This document describes Chicken's R5RS support, with a heavy emphasis
on syntax and procedures.  It is based directly on the
''Revised^5 Report on the Algorithmic Language Scheme''.
[[toc:]]
== Overview of Scheme
== Lexical conventions
== Basic concepts
== Expressions

Expression types are categorized as primitive or derived. Primitive
expression types include variables and procedure calls. Derived
expression types are not semantically primitive, but can instead be
defined as macros. With the exception of quasiquote, whose macro
definition is complex, the derived expressions are classified as
library features. Suitable definitions are given in section 7.3.

=== Primitive expression types

==== Variable references

<macro><variable></macro><br>

An expression consisting of a variable (section 3.1) is a variable
reference. The value of the variable reference is the value stored in
the location to which the variable is bound. It is an error to
reference an unbound variable.

 (define x 28)
 x           ===>  28

==== Literal expressions

<macro>(quote <datum>)</macro><br>
<macro>'<datum></macro><br>
<macro><constant></macro><br>

(quote <datum>) evaluates to <datum>. <Datum> may be any external
representation of a Scheme object (see section 3.3). This notation is
used to include literal constants in Scheme code.

 (quote a)                    ===>  a
 (quote #(a b c))             ===>  #(a b c)
 (quote (+ 1 2))              ===>  (+ 1 2)

(quote <datum>) may be abbreviated as '<datum>. The two notations are
equivalent in all respects.

 'a                           ===>  a
 '#(a b c)                    ===>  #(a b c)
 '()                          ===>  ()
 '(+ 1 2)                     ===>  (+ 1 2)
 '(quote a)                   ===>  (quote a)
 "a                           ===>  (quote a)

Numerical constants, string constants, character constants, and boolean
constants evaluate "to themselves"; they need not be quoted.

 '"abc"             ===>  "abc"
 "abc"              ===>  "abc"
 '145932            ===>  145932
 145932             ===>  145932
 '#t                ===>  #t
 #t                 ===>  #t

As noted in section 3.4, it is an error to alter a constant (i.e. the
value of a literal expression) using a mutation procedure like set-car!
or string-set!.

==== Procedure calls

<macro>(<operator> <operand[1]> ...)</macro><br>

A procedure call is written by simply enclosing in parentheses
expressions for the procedure to be called and the arguments to be
passed to it. The operator and operand expressions are evaluated (in an
unspecified order) and the resulting procedure is passed the resulting
arguments.

 (+ 3 4)                           ===>  7
 ((if #f + *) 3 4)                 ===>  12

A number of procedures are available as the values of variables in the
initial environment; for example, the addition and multiplication
procedures in the above examples are the values of the variables + and *.
New procedures are created by evaluating lambda expressions (see
section 4.1.4). Procedure calls may return any number of values (see
values in section 6.4). With the exception of values the procedures
available in the initial environment return one value or, for
procedures such as apply, pass on the values returned by a call to one
of their arguments.

Procedure calls are also called combinations.

Note:   In contrast to other dialects of Lisp, the order of
evaluation is unspecified, and the operator expression and the
operand expressions are always evaluated with the same evaluation
rules.

Note:   Although the order of evaluation is otherwise unspecified,
the effect of any concurrent evaluation of the operator and operand
expressions is constrained to be consistent with some sequential
order of evaluation. The order of evaluation may be chosen
differently for each procedure call.

Note:   In many dialects of Lisp, the empty combination, (), is a
legitimate expression. In Scheme, combinations must have at least
one subexpression, so () is not a syntactically valid expression.

==== Procedures

<macro>(lambda <formals> <body>)</macro><br>

Syntax: <Formals> should be a formal arguments list as described below,
and <body> should be a sequence of one or more expressions.

Semantics: A lambda expression evaluates to a procedure. The
environment in effect when the lambda expression was evaluated is
remembered as part of the procedure. When the procedure is later called
with some actual arguments, the environment in which the lambda
expression was evaluated will be extended by binding the variables in
the formal argument list to fresh locations, the corresponding actual
argument values will be stored in those locations, and the expressions
in the body of the lambda expression will be evaluated sequentially in
the extended environment. The result(s) of the last expression in the
body will be returned as the result(s) of the procedure call.

