String` should be a string naming a file, and `proc` should be a procedure of no arguments. For `with-input-from-file`, the file should already exist. The file is opened for input or output, an input or output port connected to it is made the default value returned by `current-input-port` or `current-output-port` (and is used by `(read)`, `(write obj)`, and so forth), and the thunk is called with no arguments. When the thunk returns, the port is closed and the previous default is restored. `With-input-from-file` and `with-output-to-file` return(s) the value(s) yielded by thunk.

The following example uses a pipe port opened for reading. It permits to read all the lines produced by an external ls command (i.e. the output of the ls command is redirected to the Scheme pipe port).
 ```(with-input-from-file "| ls -ls" (lambda () (do ((l (read-line) (read-line))) ((eof-object? l)) (display l) (newline)))) ```
Hereafter is another example of Unix command redirection. This time, it is the standard input of the Unix command which is redirected.
 ```(with-output-to-file "| mail root" (lambda () (display "A simple mail from Scheme") (newline))) ```

 `(with-error-to-file string thunk)` STklos procedure

This procedure is similar to with-output-to-file, excepted that it uses the current error port instead of the output port.

 `(with-input-from-string string thunk)` STklos procedure

A string port is opened for input from `string`. `Current-input-port` is set to the port and `thunk` is called. When `thunk` returns, the previous default input port is restored. `With-input-from-string` returns the value(s) computed by `thunk`.
 ```(with-input-from-string "123 456" (lambda () (read))) ⇒ 123 ```

 `(with-output-to-string thunk)` STklos procedure

A string port is opened for output. `Current-output-port` is set to it and `thunk` is called. When `thunk` returns, the previous default output port is restored. `With-output-to-string` returns the string containing the text written on the string port.
 ```(with-output-to-string (lambda () (write 123) (write "Hello"))) ⇒ "123\"Hello\"" ```

 `(with-input-from-port port thunk)` STklos procedure
`(with-output-to-port port thunk)`
`(with-error-to-port port thunk)`

`Port` should be a port, and `proc` should be a procedure of no arguments. These procedures do a job similar to the `with-...-file` counterparts excepted that they use an open port instead of string specifying a file name

 `(open-input-file filename)` R5RS procedure

Takes a string naming an existing file and returns an input port capable of delivering characters from the file. If the file cannot be opened, an error is signalled.

Note: if `filename` starts with the string `"| "`, this procedure returns a pipe port. Consequently, it is not possible to open a file whose name starts with those two characters.

 `(open-input-string str)` STklos procedure

Returns an input string port capable of delivering characters from `str`.

 `(open-input-virtual :key (read-char #f) (ready? #f) (eof? #f) (close #f))` STklos procedure

Returns a virtual port using the `read-char` procedure to read a character from the port, `ready?` to know if there is any data to read from the port, `eof?` to know if the end of file is reached on the port and finally `close` to close the port. All theses procedure takes one parameter which is the port from which the input takes place. `Open-input-virtual` accepts also the special value `#f` for the I/O procedures with the following conventions:
• if `read-char` or `eof?` is `#f`, any attempt to read the virtual port will return an eof object;
• if `ready?` is `#f`, the file is always ready for reading;
• if `close` is `#f`, no action is done when the port is closed.

Hereafter is a possible implementation of `open-input-string` using virtual ports:
 ```(define (open-input-string str) (let ((index 0)) (open-input-virtual :read-char (lambda (p) ;; test on eof is already done by the system (let ((res (string-ref str index))) (set! index (+ index 1)) res)) :eof? (lambda (p) (>= index (string-length str)))))) ```

 `(open-output-file filename)` R5RS procedure

Takes a string naming an output file to be created and returns an output port capable of writing characters to a new file by that name. If the file cannot be opened, an error is signalled. If a file with the given name already exists, it is rewritten.

Note: if `filename` starts with the string `"| "`, this procedure returns a pipe port. Consequently, it is not possible to open a file whose name starts with those two characters.

 `(open-output-string)` STklos procedure

Returns an output string port capable of receiving and collecting characters.

 `(open-output-virtual :key (write-char #f) (write-string #f) (flush #f) (close #f))` STklos procedure

Returns a virtual port using the `write-char` procedure to write a character to the port, `write-string` to write a string to the port, `flush` to (eventuelly) flush the characters on the port and finally `close`to close the port. `Write-char` takes two parameters: a character and the port to which the output must be done. `write-string` takes two parameters: a string and a port. `Flush` and `Close` take one parameter which is the port on which the action must be done. `Open-output-virtual` accepts also the special value `#f` for the I/O procedures. If a procedure is `#f` nothing is done on the corresponding action.

Hereafter is an (very inefficient) implementation of a variant of `open-output-string` using virtual ports. The value of the output string is printed when the port is closed:
 ```(define (open-output-string) (let ((str "")) (open-output-virtual :write-char (lambda (c p) (set! str (string-append str (string c)))) :write-string (lambda (s p) (set! str (string-append str s))) :close (lambda (p) (write str) (newline))))) ```
Note: `write-string` is mainly used for writing strings and is generally more efficient than writing the string character by character. However, if `write-string` is not provided, strings are printed with `write-char`. On the other hand, if `write-char` is absent, characters are written by successive allocation of one character strings.

Hereafter is another example: a virtual file port where all characters are converted to upper case:
 ```(define (open-output-uppercase-file file) (let ((out (open-file file "w"))) (and out (open-output-virtual :write-string (lambda (s p) (display (string-upper s) out)) :close (lambda (p) (close-port out)))))) ```

 `(open-file filename mode)` STklos procedure

Opens the file whose name is `filename` with the specified string `mode` which can be:
• `"r"` to open file for reading. The stream is positioned at the beginning of the file.
• `"r+"` to open file for reading and writing. The stream is positioned at the beginning of the file.
• `"w"` to truncate file to zero length or create file for writing. The stream is positioned at the beginning of the file.
• `"w+"` to open file for reading and writing. The file is created if it does not exist, otherwise it is truncated. The stream is positioned at the beginning of the file.
• `"a"` to open for writing. The file is created if it does not exist. The stream is positioned at the end of the file.
• `"a+"` to open file for reading and writing. The file is created if it does not exist. The stream is positioned at the end of the file.
If the file can be opened, `open-file` returns the port associated with the given file, otherwise it returns `#f`. Here again, the ``magic'' string "| " permits to open a pipe port (in this case mode can only be `"r"` or `"w"`).

 `(get-output-string port)` STklos procedure

Returns a string containing all the text that has been written on the output string `port`.
 ``` (let ((p (open-output-string))) (display "Hello, world" p) (get-output-string p)) ⇒ "Hello, world" ```

 `(close-input-port port)` R5RS procedure
`(close-output-port port)`

Closes the port associated with `port`, rendering the port incapable of delivering or accepting characters. These routines have no effect if the port has already been closed. The value returned is void.

 `(close-port port)` STklos procedure

Closes the port associated with `port`.

 `(port-rewind port)` STklos procedure

Sets the port position to the beginning of `port`. The value returned by `port-rewind` is void.

 `(port-seek port pos)` STklos procedure
`(port-seek port pos whence)`

Sets the file position for the given `port` to the position `pos`. The new position, measured in bytes, is obtained by adding `pos` bytes to the position specified by `whence`. If passed, `whence` must be one of `:start`, `:current` or `:end`. The resulting position is relative to the start of the file, the current position indicator, or end-of-file, respectively. If `whence` is omitted, it defaults to `:start`.

Note: After using port-seek, the value returned by `port-current-line` may be incorrect.

 `(port-current-line)` STklos procedure
`(port-current-line port)`

Returns the current line number associated to the given input `port` as an integer. The `port` argument may be omitted, in which case it defaults to the value returned by `current-input-port`.

Note: The `port-seek`, `read-chars` and `read-chars!` procedures generally break the line-number. After using one of theses procedures, the value returned by `port-current-line` will be `-1` (except a `port-seek` at the beginning of the port reinitializes the line counter).

 `(port-current-position)` STklos procedure
`(port-current-position port)`

Returns the position associated to the given `port` as an integer (i.e. number of characters from the beginning of the port). The `port` argument may be omitted, in which case it defaults to the value returned by `current-input-port`.

 `(port-file-name port)` STklos procedure

Returns the file name used to open `port`; `port` must be a file port.

 `(port-idle-register! port thunk)` STklos procedure
`(port-idle-unregister! port thunk)`
`(port-idle-reset! port)`

`port-idle-register!` allows to register `thunk` as an idle handler when reading on port. That means that `thunk` will be called continuously while waiting an input on `port` (and only while using a reading primitive on this port). `port-idle-unregister!` can be used to unregister a handler previously set by `port-idle-register!`. The primitive `port-idle-reset!` unregisters all the handlers set on `port`. Hereafter is a (not too realistic) example: a message will be displayed repeatedly until a sexpr is read on the current input port.
 ```(let ((idle (lambda () (display "Nothing to read!\n")))) (port-idle-register! (current-input-port) idle) (let ((result (read))) (port-idle-unregister! (current-input-port) idle) result)) ```

 `(port-closed? port)` STklos procedure

Returns `#t` if `port` is closed and `#f` otherwise.

#### 4.12.2 Input

 `(read)` R5RS procedure
`(read port)`

`Read` converts external representations of Scheme objects into the objects themselves. `Read` returns the next object parsable from the given input port, updating port to point to the first character past the end of the external representation of the object.

If an end of file is encountered in the input before any characters are found that can begin an object, then an end of file object is returned. The port remains open, and further attempts to read will also return an end of file object. If an end of file is encountered after the beginning of an object's external representation, but the external representation is incomplete and therefore not parsable, an error is signalled.

The port argument may be omitted, in which case it defaults to the value returned by `current-input-port`. It is an error to read from a closed port.

STklos `read` supports the SRFI-10 (Sharp Comma External Form) `#` form that can be used to denote values that do not have a convenient printed representation. See the SRFI document for more information.

 `(read-with-shared-structure)` STklos procedure
`(read-with-shared-structure port)`
`(read/ss)`
`(read/ss port)`

`read-with-shared-structure` is identical to `read`. It has been added to be compatible with SRFI-38 (External representation of shared structures). STklos always knew how to deal with recursive input data. `read/ss` is only a shorter name for `read-with-shared-structure`.

 `(define-reader-ctor tag proc)` STklos procedure

This procedure permits to define a new user to reader constructor procedure at run-time. It is defined in SRFI-10 (Sharp Comma External Form) document. See SRFI document for more information.
 ```(define-reader-ctor 'rev (lambda (x y) (cons y x))) (with-input-from-string "#,(rev 1 2)" read) ⇒ (2 . 1) ```

 `(read-char)` R5RS procedure
`(read-char port)`

Returns the next character available from the input `port`, updating the `port` to point to the following character. If no more characters are available, an end of file object is returned. `Port` may be omitted, in which case it defaults to the value returned by `current-input-port`.

 `(read-chars size)` STklos procedure
`(read-chars size port)`

Returns a newly allocated string made of `size` characters read from `port`. If less than `size` characters are available on the input port, the returned string is smaller than `size` and its size is the number of available characters. `Port` may be omitted, in which case it defaults to the value returned by `current-input-port`.

 `(read-chars! str)` STklos procedure
`(read-chars! str port)`

This function reads the characters available from `port` in the string `str` by chuncks whose size is equal to the length of `str`. The value returned by `read-chars!`is an integer indicating the number of characters read. `Port` may be omitted, in which case it defaults to the value returned by `current-input-port`.

This function is similar to `read-chars` except that it avoids to allocate a new string for each read.
 ```(define (copy-file from to) (let* ((size 1024) (in (open-input-file from)) (out (open-output-file to)) (s (make-string size))) (let Loop () (let ((n (read-chars! s in))) (cond ((= n size) (write-chars s out) (Loop)) (else (write-chars (substring s 0 n) out) (close-port out))))))) ```

 `(read-byte)` STklos procedure
`(read-byte port)`

Returns the next character available from the input `port` as an integer. If the end of file is reached, this function returns the end of file object.

 `(peek-char)` R5RS procedure
`(peek-char port)`

Returns the next character available from the input `port`, without updating the port to point to the following character. If no more characters are available, an end of file object is returned. `Port` may be omitted, in which case it defaults to the value returned by `current-input-port`.

Note: The value returned by a call to `peek-char` is the same as the value that would have been returned by a call to `read-char` with the same port. The only difference is that the very next call to `read-char` or `peek-char` on that port will return the value returned by the preceding call to `peek-char`. In particular, a call to `peek-char` on an interactive port will hang waiting for input whenever a call to `read-char` would have hung.

 `(peek-byte)` STklos procedure
`(peek-byte port)`

Returns the next character available from the input `port`, without updating the port to point to the following character. Whereas `peek-char` returns a character, this function returns an integer between 0and 255.

 `(eof-object? obj)` R5RS procedure

Returns `#t` if `obj` is an end of file object, otherwise returns `#f`.

 `(eof-object)` STklos procedure

Returns an end of file object. Note that the special notation `#eof` is another way to return such an end of file object.

 `(char-ready?)` R5RS procedure
`(char-ready? port)`

Returns `#t` if a character is ready on the input port and returns `#f` otherwise. If char-ready returns `#t` then the next read-char operation on the given port is guaranteed not to hang. If the port is at end of file then `char-ready?` returns `#t`. Port may be omitted, in which case it defaults to the value returned by `current-input-port`.

 `(read-line)` STklos procedure
`(read-line port)`

Reads the next line available from the input port `port`. This function returns 2 values: the first one is is the string which contains the line read, and the second one is the end of line delimiter. The end of line delimiter can be an end of file object, a character or a string in case of a multiple character delimiter. If no more characters are available on `port`, an end of file object is returned. `Port` may be omitted, in which case it defaults to the value returned by `current-input-port`.

Note: As said in values, if `read-line` is not used in the context of `call-with-values`, the second value returned by this procedure is ignored.

 `(read-from-string str)` STklos procedure

Performs a read from the given `str`. If `str` is the empty string, an end of file object is returned.
 ```(read-from-string "123 456") ⇒ 123 (read-from-string "") ⇒ an eof object ```

 `(port->string port)` STklos procedure
`(port->sexp-list port)`
`(port->string-list port)`

All these procedure take a port opened for reading. `Port->string` reads `port` until the it reads an end of file object and returns all the characters read as a string. `Port->sexp-list)` and `port->string-list` do the same things except that they return a list of S-expressions and a list of strings respectively. For the following example we suppose that file `"foo"` is formed of two lines which contains respectively the number `100` and the string `"bar"`.
 ```(port->sexp-list (open-input-file "foo")) ⇒ (100 "bar") (port->string-list (open-input-file "foo")) ⇒ ("100" ""bar"") ```

#### 4.12.3 Output

 `(write obj)` R5RS procedure
`(write obj port)`

Writes a written representation of `obj` to the given `port`. Strings that appear in the written representation are enclosed in doublequotes, and within those strings backslash and doublequote characters are escaped by backslashes. Character objects are written using the #\ notation. `Write` returns an unspecified value. The `port` argument may be omitted, in which case it defaults to the value returned by `current-output-port`.

 `(write* obj)` STklos procedure
`(write* obj port)`

Writes a written representation of `obj` to the given port. The main difference with the `write` procedure is that `write*` handles data structures with cycles. Circular structure written by this procedure use the `"#n="` and `"#n#"` notations (see Circular-structure).

 `(write-with-shared-structure obj)` STklos procedure
`(write-with-shared-structure obj port)`
`(write-with-shared-structure obj port optarg)`
`(write/ss obj)`
`(write/ss obj port)`
`(write/ss obj port optarg)`

`write-with-shared-structure` has been added to be compatible with SRFI-38 (External representation of shared structures). It is is identical to `write*`, except that it accepts one more parameter (`optarg`). This parameter, which is not specified in SRFI-38 (External representation of shared structures), is always ignored. `write/ss` is only a shorter name for `write-with-shared-structure`.

 `(display obj)` R5RS procedure
`(display obj port)`

Writes a representation of `obj` to the given `port`. Strings that appear in the written representation are not enclosed in doublequotes, and no characters are escaped within those strings. Character objects appear in the representation as if written by `write-char` instead of by `write`. `Display` returns an unspecified value. The `port` argument may be omitted, in which case it defaults to the value returned by `current-output-port`.

Rationale: `Write` is intended for producing machine-readable output and `display` is for producing human-readable output.

 `(newline)` R5RS procedure
`(newline port)`

Writes an end of line to `port`. Exactly how this is done differs from one operating system to another. Returns an unspecified value. The `port` argument may be omitted, in which case it defaults to the value returned by `current-output-port`.

 `(write-char char)` R5RS procedure
`(write-char char port)`

Writes the character `char` (not an external representation of the character) to the given `port` and returns an unspecified value. The `port` argument may be omitted, in which case it defaults to the value returned by `current-output-port`.

 `(write-chars str)` STklos procedure
`(write-char str port)`

Writes the character of string `str` to the given `port` and returns an unspecified value. The `port` argument may be omitted, in which case it defaults to the value returned by `current-output-port`. Note: This function is generally faster than `display` for strings. Furthermore, this primitive does not use the buffer associated to `port`.

 `(write-byte b)` STklos procedure
`(write-byte b port)`

Write byte `b` to the port. `b` must be an exact integer in range between 0 and 255.

 `(format port str obj ...)` STklos procedure
`(format str obj)`

Writes the `obj`s to the given `port`, according to the format string `str`. `Str` is written literally, except for the following sequences:
• `~a` or `~A` is replaced by the printed representation of the next `obj`.
• `~s` or `~S` is replaced by the ``slashified'' printed representation of the next `obj`.
• `~w` or `~W` is replaced by the printed representation of the next `obj` (circular structures are correctly handled and printed using `write*`).
• `~d` or `~D` is replaced by the decimal printed representation of the next `obj` (which must be a number).
• `~x` or `~X` is replaced by the hexadecimal printed representation of the next `obj` (which must be a number).
• `~o` or `~O` is replaced by the octal printed representation of the next `obj` (which must be a number).
• `~b` or `~B` is replaced by the binary printed representation of the next `obj` (which must be a number).
• `~c` or `~C` is replaced by the printed representation of the next `obj` (which must be a character).
• `~y` or `~Y` is replaced by the pretty-printed representation of the next `obj`. The standard pretty-printer is used here.
• `~?` is replaced by the result of the recursive call of `format` with the two next `obj`.
• `~k` or `~K` is another name for `~?`
• `~[w[,d]]f` or `~[w[,d]]F` is replaced by the printed representation of next `obj` (which must be a number) with width `w` and `d` digits after the decimal. Eventually, `d` may be omitted.
• `~~` is replaced by a single tilde character.
• `~%` is replaced by a newline
• `~t` or `~t` is replaced by a tabulation character.
• `~&` is replaced by a newline character if it is known that the previous character was not a newline
• `~_` is replaced by a space
• `~h` or `~H` provides some help
`Port` can be a boolean or a port. If `port` is `#t`, output goes to the current output port; if `port` is `#f`, the output is returned as a string. Otherwise, the output is printed on the specified port.
 ``` (format #f "A test.") ⇒ "A test." (format #f "A ~a." "test") ⇒ "A test." (format #f "A ~s." "test") ⇒ "A \"test\"." (format "~8,2F" 1/3) ⇒ " 0.33" (format "~6F" 32) ⇒ " 32" (format "~1,2F" 4321) ⇒ "4321.00" (format "~1,2F" (sqrt -3.9)) ⇒ "0.00+1.97i" (format "#d~d #x~x #o~o #b~b~%" 32 32 32 32) ⇒ "#d32 #x20 #o40 #b100000\n" (format #f "~&1~&~&2~&~&~&3~%") ⇒ "1\n2\n3\n" (format "~a ~? ~a" 'a "~s" '(new) 'test) ⇒ "a new test" ```
Note: The second form of `format` is compliant with SRFI-28 (Basic Format Strings). That is, when `port` is omitted, the output is returned as a string as if `port` was given the value `#f`.