 (lambda (x) (+ x x))              ===>  a procedure
 ((lambda (x) (+ x x)) 4)          ===>  8
 
 (define reverse-subtract
   (lambda (x y) (- y x)))
 (reverse-subtract 7 10)           ===>  3
 
 (define add4
   (let ((x 4))
     (lambda (y) (+ x y))))
 (add4 6)                          ===>  10

<Formals> should have one of the following forms:

*   (<variable[1]> ...): The procedure takes a fixed number of
    arguments; when the procedure is called, the arguments will be
    stored in the bindings of the corresponding variables.

*   <variable>: The procedure takes any number of arguments; when the
    procedure is called, the sequence of actual arguments is converted
    into a newly allocated list, and the list is stored in the binding
    of the <variable>.

*   (<variable[1]> ... <variable[n]> . <variable[n+1]>): If a
    space-delimited period precedes the last variable, then the
    procedure takes n or more arguments, where n is the number of
    formal arguments before the period (there must be at least one).
    The value stored in the binding of the last variable will be a
    newly allocated list of the actual arguments left over after all
    the other actual arguments have been matched up against the other
    formal arguments.

It is an error for a <variable> to appear more than once in <formals>.

 ((lambda x x) 3 4 5 6)                  ===>  (3 4 5 6)
 ((lambda (x y . z) z)
  3 4 5 6)                               ===>  (5 6)

Each procedure created as the result of evaluating a lambda expression
is (conceptually) tagged with a storage location, in order to make eqv?
and eq? work on procedures (see section 6.1).

==== Conditionals

<macro>(if <test> <consequent> <alternate>)</macro><br>
<macro>(if <test> <consequent>)</macro><br>

Syntax: <Test>, <consequent>, and <alternate> may be arbitrary
expressions.

Semantics: An if expression is evaluated as follows: first, <test> is
evaluated. If it yields a true value (see section 6.3.1), then
<consequent> is evaluated and its value(s) is(are) returned. Otherwise
<alternate> is evaluated and its value(s) is(are) returned. If <test>
yields a false value and no <alternate> is specified, then the result
of the expression is unspecified.

 (if (> 3 2) 'yes 'no)                   ===>  yes
 (if (> 2 3) 'yes 'no)                   ===>  no
 (if (> 3 2)
     (- 3 2)
     (+ 3 2))                            ===>  1

==== Assignments

<macro>(set! <variable> <expression>)</macro><br>

<Expression> is evaluated, and the resulting value is stored in the
location to which <variable> is bound. <Variable> must be bound either
in some region enclosing the set! expression or at top level. The
result of the set! expression is unspecified.

 (define x 2)
 (+ x 1)                         ===>  3
 (set! x 4)                      ===>  unspecified
 (+ x 1)                         ===>  5

=== Derived expression types

The constructs in this section are hygienic, as discussed in section 
4.3. For reference purposes, section 7.3 gives macro definitions that
will convert most of the constructs described in this section into the
primitive constructs described in the previous section.

==== Conditionals

<macro>(cond <clause[1]> <clause[2]> ...)</macro><br>

Syntax: Each <clause> should be of the form

 (<test> <expression[1]> ...)

where <test> is any expression. Alternatively, a <clause> may be of the
form

 (<test> => <expression>)

The last <clause> may be an "else clause," which has the form

 (else <expression[1]> <expression[2]> ...).

Semantics: A cond expression is evaluated by evaluating the <test>
expressions of successive <clause>s in order until one of them
evaluates to a true value (see section 6.3.1). When a <test> evaluates
to a true value, then the remaining <expression>s in its <clause> are
evaluated in order, and the result(s) of the last <expression> in the
<clause> is(are) returned as the result(s) of the entire cond
expression. If the selected <clause> contains only the <test> and no
<expression>s, then the value of the <test> is returned as the result.
If the selected <clause> uses the => alternate form, then the
<expression> is evaluated. Its value must be a procedure that accepts
one argument; this procedure is then called on the value of the <test>
and the value(s) returned by this procedure is(are) returned by the
cond expression. If all <test>s evaluate to false values, and there is
no else clause, then the result of the conditional expression is
unspecified; if there is an else clause, then its <expression>s are
evaluated, and the value(s) of the last one is(are) returned.