Note: Since version 0.58, `format` is also compliant with SRFI-48 (Intermediate Format Strings).

 `(flush-output-port)` STklos procedure
`(flush-output-port port)`

Flushes the buffer associated with the given output `port`. The `port` argument may be omitted, in which case it defaults to the value returned by `current-output-port`

 `(print obj ...)` STklos procedure
`(printerr obj ...)`

These procedures display all their arguments followed by a newline. The procedure `print` uses the standard output port, whereas `printerr` uses the current error port

 `(printf fmt obj ...)` STklos procedure
`(fprintf port fmt obj ...)`
`(eprintf fmt obj ...)`

These procedures are specialized version of `format`. In these procedures, `fmt` is a string using the `format` conventions. `printf` outputs go on the current output port. `fprintf` outputs go on the specified `port`. `eprintf` outputs go on the current error port (note that eprintf always flushes the characters printed).

#### 4.12.4 System interface

 `(load filename)` R5RS procedure

`Filename` should be a string naming an existing file containing Scheme expressions. `Load` has been extended in STklos to allow loading of file containing Scheme compiled code as well as object files (aka shared objects). The loading of object files is not available on all architectures. The value returned by `load` is void.

If the file whose name is `filename` cannot be located, `load` will try to find it in one of the directories given by `load-path` with the suffixes given by `load-suffixes`.

 `(try-load filename)` STklos procedure

`try-load` tries to load the file named `filename`. As `load`, `try-load` tries to find the file given the current load path and a set of suffixes if `filename` cannot be loaded. If `try-load` is able to find a readable file, it is loaded, and `try-load` returns `#t`. Otherwise, `try-load` retuns `#f`.

 `(find-path str)` STklos procedure
`(find-path str path)`
`(find-path str path suffixes)`

In its first form, `find-path` returns the path name of the file that should be loaded by the procedure `load` given the name `str`. The string returned depends of the current load path and of the currently accepted suffixes.

The other forms of `find-path` are more general and allow to give a path list (a list of strings representing supposed directories) and a set of suffixes (given as a list of strings too) to try for finding a file. If no file is found, `find-path` returns `#f`.

For instance, on a "classical" Unix box:
 ```(find-path "passwd" '("/bin" "/etc" "/tmp")) ⇒ "/etc/passwd" (find-path "stdio" '("/usr" "/usr/include") '("c" "h" "stk")) ⇒ "/usr/include/stdio.h" ```

 `(current-loading-file)` STklos procedure

Returns the path of the file that is currently being load.

 `(require string)` STklos procedure
`(provide string)`
`(require/provide string)`
`(provided? string)`

`Require` loads the file whose name is `string` if it was not previously "provided". `Provide` permits to store `string` in the list of already provided files. Providing a file permits to avoid subsequent loads of this file. `Require/provide` is more or less equivalent to a `require` followed by a `provide`. `Provided?` returns `#t` if `string` was already provided; it returns `#f` otherwise.

### 4.13 Keywords

Keywords are symbolic constants which evaluate to themselves. A keyword is a symbol whose first (or last) character is a colon (":").

 `(keyword obj)` STklos procedure

Returns `#t` if `obj` is a keyword, otherwise returns `#f`.
 ```(keyword? 'foo) ⇒ #f (keyword? ':foo) ⇒ #t (keyword? 'foo:) ⇒ #t (keyword? :foo) ⇒ #t (keyword? foo:) ⇒ #t ```

 `(make-keyword s)` STklos procedure

Builds a keyword from the given `s`. The parameter `s` must be a symbol or a string.
 ```(make-keyword "test") ⇒ :test (make-keyword 'test) ⇒ :test (make-keyword ":hello") ⇒ ::hello ```

 `(keyword->string key)` STklos procedure

Returns the name of `key` as a string. The result does not contain a colon.

 `(string->keyword str)` STklos procedure

This function function has been added to be compatibe with SRFI-88. It is equivalent to make-keyword, except that the parameter cannot be a symbol.

 `(key-get list key)` STklos procedure
`(key-get list key default)`

`List` must be a list of keywords and their respective values. `key-get` scans the `list` and returns the value associated with the given `key`. If `key` does not appear in an odd position in `list`, the specified `default` is returned, or an error is raised if no `default` was specified.
 ```(key-get '(:one 1 :two 2) :one) ⇒ 1 (key-get '(:one 1 :two 2) :four #f) ⇒ #f (key-get '(:one 1 :two 2) :four) ⇒ error ```

 `(key-set! list key value)` STklos procedure

`List` must be a list of keywords and their respective values. `key-set!` sets the value associated to `key` in the keyword list. If the key is already present in `list`, the keyword list is physically changed.
 ```(let ((l (list :one 1 :two 2))) (set! l (key-set! l :three 3)) (cons (key-get l :one) (key-get l :three))) ⇒ (1 . 3) ```

 `(key-delete list key)` STklos procedure
`(key-delete! list key)`

`List` must be a list of keywords and their respective values. `key-delete` remove the `key` and its associated value of the keyword list. The key can be absent of the list.
`key-delete!` does the same job than `key-delete` by physically modifying its `list` argument.
 ```(key-delete '(:one 1 :two 2) :two) ⇒ (:one 1) (key-delete '(:one 1 :two 2) :three) ⇒ (:one 1 :two 2) ```

### 4.14 Hash Tables

A hash table consists of zero or more entries, each consisting of a key and a value. Given the key for an entry, the hashing function can very quickly locate the entry, and hence the corresponding value. There may be at most one entry in a hash table with a particular key, but many entries may have the same value.

STklos hash tables grow gracefully as the number of entries increases, so that there are always less than three entries per hash bucket, on average. This allows for fast lookups regardless of the number of entries in a table.

STklos hash tables procedures are identical to the ones defined in SRFI-69 (Basic Hash Tables). Note that the default comparison function is `eq?` whereas it is `equal?` in this SRFI. See SRFIs for more information.

 `(make-hash-table)` STklos procedure
`(make-hash-table comparison)`
`(make-hash-table comparison hash)`

`Make-hash-table` admits three different forms. The most general form admit two arguments. The first argument is a comparison function which determines how keys are compared; the second argument is a function which computes a hash code for an object and returns the hash code as a non negative integer. Objets with the same hash code are stored in an A-list registered in the bucket corresponding to the key.

If omitted,
• `hash` defaults to the `hash-table-hash` procedure (see hash-table-hash).
• `comparison` defaults to the `eq?` procedure (see eq-).
Consequently,
 ```(define h (make-hash-table)) ```
is equivalent to
 ```(define h (make-hash-table eq? hash-table-hash)) ```
An interesting example is
 ```(define h (make-hash-table string-ci=? string-length)) ```
which defines a new hash table which uses `string-ci=?` for comparing keys. Here, we use the string-length as a (very simple) hashing function. Of course, a function which gives a key depending of the characters composing the string gives a better repartition and should probably enhance performances. For instance, the following call to `make-hash-table` should return a more efficient, even if not perfect, hash table:
 ```(make-hash-table string-ci=? (lambda (s) (let ((len (string-length s))) (do ((h 0) (i 0 (+ i 1))) ((= i len) h) (set! h (+ h (char->integer (char-downcase (string-ref s i))))))))) ```
Note: Hash tables with a comparison function equal to `eq?` or `string=?` are handled in an more efficient way (in fact, they don't use the `hash-table-hash` function to speed up hash table retrievals).

 `(hash-table? obj)` STklos procedure

Returns `#t` if `obj` is a hash table, returns `#f` otherwise.

 `(hash-table-hash obj)` STklos procedure

Computes a hash code for an object and returns this hash code as a non negative integer. A property of `hash-table-hash` is that
 ```(equal? x y) ⇒ (equal? (hash-table-hash x) (hash-table-hash y) ```
as the the Common Lisp `sxhash` function from which this procedure is modeled.

 `(alist->hash-table alist)` STklos procedure
`(alist->hash-table alist comparison)`
`(alist->hash-table alist comparison hash)`

Returns hash-table built from the ``association list'' `alist`. This function maps the `car` of every element in `alist` to the `cdr` of corresponding elements in `alist`. the `comparison` and `hash` functions are interpreted as in `make-hash-table`. If some key occurs multiple times in `alist`, the value in the first association will take precedence over later ones.

 `(hash-table->alist hash)` STklos procedure

Returns an ``association list'' built from the entries in `hash`. Each entry in `hash` will be represented as a pair whose `car` is the entry's key and whose `cdr` is its value.

Note: the order of pairs in the resulting list is unspecified.
 ```(let ((h (make-hash-table))) (dotimes (i 5) (hash-table-set! h i (number->string i))) (hash-table->alist h)) ⇒ ((3 . "3") (4 . "4") (0 . "0") (1 . "1") (2 . "2")) ```

 `(hash-table-set! hash key value)` STklos procedure

Enters an association between `key` and `value` in the`hash` table. The value returned by `hash-table-set!` is void.

 `(hash-table-ref hash key)` STklos procedure
`(hash-table-ref hash key thunk)`

Returns the value associated with `key` in the given `hash` table. If no value has been associated with `key` in `hash`, the specified `thunk` is called and its value is returned; otherwise an error is raised.
 ```(define h1 (make-hash-table)) (hash-table-set! h1 'foo (list 1 2 3)) (hash-table-ref h1 'foo) ⇒ (1 2 3) (hash-table-ref h1 'bar (lambda () 'absent)) ⇒ absent (hash-table-ref h1 'bar) ⇒ error (hash-table-set! h1 '(a b c) 'present) (hash-table-ref h1 '(a b c) (lambda () 'absent)) ⇒ absent (define h2 (make-hash-table equal?)) (hash-table-set! h2 '(a b c) 'present) (hash-table-ref h2 '(a b c)) ⇒ present ```

 `(hash-table-ref/default hash key default)` STklos procedure

This function is equivalent to
 ```(hash-table-ref hash key (lambda () default)) ```

 `(hash-table-delete! hash key)` STklos procedure

Deletes the entry for `key` in `hash`, if it exists. Result of `hash-table-delete!` is void.
 ```(define h (make-hash-table)) (hash-table-set! h 'foo (list 1 2 3)) (hash-table-ref h 'foo) ⇒ (1 2 3) (hash-table-delete! h 'foo) (hash-table-ref h 'foo (lambda () 'absent) ⇒ absent ```

 `(hash-table-exists? hash key)` STklos procedure

Returns `#t` if there is any association of `key` in `hash`. Returns `#f` otherwise.

 `(hash-table-update! hash key update-fun thunk)` STklos procedure
`(hash-table-update!/default hash key update-fun default)`

Update the value associated to `key` in table `hash` if key is already in table with the value `(update-fun current-value)`. If no value is associated to `key`, a new entry in the table is first inserted before updating it (this new entry being the result of calling `thunk`).

Note that the expression
 ```(hash-table-update!/default hash key update-fun default) ```
is equivalent to
 ```(hash-table-update! hash key update-fun (lambda () default)) ```
 ```(let ((h (make-hash-table)) (1+ (lambda (n) (+ n 1)))) (hash-table-update!/default h 'test 1+ 100) (hash-table-update!/default h 'test 1+) (hash-table-ref h 'test)) ⇒ 102 ```

 `(hash-table-for-each hash proc)` STklos procedure
`(hash-table-walk hash proc)`

`Proc` must be a procedure taking two arguments. `Hash-table-for-each` calls `proc` on each key/value association in `hash`, with the key as the first argument and the value as the second. The value returned by `hash-table-for-each` is void.
Note: The order of application of `proc` is unspecified.
Note: `hash-table-walk` is another name for `hash-table-for-each` (this is the name used in SRFI-69 (Basic Hash Tables)).
 ```(let ((h (make-hash-table)) (sum 0)) (hash-table-set! h 'foo 2) (hash-table-set! h 'bar 3) (hash-table-for-each h (lambda (key value) (set! sum (+ sum value)))) sum) ⇒ 5 ```

 `(hash-table-map hash proc)` STklos procedure

`Proc` must be a procedure taking two arguments. `Hash-table-map` calls `proc` on each key/value association in `hash`, with the key as the first argument and the value as the second. The result of `hash-table-map` is a list of the values returned by `proc`, in an unspecified order.
Note: The order of application of `proc` is unspecified.
 ```(let ((h (make-hash-table))) (dotimes (i 5) (hash-table-set! h i (number->string i))) (hash-table-map h (lambda (key value) (cons key value)))) ⇒ ((3 . "3") (4 . "4") (0 . "0") (1 . "1") (2 . "2")) ```

 `(hash-table-keys hash)` STklos procedure
`(hash-table-values hash)`

Returns the keys or the values of `hash`.

 `(hash-table-fold hash func init-value)` STklos procedure

This procedure calls `func` for every association in `hash` with three arguments: the key of the association key, the value of the association value, and an accumulated value, `val`. `Val` is init-value for the first invocation of `func`, and for subsequent invocations of `func`, the return value of the previous invocation of `func`. The value `final-value` returned by `hash-table-fold` is the return value of the last invocation of `func`. The order in which `func` is called for different associations is unspecified.

For instance, the following expression
 ```(hash-table-fold ht (lambda (k v acc) (+ acc 1)) 0) ```
computes the number of associations present in the `ht` hash table.

 `(hash-table-copy hash)` STklos procedure

Returns a copy of `hash`.

 `(hash-table-merge! hash1 hash2)` STklos procedure

Adds all mappings in `hash2` into `hash1` and returns the resulting hash table. This function may modify `hash1` destructively.

 `(hash-table-equivalence-function hash)` STklos procedure

Returns the equivalence predicate used for keys in `hash`.

 `(hash-table-hash-function hash)` STklos procedure

Returns the hash function used for keys in `hash`.

 `(hash-table-size hash)` STklos procedure

Returns the number of entries in the `hash`.

 `(hash-table-stats hash)` STklos procedure
`(hash-table-stats hash port)`

Prints overall information about `hash`, such as the number of entries it contains, the number of buckets in its hash array, and the utilization of the buckets. Informations are printed on `port`. If no `port` is given to `hash-table-stats`, information are printed on the current output port (see current-output-port).

### 4.15 Dates and Times

STklos stores dates and times with a compact representation which consists is an integer which represents the number of seconds elapsed since the Epoch (00:00:00 on January 1, 1970, Coordinated Universal Time --UTC). Dates can also be represented with date structures.

 `(current-seconds)` STklos procedure

Returns the time since the Epoch (that is 00:00:00 UTC, January 1, 1970), measured in seconds.

 `(current-time)` R5RS procedure

Returns a time object corresponding to the current time.

 `(time? obj)` STklos procedure

Return `#t` if `obj` is a time object, othererwise returns `#f`.

 `(time->seconds time)` STklos procedure

Convert the time object `time` into an inexact real number representing the number of seconds elapsed since the Epoch.
 ```(time->seconds (current-time)) =⇒ 1138983411.09337 ```