 (cond ((> 3 2) 'greater)
       ((< 3 2) 'less))           ===>  greater
 (cond ((> 3 3) 'greater)
       ((< 3 3) 'less)
       (else 'equal))             ===>  equal
 (cond ((assv 'b '((a 1) (b 2))) => cadr)
       (else #f))                 ===>  2

<macro>(case <key> <clause[1]> <clause[2]> ...)</macro><br>

Syntax: <Key> may be any expression. Each <clause> should have the form

 ((<datum[1]> ...) <expression[1]> <expression[2]> ...),

where each <datum> is an external representation of some object. All
the <datum>s must be distinct. The last <clause> may be an "else
clause," which has the form

 (else <expression[1]> <expression[2]> ...).

Semantics: A case expression is evaluated as follows. <Key> is
evaluated and its result is compared against each <datum>. If the
result of evaluating <key> is equivalent (in the sense of eqv?; see
section 6.1) to a <datum>, then the expressions in the corresponding
<clause> are evaluated from left to right and the result(s) of the last
expression in the <clause> is(are) returned as the result(s) of the
case expression. If the result of evaluating <key> is different from
every <datum>, then if there is an else clause its expressions are
evaluated and the result(s) of the last is(are) the result(s) of the
case expression; otherwise the result of the case expression is
unspecified.

 (case (* 2 3)
   ((2 3 5 7) 'prime)
   ((1 4 6 8 9) 'composite))             ===>  composite
 (case (car '(c d))
   ((a) 'a)
   ((b) 'b))                             ===>  unspecified
 (case (car '(c d))
   ((a e i o u) 'vowel)
   ((w y) 'semivowel)
   (else 'consonant))                    ===>  consonant

<macro>(and <test[1]> ...)</macro><br>

The <test> expressions are evaluated from left to right, and the value
of the first expression that evaluates to a false value (see section 
6.3.1) is returned. Any remaining expressions are not evaluated. If all
the expressions evaluate to true values, the value of the last
expression is returned. If there are no expressions then #t is
returned.

 (and (= 2 2) (> 2 1))                   ===>  #t
 (and (= 2 2) (< 2 1))                   ===>  #f
 (and 1 2 'c '(f g))                     ===>  (f g)
 (and)                                   ===>  #t

<macro>(or <test[1]> ...)</macro><br>

The <test> expressions are evaluated from left to right, and the value
of the first expression that evaluates to a true value (see section 
6.3.1) is returned. Any remaining expressions are not evaluated. If all
expressions evaluate to false values, the value of the last expression
is returned. If there are no expressions then #f is returned.

 (or (= 2 2) (> 2 1))                    ===>  #t
 (or (= 2 2) (< 2 1))                    ===>  #t
 (or #f #f #f)         ===>  #f
 (or (memq 'b '(a b c)) 
     (/ 3 0))                            ===>  (b c)

==== Binding constructs

The three binding constructs let, let*, and letrec give Scheme a block
structure, like Algol 60. The syntax of the three constructs is
identical, but they differ in the regions they establish for their
variable bindings. In a let expression, the initial values are computed
before any of the variables become bound; in a let* expression, the
bindings and evaluations are performed sequentially; while in a letrec
expression, all the bindings are in effect while their initial values
are being computed, thus allowing mutually recursive definitions.

<macro>(let <bindings> <body>)</macro><br>

Syntax: <Bindings> should have the form

 ((<variable[1]> <init[1]>) ...),

where each <init> is an expression, and <body> should be a sequence of
one or more expressions. It is an error for a <variable> to appear more
than once in the list of variables being bound.