 `(seconds->time x)` STklos procedure

Converts into a time object the real number `x` representing the number of seconds elapsed since the Epoch.
 ```(seconds->time (+ 10 (time->seconds (current-time))) =⇒ a time object representing 10 seconds in the future ```

 `(current-date)` STklos procedure

Returns the current system date.

 `(make-date :key second minute hour day month year)` STklos procedure

Build a date from its argument. `hour`, `minute`, `second` default to 0; `day` and `month` default to 1; `year` defaults to 1970

 `(date? obj)` STklos procedure

Return `#t` if `obj` is a date, and otherwise returns `#f`.

 `(date-second d)` STklos procedure

Return the second of date `d`, in the range 0 to 59.

 `(date-minute d)` STklos procedure

Return the minute of date `d`, in the range 0 to 59.

 `(date-hour d)` STklos procedure

Return the hour of date `d`, in the range 0 to 23.

 `(date-day d)` STklos procedure

Return the day of date `d`, in the range 1 to 31

 `(date-month d)` STklos procedure

Return the month of date `d`, in the range 1 to 12

 `(date-year d)` STklos procedure

Return the year of date `d`.

 `(date-week-day d)` STklos procedure

Return the week day of date `d`, in the range 0 to 6 (0 is Sunday).

 `(date-year-day d)` STklos procedure

Return the the number of days since January 1 of date `d`, in the range 1 to 366.

 `(date-dst d)` STklos procedure

Return an indication about daylight saving adjustment:
• 0 if no daylight saving adjustment
• 1 if daylight saving adjustment
• -1 if the information is not available

 `(date-tz d)` STklos procedure

Return the time zone of date `d`.

 `(date->seconds d)` STklos procedure

Convert the date `d` to the number of seconds since the Epoch.

 `(date->string format d)` STklos procedure

Convert the date `d` using the string `format` as a specification. Conventions for format are the same as the one of seconds--string.

 `(seconds->date n)` STklos procedure

Convert the date `n` expressed as a number of seconds since the Epoch to a date.

 `(seconds->string format n)` STklos procedure

Convert a date expressed in seconds using the string `format` as a specification. Conventions for `format` are given below:
• ~~ a literal ~
• ~a locale's abbreviated weekday name (Sun...Sat)
• ~A locale's full weekday name (Sunday...Saturday)
• ~b locale's abbreviate month name (Jan...Dec)
• ~B locale's full month day (January...December)
• ~c locale's date and time (e.g., `Fri Jul 14 20:28:42-0400 2000`)
• ~d day of month, zero padded (01...31)
• ~D date (mm/dd/yy)
• ~e day of month, blank padded ( 1...31)
• ~f seconds+fractional seconds, using locale's decimal separator (e.g. 5.2).
• ~h same as ~b
• ~H hour, zero padded, 24-hour clock (00...23)
• ~I hour, zero padded, 12-hour clock (01...12)
• ~j day of year, zero padded
• ~k hour, blank padded, 24-hour clock (00...23)
• ~l hour, blank padded, 12-hour clock (01...12)
• ~m month, zero padded (01...12)
• ~M minute, zero padded (00...59)
• ~n new line
• ~p locale's AM or PM
• ~r time, 12 hour clock, same as `~I:~M:~S ~p`
• ~s number of full seconds since "the epoch" (in UTC)
• ~S second, zero padded (00...61)
• ~t horizontal tab
• ~T time, 24 hour clock, same as `~H:~M:~S`
• ~U week number of year with Sunday as first day of week (00...53)
• ~V weekISO 8601:1988 week number of year (01...53) (week 1 is the first week that has at least 4 days in the current year, and with Monday as the first day of the week)
• ~w day of week (1...7, 1 is Monday)
• ~W week number of year with Monday as first day of week (01...52)
• ~x week number of year with Monday as first day of week (00...53)
• ~X locale's date representation, for example: "07/31/00"
• ~y last two digits of year (00...99)
• ~Y year
• ~z time zone in RFC-822 style
• ~Z symbol time zone

 `(seconds->list sec)` STklos procedure

Returns a keyword list for the date given by `sec` (a date based on the Epoch). The keyed values returned are
• second : 0 to 59 (but can be up to 61 to allow for leap seconds)
• minute : 0 to 59
• hour : 0 to 23
• day : 1 to 31
• month : 1 to 12
• year : e.g., 2002
• week-day : 0 (Sunday) to 6 (Saturday)
• year-day : 0 to 365 (365 in leap years)
• dst : indication about daylight savings time. See date-dst
• tz : the difference between Coordinated Universal Time (UTC) and local standard time in seconds.
 ```(seconds->list (current-second)) ⇒ (:second 51 :minute 26 :hour 19 :day 5 :month 11 :year 2004 :week-day 5 :year-day 310 :dst 0 :tz -3600) ```

 `(date)` STklos procedure

Returns the current date in a string

### 4.16 Boxes

Boxes are records that have a single field. A box may be constructed with the make-box, make-constant-box or the read primitives. A box produced by read (using the "#&" notation) is mutable. Note that two boxes are equal? iff their content are equal?.

 `(make-box obj)` STklos procedure

Returns a new box that contains `obj`. The box is mutable.
 ```(let ((x (make-box 10))) (list 10 x)) ⇒ (10 #&10) ```

 `(make-constant-box obj)` STklos procedure

Returns a new box that contains `obj`. The box is immutable.

 `(box? obj)` STklos procedure

Returns `#t` if `obj`is box, `#f` otherwise.

 `(box-mutable? obj)` STklos procedure

Returns `#t` if `obj`is mutable box, `#f` otherwise.

 `(box-set! box x)` STklos procedure

Sets the content of `box` to `x`. The box must be mutable.

 `(unbox box)` STklos procedure

Returns the content of `box`. For any `obj`, `(unbox (make-box obj))` returns `obj`.

### 4.17 Processes

STklos provides access to Unix processes as first class objects. Basically, a process contains several informations such as the standard system process identification (aka PID on Unix Systems), the files where the standard files of the process are redirected.

 `(run-process command p1 p2 ...)` STklos procedure

`run-process` creates a new process and run the executable specified in `command`. The `p` correspond to the command line arguments. The following values of `p` have a special meaning:
• `:input` permits to redirect the standard input file of the process. Redirection can come from a file or from a pipe. To redirect the standard input from a file, the name of this file must be specified after `:input`. Use the special keyword `:pipe` to redirect the standard input from a pipe.
• `:output` permits to redirect the standard output file of the process. Redirection can go to a file or to a pipe. To redirect the standard output to a file, the name of this file must be specified after `:output`. Use the special keyword `:pipe` to redirect the standard output to a pipe.
• `:error` permits to redirect the standard error file of the process. Redirection can go to a file or to a pipe. To redirect the standard error to a file, the name of this file must be specified after `error`. Use the special keyword `:pipe` to redirect the standard error to a pipe.
• `:wait` must be followed by a boolean value. This value specifies if the process must be run asynchronously or not. By default, the process is run asynchronously (i.e. `:wait` is `#f`).
• `:host` must be followed by a string. This string represents the name of the machine on which the command must be executed. This option uses the external command `rsh`. The shell variable `PATH` must be correctly set for accessing it without specifying its abolute path.
• `:fork` must be followed by a boolean value. This value specifies if a fork system call must be done before running the process. If the process is run without fork the Scheme program is lost. This feature mimics the ```exec`'' primitive of the Unix shells. By default, a fork is executed before running the process (i.e. `:fork` is `#t`). This option works on Unix implementations only.
The following example launches a process which executes the Unix command `ls` with the arguments `-l` and `/bin`. The lines printed by this command are stored in the file `/tmp/X`
 ```(run-process "ls" "-l" "/bin" :output "/tmp/X") ```

 `(process? obj)` STklos procedure

Returns `#t` if `obj` is a process , otherwise returns `#f`.

 `(process-alive? proc)` STklos procedure

Returns `#t` if process `proc` is currently running, otherwise returns `#f`.

 `(process-pid proc)` STklos procedure

Returns an integer which represents the Unix identification (PID) of the processus.

 `(process-input proc)` STklos procedure
`(process-output proc)`
`(process-error proc)`

Returns the file port associated to the standard input, output or error of `proc`, if it is redirected in (or to) a pipe; otherwise returns `#f`. Note that the returned port is opened for reading when calling `process-output` or `process-error`; it is opened for writing when calling `process-input`.

 `(process-wait proc)` STklos procedure

Stops the current process (the Scheme process) until `proc` completion. `Process-wait` returns `#f` when `proc` is already terminated; it returns `#t` otherwise.

 `(process-exit-status proc)` STklos procedure

Returns the exit status of `proc` if it has finished its execution; returns `#f` otherwise.

 `(process-signal proc sig)` STklos procedure

Sends the integer signal `sig` to `proc`. Since value of `sig` is system dependant, use the symbolic defined signal constants to make your program independant of the running system (see signals). The result of `process-signal` is void.

 `(process-kill proc)` STklos procedure

Kills (brutally) `process`. The result of `process-kill` is void. This procedure is equivalent to
 ```(process-signal process 'SIGTERM) ```

 `(process-stop proc)` STklos procedure
`(process-continue proc)`

`Process-stop` stops the execution of `proc` and `process-continue` resumes its execution. They are equivalent, respectively, to
 ```(process-signal process 'SIGSTOP) (process-signal process 'SIGCONT) ```

 `(process-list)` STklos procedure

Returns the list of processes which are currently running (i.e. alive).

 `(fork)` STklos procedure
`(fork thunk)`

This procedure is a wrapper around the standard Unix `fork` system call which permits to create a new (heavy) process. When called without parameter, this procedure returns two times (one time in the parent process and one time in the child process). The value returned in the parent process is a process object representing the child process and the value returned in the child process is always the value `#f`. When called with a parameter (which must be a thunk), the new process excutes `thunk` and terminate it execution when `thunk` returns. The value returned in the parent process is a process object representing the child process.

### 4.18 Sockets

STklos defines sockets, on systems which support them, as first class objects. Sockets permits processes to communicate even if they are on different machines. Sockets are useful for creating client-server applications.

 ` (make-client-socket hostname port-number)` STklos procedure
` (make-client-socket hostname port_number line-buffered)`

`make-client-socket` returns a new socket object. This socket establishes a link between the running program and the application listening on port `port-number` of `hostname`. If the optional argument `line-buffered` has a true value, a line buffered policy is used when writing to the client socket (i.e. characters on the socket are tranmitted as soon as a `#\newline` character is encountered). The default value of `line-buffered` is `#t`.

 `(make-server-socket)` STklos procedure
`(make-server-socket port-number)`

`make-server-socket` returns a new socket object. If `port-number` is specified, the socket is listening on the specified port; otherwise, the communication port is chosen by the system.

 `(socket-shutdown sock)` STklos procedure
`(socket-shutdown sock close)`

`Socket-shutdown` shutdowns the connection associated to `socket`. If the socket is a server socket, `socket-shutdown` is called on all the client sockets connected to this server. `Close` indicates if the the socket must be closed or not, when the connection is destroyed. Closing the socket forbids further connections on the same port with the `socket-accept` procedure. Omitting a value for `close` implies the closing of socket.

The following example shows a simple server: when there is a new connection on the port number 12345, the server displays the first line sent to it by the client, discards the others and go back waiting for further client connections.
 ```(let ((s (make-server-socket 12345))) (let loop () (let ((ns (socket-accept s))) (format #t "I've read: ~A\n" (read-line (socket-input ns))) (socket-shutdown ns #f) (loop)))) ```

 `(socket-accept socket)` STklos procedure
`(socket-accept socket line-buffered)`

`socket-accept` waits for a client connection on the given `socket`. If no client is already waiting for a connection, this procedure blocks its caller; otherwise, the first connection request on the queue of pending connections is connected and `socket-accept` returns a new client socket to serve this request. This procedure must be called on a server socket created with `make-server-socket`. The result of `socket-accept` is undefined. `Line-buffered` indicates if the port should be considered as a line buffered. If `line-buffered` is omitted, it defaults to `#t`.

The following example is a simple server which waits for a connection on the port 12345 1. Once the connection with the distant program is established, we read a line on the input port associated to the socket and we write the length of this line on its output port.
 ```(let* ((server (make-server-socket 13345)) (client (socket-accept server)) (l (read-line (socket-input client)))) (format (socket-output client) "Length is: ~an" (string-length l)) (socket-shutdown server)) ```
Note that shutting down the `server` socket suffices here to close also the connection to `client`.

 `(socket? obj)` STklos procedure

Returns `#t` if `socket` is a socket, otherwise returns `#f`.

 `(socket-server? obj)` STklos procedure

Returns `#t` if `socket` is a server socket, otherwise returns `#f`.

 `(socket-client? obj)` STklos procedure

Returns `#t` if `socket` is a client socket, otherwise returns `#f`.

 `(socket-host-name socket)` STklos procedure

Returns a string which contains the name of the distant host attached to `socket`. If `socket` has been created with `make-client-socket` this procedure returns the official name of the distant machine used for connection. If `socket` has been created with `make-server-socket`, this function returns the official name of the client connected to the socket. If no client has used yet `socket`, this function returns `#f`.

 `(socket-host-address socket)` STklos procedure

Returns a string which contains the IP number of the distant host attached to `socket`. If `socket` has been created with `make-client-socket` this procedure returns the IP number of the distant machine used for connection. If `socket` has been created with `make-server-socket`, this function returns the address of the client connected to the socket. If no client has used yet `socket`, this function returns `#f`.

 `(socket-local-address socket)` STklos procedure

Returns a string which contains the IP number of the local host attached to `socket`.

 `(socket-port-number socket)` STklos procedure

Returns the integer number of the port used for `socket`.

 `(socket-input socket)` STklos procedure
`(socket-output socket)`

Returns the file port associated for reading or writing with the program connected with `socket`. If no connection has already been established, these functions return `#f`. The following example shows how to make a client socket. Here we create a socket on port 13 of the machine `kaolin.unice.fr`2:
 ```(let ((s (make-client-socket "kaolin.unice.fr" 13))) (format #t "Time is: ~A~%" (read-line (socket-input s))) (socket-shutdown s)) ```

### 4.19 System Procedures

#### 4.19.1 File Primitives

 `(temporary-file-name)` STklos procedure

Generates a unique temporary file name. The value returned by `temporary-file-name` is the newly generated name of `#f` if a unique name cannot be generated.

 `(rename-file string1 string2)` STklos procedure

Renames the file whose path-name is `string1` to a file whose path-name is `string2`. The result of `rename-file` is void.

 `(remove-file string)` STklos procedure

Removes the file whose path name is given in `string`. The result of `remove-file` is void.

 `(copy-file string1 string2)` STklos procedure

Copies the file whose path-name is `string1` to a file whose path-name is `string2`. If the file `string2` already exists, its content prior the call to `copy-file` is lost. The result of `copy-file` is void.

 `(copy-port in out)` STklos procedure
`(copy-port in out max)`

Copy the content of port `in`, which must be opened for reading, on port `out`, which must be opened for writing. If `max` is not specified, All the characters from the input port are copied on ouput port. If `max` is specified, it must be an integer indicating the maximum number of characters which are copied from `in` to `out`.

 `(file-is-directory? string)` STklos procedure
`(file-is-regular? string)`
`(file-is-readable? string)`
`(file-is-writable? string)`
`(file-is-executable? string)`
`(file-exists? string)`

Returns `#t` if the predicate is true for the path name given in `string`; returns `#f` otherwise (or if `string` denotes a file which does not exist).

 `(file-size string)` STklos procedure

Returns the size of the file whose path name is given in `string`.If `string` denotes a file which does not exist, `file-size` returns `#f`.

 `(getcwd)` STklos procedure

Returns a string containing the current working directory.

 `(chmod str)` STklos procedure
`(chmod str option1 ...)`

Change the access mode of the file whose path name is given in `string`. The options must be composed of either an integer or one of the following symbols `read`, `write` or `execute`. Giving no option to `chmod` is equivalent to pass it the integer `0`. If the operation succeeds, `chmod` returns `#t`; otherwise it returns `#f`.
 ```(chmod "~/.stklos/stklosrc" 'read 'execute) (chmod "~/.stklos/stklosrc" #o644) ```

 `(chdir dir)` STklos procedure

Changes the current directory to the directory given in string `dir`.

 `(make-directory dir)` STklos procedure

Create a directory with name `dir`.

 `(make-directories str)` STklos procedure

Create a directory with name `dir`. No error is signaled if `dir` already exists, Parent directories of `dir` are created as needed.

 `(ensure-directories-exist path)` STklos procedure

Create a directory with name `dir` (and its parent directories if needed), if it does not exist yet.

 `(remove-directory dir)` STklos procedure

Delete the directory with name `dir`.

 `(directory-files path)` STklos procedure

Returns the list of the files in the directory `path`. Directories "." and ".." don't appear in the result.

 `(expand-file-name path)` STklos procedure

`Expand-file-name` expands the filename given in `path` to an absolute path.
 ``` ;; Current directory is ~eg/stklos (i.e. /users/eg/stklos) (expand-file-name "..") ⇒ "/users/eg" (expand-file-name "~eg/../eg/bin") ⇒ "/users/eg/bin" (expand-file-name "~/stklos)" ⇒ "/users/eg/stk" ```

 `(canonical-file-name path)` STklos procedure

Expands all symbolic links in `path` and returns its canonicalized absolute path name. The resulting path does not have symbolic links. If `path` doesn't designate a valid path name, `canonical-file-name` returns `#f`.

 `(decompose-file-name string)` STklos procedure

Returns an ``exploded'' list of the path name components given in `string`. The first element in the list denotes if the given `string` is an absolute path or a relative one, being "/" or "." respectively. Each component of this list is a string.
 ```(decompose-file-name "/a/b/c.stk") ⇒ ("/" "a" "b" "c.stk") (decompose-file-name "a/b/c.stk") ⇒ ("." "a" "b" "c.stk") ```

 `(winify-file-name fn)` STklos procedure

On Win32 system, when compiled with the Cygwin environment, file names are internally represented in a POSIX-like internal form. `Winify-file-bame` permits to obtain back the Win32 name of an interned file name
 ```(winify-file-name "/tmp") ⇒ "C:\\cygwin\\tmp" (list (getcwd) (winify-file-name (getcwd))) ⇒ ("//saxo/homes/eg/Projects/,(stklos)" "\\\\saxo\\homes\\eg\\Projects\\,(stklos)") ```

 `(posixify-file-name fn)` STklos procedure

On Win32 system, when compiled with the Cygwin environment, file names are internally represented in a POSIX-like internal form. `posixify-file-bame` permits to obtain the interned file name from its external form. file name
 ```(posixify-file-name "C:\\cygwin\\tmp") ⇒ "/tmp" ```

 `(basename str)` STklos procedure

Returns a string containing the last component of the path name given in `str`.
 ```(basename "/a/b/c.stk") ⇒ "c.stk" ```

 `(dirname str)` STklos procedure

Returns a string containing all but the last component of the path name given in `str`.
 ```(dirname "/a/b/c.stk") ⇒ "/a/b" ```

 `(file-suffix pathname)` STklos procedure

Returns the suffix of given `pathname`. If no suffix is found, `file-suffix` returns an empty string.
 ```(file-suffix "./foo.tar.gz") ⇒ "gz" (file-suffix "./a.b/c") ⇒ "" ```

 `(file-prefix pathname)` STklos procedure

Returns the suffix of given `pathname`.
 ```(file-prefix "./foo.tar.gz") ⇒ "./foo.tar" (file-prefix "./a.b/c") ⇒ "./a.b/c" ```

 `(file-separator)` STklos procedure

Retuns the operating system file separator as a character. This is typically #\/ on Unix (or Cygwin) systems and #\\ on Windows.