Semantics: The <init>s are evaluated in the current environment (in
some unspecified order), the <variable>s are bound to fresh locations
holding the results, the <body> is evaluated in the extended
environment, and the value(s) of the last expression of <body> is(are)
returned. Each binding of a <variable> has <body> as its region.

 (let ((x 2) (y 3))
   (* x y))                              ===>  6
 
 (let ((x 2) (y 3))
   (let ((x 7)
         (z (+ x y)))
     (* z x)))                           ===>  35

See also named let, section 4.2.4.

<macro>(let* <bindings> <body>)</macro><br>

Syntax: <Bindings> should have the form

 ((<variable[1]> <init[1]>) ...),

and <body> should be a sequence of one or more expressions.

Semantics: Let* is similar to let, but the bindings are performed
sequentially from left to right, and the region of a binding indicated
by (<variable> <init>) is that part of the let* expression to the right
of the binding. Thus the second binding is done in an environment in
which the first binding is visible, and so on.

 (let ((x 2) (y 3))
   (let* ((x 7)
          (z (+ x y)))
     (* z x)))                     ===>  70

<macro>(letrec <bindings> <body>)</macro><br>

Syntax: <Bindings> should have the form

 ((<variable[1]> <init[1]>) ...),

and <body> should be a sequence of one or more expressions. It is an
error for a <variable> to appear more than once in the list of
variables being bound.

Semantics: The <variable>s are bound to fresh locations holding
undefined values, the <init>s are evaluated in the resulting
environment (in some unspecified order), each <variable> is assigned to
the result of the corresponding <init>, the <body> is evaluated in the
resulting environment, and the value(s) of the last expression in
<body> is(are) returned. Each binding of a <variable> has the entire
letrec expression as its region, making it possible to define mutually
recursive procedures.

 (letrec ((even?
           (lambda (n)
             (if (zero? n)
                 #t
                 (odd? (- n 1)))))
          (odd?
           (lambda (n)
             (if (zero? n)
                 #f
                 (even? (- n 1))))))
   (even? 88))
                         ===>  #t

One restriction on letrec is very important: it must be possible to
evaluate each <init> without assigning or referring to the value of any
<variable>. If this restriction is violated, then it is an error. The
restriction is necessary because Scheme passes arguments by value
rather than by name. In the most common uses of letrec, all the <init>s
are lambda expressions and the restriction is satisfied automatically.

==== Sequencing

<macro>(begin <expression[1]> <expression[2]> ...)</macro><br>

The <expression>s are evaluated sequentially from left to right, and
the value(s) of the last <expression> is(are) returned. This expression
type is used to sequence side effects such as input and output.

 (define x 0)
 
 (begin (set! x 5)
        (+ x 1))                          ===>  6
 
 (begin (display "4 plus 1 equals ")
        (display (+ 4 1)))                ===>  unspecified
   and prints  4 plus 1 equals 5

==== Iteration

<macro>(do ((<variable[1]> <init[1]> <step[1]>) ...) (<test> <expression> ...) <command> ...)</macro><br>

Do is an iteration construct. It specifies a set of variables to be
bound, how they are to be initialized at the start, and how they are to
be updated on each iteration. When a termination condition is met, the
loop exits after evaluating the <expression>s.

Do expressions are evaluated as follows: The <init> expressions are
evaluated (in some unspecified order), the <variable>s are bound to
fresh locations, the results of the <init> expressions are stored in
the bindings of the <variable>s, and then the iteration phase begins.

Each iteration begins by evaluating <test>; if the result is false (see
section 6.3.1), then the <command> expressions are evaluated in order
for effect, the <step> expressions are evaluated in some unspecified
order, the <variable>s are bound to fresh locations, the results of the
<step>s are stored in the bindings of the <variable>s, and the next
iteration begins.

If <test> evaluates to a true value, then the <expression>s are
evaluated from left to right and the value(s) of the last <expression>
is(are) returned. If no <expression>s are present, then the value of
the do expression is unspecified.

The region of the binding of a <variable> consists of the entire do
expression except for the <init>s. It is an error for a <variable> to
appear more than once in the list of do variables.