 `(make-path dirname . names)` STklos procedure

Builds a file name from the directory `dirname` and `names`. For instance, on a Unix system:
 ```(make-path "a" "b" "c") ⇒ "a/b/c" ```

 `(glob pattern ...)` STklos procedure

`Glob` performs file name ``globbing'' in a fashion similar to the csh shell. `Glob` returns a list of the filenames that match at least one of `pattern` arguments. The `pattern` arguments may contain the following special characters:
• `?` Matches any single character.
• `*` Matches any sequence of zero or more characters.
• `[chars]` Matches any single character in `chars`. If chars contains a sequence of the form `a-b` then any character between `a` and `b` (inclusive) will match.
• `\x` Matches the character `x`.
• `{a,b,...}` Matches any of the strings `a`, `b`, etc.
As with csh, a '.' at the beginning of a file's name or just after a '/ must be matched explicitly or with a `@{@}` construct. In addition, all '/' characters must be matched explicitly.

If the first character in a pattern is '~' then it refers to the home directory of the user whose name follows the '~'. If the '~' is followed immediately by `/' then the value of the environment variable HOME is used.

`Glob` differs from csh globbing in two ways. First, it does not sort its result list (use the `sort` procedure if you want the list sorted). Second, `glob` only returns the names of files that actually exist; in csh no check for existence is made unless a pattern contains a `?`, `*`, or `[]` construct.

#### 4.19.2 Environment

 `(getenv str)` STklos procedure
`(getenv)`

Looks for the environment variable named `str` and returns its value as a string, if it exists. Otherwise, `getenv` returns `#f`. If `getenv` is called without parameter, it returns the list of all the environment variables accessible from the program as an A-list.
 ```(getenv "SHELL") ⇒ "/bin/zsh" (getenv) ⇒ (("TERM" . "xterm") ("PATH" . "/bin:/usr/bin") ...) ```

 `(setenv! var value)` STklos procedure

Sets the environment variable `var` to `value`. `Var` and `value` must be strings. The result of `setenv!` is void.

 `(unsetenv! var)` STklos procedure

Unsets the environment variable `var`. `Var` must be a string. The result of `unsetenv!` is void.

#### 4.19.3 System Informations

 `(running-os)` STklos procedure

Returns the name of the underlying Operating System which is running the program. The value returned by `runnin-os` is a symbol. For now, this procedure returns either `unix`, `windows`, or `cygwin-windows`.

 `(hostname)` STklos procedure

Return the host name of the current processor as a string.

 `(argc)` STklos procedure

Returns the number of argument present on the command line

 `(argv)` STklos procedure

Returns a list of the arguments given on the shell command line. The interpreter options are no included in the result

 `(program-name)` STklos procedure

Returns the invocation name of the current program as a string.

 `(version)` STklos procedure

Returns a string identifying the current version of the system. A version is constituted of three numbers separated by a point: the version, the release and sub-release numbers.

 `(machine-type)` STklos procedure

Returns a string identifying the kind of machine which is running the program. The result string is of the form ``(os-name)-`(os-version)-`(processor-type)`.

 `(clock)` STklos procedure

Returns an approximation of processor time, in milliseconds, used so far by the program.

 `(sleep n)` STklos procedure

Suspend the execution of the program for at `ms` milliseconds. Note that due to system clock resolution, the pause may be a little bit longer. If a signal arrives during the pause, the execution may be resumed.

 `(time expr1 expr2 ...)` STklos syntax

Evaluates the expressions `expr1`, `expr2`, ... and returns the result of the last expression. This form prints also the time spent for this evaluation on the current error port.

 `(getpid)` STklos procedure

Returns the system process number of the current program (i.e. the Unix pid) as an integer.

#### 4.19.4 Program Arguments Parsing

STklos provides a simple way to parse program arguments with the |parse-arguments| special form. This form is generally used into the |main| function in a Scheme script. See SRFI-22 (Running Scheme Scripts on Unix) on how to use a |main| function in a Scheme program.

 `(parse-arguments ...)` STklos procedure

The `parse-arguments` special form is used to parse the command line arguments of a Scheme script. The implementation of this form internally uses the GNU C `getopt` function. As a consequence `parse-arguments` accepts options which start with the '-' (short option) or '--' characters (long option).
The first argument of `parse-arguments` is a list of the arguments given to the program (comprising the program name in the CAR of this list). Following arguments are clauses. Clauses are described later.
By default, `parse-arguments` permutes the contents of (a copy) of the arguments as it scans, so that eventually all the non-options are at the end. However, if the shell environment variable `POSIXLY_CORRECT` is set, then option processing stops as soon as a non-option argument is encountered.
A clause must follow the syntax:
 ``` ⇒ string | ⇒ (
A string clause is used to build the help associated to the command. A list clause must follow the syntax describes an option. The `<expr>`s associated to a list clauses are executed when the option is recognized. The `else` clauses is executed when all parameters have been parsed. The `:alternate` key permits to have an alternate name for an option (generally a short or long name if the option name is a short or long name). The `:help` is used to provide help about the the option. The `:arg` is used when the option admit a parameter: the symbol given after `:arg` will be bound to the value of the option argument when the corresponding `<expr>`s will be executed.
In an `else` clause the symbol `other-arguments` is bound to the list of the arguments which are not options.
The following example shows a rather complete usage of the `parse-arguments` form
 ```#!/usr/bin/env stklos-script (define (main args) (parse-arguments args "Usage: foo [options] [parameter ...]" "General options:" (("verbose" :alternate "v" :help "be more verbose") (format #t "Seen the verbose option~%")) (("long" :help "a long option alone") (format #t "Seen the long option~%")) (("s" :help "a short option alone") (format #t "Seen the short option~%")) "File options:" (("input" :alternate "f" :arg file :help "use as input") (format #t "Seen the input option with ~S argument~%" file)) (("output" :alternate "o" :arg file :help "use as output") (format #t "Seen the output option with ~S argument~%" file)) "Misc:" (("help" :alternate "h" :help "provides help for the command") (arg-usage (current-error-port)) (exit 1)) (else (format #t "All options parsed. Remaining arguments are ~S~%" other-arguments)))) ```
The following program invocation
 ```foo -vs --input in -o out arg1 arg2 ```
produces the following output
 ```Seen the verbose option Seen the short option Seen the input option with "in" argument Seen the output option with "out" argument All options parsed. Remaining arguments are ("arg1" "arg2") ```
Finally, the program invocation
 ```foo --help ```
produces the following output
 ```Usage: foo `(options) `(parameter ...) General options: --verbose, -v be more verbose --long a long option alone -s a short option alone File options: --input=, -f use as input --output=, -o use as output Misc: --help, -h provides help for the command ```
Note:
• Short option can be concatenated. That is,
 ```prog -abc ```
is equivalent to the following program call
 ```prog -a -b -c ```
• Any argument following a '--' argument is no more considered as an option, even if it starts with a '-' or '--'.
• Option with a parameter can be written in several ways. For instance to set the output in the `bar` file for the previous example can be expressed as
• `--output=bar`
• `-o bar`
• `-obar`

 `(arg-usage port)` STklos procedure
`(arg-usage port as-sexpr)`

This procedure is only bound inside a `parse-arguments` form. It pretty prints the help associated to the clauses of the `parse-arguments` form on the given port. If the argument `as-sexpr` is passed and is not` #f`, the help strings are printed on `port` as S-exprs. This is useful if the help strings need to be manipulated by a program.

#### 4.19.5 Misc. System Procedures

 `(system string)` STklos procedure

Sends the given `string` to the system shell `/bin/sh`. The result of `system` is the integer status code the shell returns.

 `(exec str)` STklos procedure
`(exec-list str)`

These procedures execute the command given in `str`. The command given in `str` is passed to `/bin/sh`. `Exec` returns a strings which contains all the characters that the command `str` has printed on it's standard output, whereas `exec-list` returns a list of the lines which constitute the output of `str`.
 ```(exec "echo A; echo B") ⇒ "A\nB\n" (exec-list "echo A; echo B") ⇒ ("A" "B") ```

 `(address-of obj)` R5RS procedure

Returns the address of the object `obj` as an integer.

 `(exit)` STklos procedure
`(exit ret-code)`

Exits the program with the specified integer return code. If `ret-code` is omitted, the program terminates with a return code of 0. If program has registered exit functions with `register-exit-function!`, they are called (in an order which is the reverse of their call order).

 `(die message)` STklos procedure
`(die message status)`

`Die` prints the given `message` on the current error port and exits the program with the `status` value. If `status` is omitted, it defaults to 1.

 `(get-password)` STklos procedure

This primitive permits to enter a password (character echoing being turned off). The value returned by `get-password` is the entered password as a string.

 `(register-exit-function! proc)` STklos procedure

This function registers `proc` as an exit function. This function will be called when the program exits. When called, `proc` will be passed one parmater which is the status given to the `exit` function (or 0 if the programe terminates normally). The result of `register-exit-function!` is undefined.
 ```(let* ((tmp (temporary-file-name)) (out (open-output-file tmp))) (register-exit-function! (lambda (n) (when (zero? n) (remove-file tmp)))) out) ```

### 4.21 Parameter Objects

STklos parameters correspond to the ones defined in SRFI-39 (Parameters objects). See SRFI document for more information.

 `(make-parameter init)` STklos procedure
`(make-parameter init converter)`

Returns a new parameter object which is bound in the global dynamic environment to a cell containing the value returned by the call `(converter init)`. If the conversion procedure `converter` is not specified the identity function is used instead.