A <step> may be omitted, in which case the effect is the same as if
(<variable> <init> <variable>) had been written instead of (<variable>
<init>).

 (do ((vec (make-vector 5))
      (i 0 (+ i 1)))
     ((= i 5) vec)
   (vector-set! vec i i))                    ===>  #(0 1 2 3 4)
 
 (let ((x '(1 3 5 7 9)))
   (do ((x x (cdr x))
        (sum 0 (+ sum (car x))))
       ((null? x) sum)))                     ===>  25

<macro>(let <variable> <bindings> <body>)</macro><br>

"Named let" is a variant on the syntax of let which provides a more
general looping construct than do and may also be used to express
recursions. It has the same syntax and semantics as ordinary let except
that <variable> is bound within <body> to a procedure whose formal
arguments are the bound variables and whose body is <body>. Thus the
execution of <body> may be repeated by invoking the procedure named by
<variable>.

 (let loop ((numbers '(3 -2 1 6 -5))
            (nonneg '())
            (neg '()))
   (cond ((null? numbers) (list nonneg neg))
         ((>= (car numbers) 0)
          (loop (cdr numbers)
                (cons (car numbers) nonneg)
                neg))
         ((< (car numbers) 0)
          (loop (cdr numbers)
                nonneg
                (cons (car numbers) neg)))))
                 ===>  ((6 1 3) (-5 -2))

==== Delayed evaluation

<macro>(delay <expression>)</macro><br>

The delay construct is used together with the procedure force to
implement lazy evaluation or call by need. (delay <expression>) returns
an object called a promise which at some point in the future may be
asked (by the force procedure) to evaluate <expression>, and deliver
the resulting value. The effect of <expression> returning multiple
values is unspecified.

See the description of force (section 6.4) for a more complete
description of delay.

==== Quasiquotation

<macro>(quasiquote <qq template>)</macro><br>
<macro>`<qq template></macro><br>

"Backquote" or "quasiquote" expressions are useful for constructing
a list or vector structure when most but not all of the desired
structure is known in advance. If no commas appear within the <qq
template>, the result of evaluating `<qq template> is equivalent to the
result of evaluating '<qq template>. If a comma appears within the <qq
template>, however, the expression following the comma is evaluated
("unquoted") and its result is inserted into the structure instead of
the comma and the expression. If a comma appears followed immediately
by an at-sign (@), then the following expression must evaluate to a
list; the opening and closing parentheses of the list are then
"stripped away" and the elements of the list are inserted in place of
the comma at-sign expression sequence. A comma at-sign should only
appear within a list or vector <qq template>.

 `(list ,(+ 1 2) 4)          ===>  (list 3 4)
 (let ((name 'a)) `(list ,name ',name))           
                 ===>  (list a (quote a))
 `(a ,(+ 1 2) ,@(map abs '(4 -5 6)) b)           
                 ===>  (a 3 4 5 6 b)
 `(( foo ,(- 10 3)) ,@(cdr '(c)) . ,(car '(cons)))           
                 ===>  ((foo 7) . cons)
 `#(10 5 ,(sqrt 4) ,@(map sqrt '(16 9)) 8)           
                 ===>  #(10 5 2 4 3 8)

Quasiquote forms may be nested. Substitutions are made only for
unquoted components appearing at the same nesting level as the
outermost backquote. The nesting level increases by one inside each
successive quasiquotation, and decreases by one inside each
unquotation.

 `(a `(b ,(+ 1 2) ,(foo ,(+ 1 3) d) e) f)           
                 ===>  (a `(b ,(+ 1 2) ,(foo 4 d) e) f)
 (let ((name1 'x)
       (name2 'y))
   `(a `(b ,,name1 ,',name2 d) e))           
                 ===>  (a `(b ,x ,'y d) e)

The two notations `<qq template> and (quasiquote <qq template>) are
identical in all respects. ,<expression> is identical to (unquote
<expression>), and ,@<expression> is identical to (unquote-splicing
<expression>). The external syntax generated by write for two-element
lists whose car is one of these symbols may vary between
implementations.

 (quasiquote (list (unquote (+ 1 2)) 4))           
                 ===>  (list 3 4)
 '(quasiquote (list (unquote (+ 1 2)) 4))           
                 ===>  `(list ,(+ 1 2) 4)
      i.e., (quasiquote (list (unquote (+ 1 2)) 4))

Unpredictable behavior can result if any of the symbols quasiquote,
unquote, or unquote-splicing appear in positions within a <qq template>
otherwise than as described above.

=== Macros

Scheme programs can define and use new derived expression types, called
macros. Program-defined expression types have the syntax

 (<keyword> <datum> ...)

where <keyword> is an identifier that uniquely determines the
expression type. This identifier is called the syntactic keyword, or
simply keyword, of the macro. The number of the <datum>s, and their
syntax, depends on the expression type.

Each instance of a macro is called a use of the macro. The set of rules
that specifies how a use of a macro is transcribed into a more
primitive expression is called the transformer of the macro.

The macro definition facility consists of two parts:

*   A set of expressions used to establish that certain identifiers are
    macro keywords, associate them with macro transformers, and control
    the scope within which a macro is defined, and

*   a pattern language for specifying macro transformers.

The syntactic keyword of a macro may shadow variable bindings, and
local variable bindings may shadow keyword bindings. All macros defined
using the pattern language are "hygienic" and "referentially
transparent" and thus preserve Scheme's lexical scoping:

*   If a macro transformer inserts a binding for an identifier
    (variable or keyword), the identifier will in effect be renamed
    throughout its scope to avoid conflicts with other identifiers.
    Note that a define at top level may or may not introduce a binding;
    see section 5.2.

*   If a macro transformer inserts a free reference to an identifier,
    the reference refers to the binding that was visible where the
    transformer was specified, regardless of any local bindings that
    may surround the use of the macro.

==== Binding constructs for syntactic keywords

Let-syntax and letrec-syntax are analogous to let and letrec, but they
bind syntactic keywords to macro transformers instead of binding
variables to locations that contain values. Syntactic keywords may also
be bound at top level; see section 5.3.

<macro>(let-syntax <bindings> <body>)</macro><br>

Syntax: <Bindings> should have the form

 ((<keyword> <transformer spec>) ...)

Each <keyword> is an identifier, each <transformer spec> is an instance
of syntax-rules, and <body> should be a sequence of one or more
expressions. It is an error for a <keyword> to appear more than once in
the list of keywords being bound.

Semantics: The <body> is expanded in the syntactic environment obtained
by extending the syntactic environment of the let-syntax expression
with macros whose keywords are the <keyword>s, bound to the specified
transformers. Each binding of a <keyword> has <body> as its region.

 (let-syntax ((when (syntax-rules ()
                      ((when test stmt1 stmt2 ...)
                       (if test
                           (begin stmt1
                                  stmt2 ...))))))
   (let ((if #t))
     (when if (set! if 'now))
     if))                                   ===>  now
 
 (let ((x 'outer))
   (let-syntax ((m (syntax-rules () ((m) x))))
     (let ((x 'inner))
       (m))))                               ===>  outer

<macro>(letrec-syntax <bindings> <body>)</macro><br>

Syntax: Same as for let-syntax.

Semantics: The <body> is expanded in the syntactic environment obtained
by extending the syntactic environment of the letrec-syntax expression
with macros whose keywords are the <keyword>s, bound to the specified
transformers. Each binding of a <keyword> has the <bindings> as well as
the <body> within its region, so the transformers can transcribe
expressions into uses of the macros introduced by the letrec-syntax
expression.

 (letrec-syntax
   ((my-or (syntax-rules ()
             ((my-or) #f)
             ((my-or e) e)
             ((my-or e1 e2 ...)
              (let ((temp e1))
                (if temp
                    temp
                    (my-or e2 ...)))))))
   (let ((x #f)
         (y 7)
         (temp 8)
         (let odd?)
         (if even?))
     (my-or x
            (let temp)
            (if y)
            y)))                ===>  7

==== Pattern language

A <transformer spec> has the following form:

 (syntax-rules <literals> <syntax rule> ...)