The parameter object is a procedure which accepts zero or one argument. When it is called with no argument, the content of the cell bound to this parameter object in the current dynamic environment is returned. When it is called with one argument, the content of the cell bound to this parameter object in the current dynamic environment is set to the result of the call `(converter arg)`, where `arg` is the argument passed to the parameter object, and an unspecified value is returned.
 ```(define radix (make-parameter 10)) (define write-shared (make-parameter #f (lambda (x) (if (boolean? x) x (error 'write-shared "bad boolean ~S" x))))) (radix) ⇒ 10 (radix 2) (radix) ⇒ 2 (write-shared 0) ⇒ error (define prompt (make-parameter 123 (lambda (x) (if (string? x) x (with-output-to-string (lambda () (write x))))))) (prompt) ⇒ "123" (prompt ">") (prompt) ⇒ ">" ```

 `(parameterize ((expr1 expr2) ...) )` STklos syntax

The expressions `expr1` and `expr2` are evaluated in an unspecified order. The value of the `expr1` expressions must be parameter objects. For each `expr1` expression and in an unspecified order, the local dynamic environment is extended with a binding of the parameter object `expr1` to a new cell whose content is the result of the call `(converter val)`, where `val` is the value of `expr2` and converter is the conversion procedure of the parameter object. The resulting dynamic environment is then used for the evaluation of `<body>` (which refers to the R5RS grammar nonterminal of that name). The result(s) of the parameterize form are the result(s) of the `<body>`.
 ```(radix) ⇒ 2 (parameterize ((radix 16)) (radix)) ⇒ 16 (radix) ⇒ 2 (define (f n) (number->string n (radix))) (f 10) ⇒ "1010" (parameterize ((radix 8)) (f 10)) ⇒ "12" (parameterize ((radix 8) (prompt (f 10))) (prompt)) ⇒ "1010" ```

 `(parameter? obj)` STklos procedure

Returns `#t` if `obj` is a parameter object, otherwise returns `#f`.

### 4.22 Misc

 `(gc)` R5RS procedure

Returns the address of the object `obj` as an integer.

 `(void)` STklos procedure
`(void arg1 ...)`

Returns the special void object. If arguments are passed to `void`, they are evalued and simply ignored.

 `(error str obj ...)` STklos procedure
`(error name str obj ...)`

`error` is used to signal an error to the user. The second form of `error` takes a symbol as first parameter; it is generally used for the name of the procedure which raises the error.

Note: The specification string may follow the tilde conventions of `format` (see format); in this case this procedure builds an error message according to the specification given in `str`. Otherwise, this procedure is conform to the `error` procedure defined in SRFI-23 (Error reporting mechanism) and `str` is printed with the `display` procedure, whereas the `obj`s are printed with the `write` procedure.

Hereafter, are some calls of the `error` procedure using a formatted string
 ```(error "bad integer ~A" "a") -| bad integer a (error 'vector-ref "bad integer ~S" "a") -| vector-ref: bad integer "a" (error 'foo "~A is not between ~A and ~A" "bar" 0 5) -| foo: bar is not between 0 and 5 ```
and some conform to SRFI-23 (Error reporting mechanism)
 ```(error "bad integer" "a") -| bad integer "a" (error 'vector-ref "bad integer" "a") -| vector-ref: bad integer "a" (error "bar" "is not between" 0 "and" 5) -| bar "is not between" 0 "and" 5 ```

 `(signal-error cond str obj ...)` STklos procedure
`(signal-error cond name str obj ...)`

This procedure is similar to error, except that the type of the error can be passed as the first parameter. The type of the error must be a condition which inherits from `&error-message`.

Note that `(error arg ...)` is equivalent to
 ```(signal-error &error-message arg ...) ```

 `(require-extension ...)` STklos syntax

The syntax of require-extension is as follows:
 ```(require-extension ...) ```
A clause has the form:
 ``` (srfi ...) ```
where `<extension-argument>`s may be any Scheme-values.

If an `<extension-argument>` is a nonnegative integer, the functionality of the indicated SRFIs is made available in the context in which the require-extension form appears. For instance,
 ```(require-extension (srfi 1 2)) ; Make the SRFI 1 and 2 available ```
This form is compatible with SRFI-55 (Require-extension). However, STklos accepts also some symbolic names for requiring some extensions. For instance,
 ```(require-extension (srfi lists and-let*)) ```
is equivalent to the previous `require-extension`. A list of available symbols as `<extension-argument>` is given in chapter SRFIs.

 `(repl)` STklos procedure
`(repl :in inport :out outport :err errport)`

This procedure launches a new Read-Eval-Print-Loop. Calls to `repl` can be embedded. The ports used for input/output as well as the error port can be passed when `repl` is called. If not passed, they default to `current-input-port`, `current-output-port` and `current-error-port`.

 `(apropos obj)` STklos procedure
`(apropos obj module)`

`Apropos` returns a list of symbols whose print name contains the characters of `obj` as a substring . The given `obj` can be a string or symbol. This function returns the list of matched symbols which can be accessed from the given `module` (defaults to the current module if not provided).

 `(help obj)` STklos procedure
`(help)`

When called with an argument, `help` tries to give some help on the given object, which could be a symbol, a procedure, a generic function or a method. Whe called called without arguments, `help` enters a read-help-print loop. The documentation for an object is searched in the object itself or, if absent, in STklos documentation. Inserting the documentation in an objet is very similar to Emacs docstrings: a documentation string is defined among the code. Exemples of such strings are given below
 ```(define (foo n) "If the function body starts with a string, it's a docstring" (+ n 1)) (define-generic bar :documentation "Generic function docsting for bar") (define-method bar ((x )) "Probably less useful: as in functions, methods can have docstrings" (- x 1)) ```

 `(trace f-name ...)` STklos syntax

Invoking `trace` with one or more function names causes the functions named to be traced. Henceforth, whenever such a function is invoked, information about the call and the returned values, if any, will be printed on the current error port. Calling `trace` with no argument returns the list of traced functions.

 `(untrace f-name ...)` STklos syntax

Invoking `untrace` with one or more function names causes the functions named not to be traced anymore. Calling `untrace` with no argument will untrace all the functions currently traced.

 `(pretty-print sexpr :key port width)` STklos procedure
`(pp sexpr :key port width)`

This function tries to obtain a pretty-printed representation of `sexpr`. The pretty-printed form is written on `port` with lines which are no more long than `width` characters. If `port` is omitted if defaults to the current error port. As a special convention, if `port` is `#t`, output goes to the current output port and if `port` is `#f`, the output is returned as a string by `pretty-print`. Note that `pp` is another name for `pretty-print`.

 `(uri-parse str)` STklos procedure

Parses the string `str` as a RFC-2396 URI and return a keyed list with the following components
• `scheme` : the scheme used as a string (defaults to `"file"`)
• `user`: the user information (generally expressed as `login:password`)
• `host` : the host as a string (defaults to "")
• `port` : the port as an integer (0 if no port specified)
• `path` : the path
• `query` : the qury part of the URI as a string (defaults to the empty string)
• `fragment` : the fragment of the URI as a string (defaults to the empty string)
 ```(uri-parse "http://google.com") ⇒ (:scheme "http" :user "" :host "google.com" :port 80 :path "/" :query "" :fragment "") (uri-parse "http://stklos.net:8080/a/file?x=1;y=2#end") ⇒ (:scheme "http" :user "" :host "stklos.net" :port 8080 :path "/a/file" :query "x=1;y=2" :fragment "end") (uri-parse "http://foo:secret@stklos.net:2000/a/file") ⇒ (:scheme "http" :user "foo:secret" :host "stklos.net" :port 2000 :path "/a/file" :query "" :fragment "") (uri-parse "/a/file") ⇒ (:scheme "file" :user "" :host "" :port 0 :path "/a/file" :query "" :fragment "") (uri-parse "") ⇒ (:scheme "file" :user "" :host "" :port 0 :path "" :query "" :fragment "") ```

 `(string->html str)` STklos procedure

This primitive is a convenience function; it returns a string where the HTML special chars are properly translated. It can easily written in Scheme, but this version is fast.
 ```(string->html "Just a ") ⇒ "Just a <test>" ```

 `(md5sum obj)` STklos procedure

Return a string contening the md5 dum of `obj`. The given parameter can be a string or an open input port.

 `(md5sum-file str)` STklos procedure

Return a string contening the md5 dum of the file whose name is `str`.

 `(base64-encode in)` STklos procedure
`(base64-encode in out)`

Encode in Base64 the characters from input port `in` to the output port `out`. If `out` is not specified, it defaults to the current output port.

 `(base64-decode in)` STklos procedure
`(base64-decode in out)`

Decode the Base64 characters from input port `in` to the output port `out`. If `out` is not specified, it defaults to the current output port.

 `(base64-encode-string str)` STklos procedure

Return a string contening the contents of `str` converted to Base64 encoded format

 `(base64-encode-string str)` STklos procedure

Decode the contents of `str` expressed in Base64.

1: Under Unix, you can simply connect to a listening socket with the `telnet` command. With the given example, this can be achieved by typing the following command in a window shell:
 ```\$ telnet localhost 12345 ```

2: Port 13 is generally used for testing: making a connection to it permits to know the distant system's idea of the time of day.

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Last update Fri Apr 20 19:24:44 2018