Syntax: <Literals> is a list of identifiers and each <syntax rule>
should be of the form

 (<pattern> <template>)

The <pattern> in a <syntax rule> is a list <pattern> that begins with
the keyword for the macro.

A <pattern> is either an identifier, a constant, or one of the
following

 (<pattern> ...)
 (<pattern> <pattern> ... . <pattern>)
 (<pattern> ... <pattern> <ellipsis>)
 #(<pattern> ...)
 #(<pattern> ... <pattern> <ellipsis>)

and a template is either an identifier, a constant, or one of the
following

 (<element> ...)
 (<element> <element> ... . <template>)
 #(<element> ...)

where an <element> is a <template> optionally followed by an <ellipsis>
and an <ellipsis> is the identifier "..." (which cannot be used as an
identifier in either a template or a pattern).

Semantics: An instance of syntax-rules produces a new macro transformer
by specifying a sequence of hygienic rewrite rules. A use of a macro
whose keyword is associated with a transformer specified by
syntax-rules is matched against the patterns contained in the <syntax
rule>s, beginning with the leftmost <syntax rule>. When a match is
found, the macro use is transcribed hygienically according to the
template.

An identifier that appears in the pattern of a <syntax rule> is a
pattern variable, unless it is the keyword that begins the pattern, is
listed in <literals>, or is the identifier "...". Pattern variables
match arbitrary input elements and are used to refer to elements of the
input in the template. It is an error for the same pattern variable to
appear more than once in a <pattern>.

The keyword at the beginning of the pattern in a <syntax rule> is not
involved in the matching and is not considered a pattern variable or
literal identifier.

Rationale:   The scope of the keyword is determined by the
expression or syntax definition that binds it to the associated
macro transformer. If the keyword were a pattern variable or
literal identifier, then the template that follows the pattern
would be within its scope regardless of whether the keyword were
bound by let-syntax or by letrec-syntax.

Identifiers that appear in <literals> are interpreted as literal
identifiers to be matched against corresponding subforms of the input.
A subform in the input matches a literal identifier if and only if it
is an identifier and either both its occurrence in the macro expression
and its occurrence in the macro definition have the same lexical
binding, or the two identifiers are equal and both have no lexical
binding.

A subpattern followed by ... can match zero or more elements of the
input. It is an error for ... to appear in <literals>. Within a pattern
the identifier ... must follow the last element of a nonempty sequence
of subpatterns.

More formally, an input form F matches a pattern P if and only if:

*   P is a non-literal identifier; or

*   P is a literal identifier and F is an identifier with the same
    binding; or

*   P is a list (P[1] ... P[n]) and F is a list of n forms that match P
    [1] through P[n], respectively; or

*   P is an improper list (P[1] P[2] ... P[n] . P[n+1]) and F is a list
    or improper list of n or more forms that match P[1] through P[n],
    respectively, and whose nth "cdr" matches P[n+1]; or

*   P is of the form (P[1] ... P[n] P[n+1] <ellipsis>) where <ellipsis>
    is the identifier ... and F is a proper list of at least n forms,
    the first n of which match P[1] through P[n], respectively, and
    each remaining element of F matches P[n+1]; or

*   P is a vector of the form #(P[1] ... P[n]) and F is a vector of n
    forms that match P[1] through P[n]; or

*   P is of the form #(P[1] ... P[n] P[n+1] <ellipsis>) where
    <ellipsis> is the identifier ... and F is a vector of n or more
    forms the first n of which match P[1] through P[n], respectively,
    and each remaining element of F matches P[n+1]; or

*   P is a datum and F is equal to P in the sense of the equal?
    procedure.

It is an error to use a macro keyword, within the scope of its binding,
in an expression that does not match any of the patterns.

When a macro use is transcribed according to the template of the
matching <syntax rule>, pattern variables that occur in the template
are replaced by the subforms they match in the input. Pattern variables
that occur in subpatterns followed by one or more instances of the
identifier ... are allowed only in subtemplates that are followed by as
many instances of .... They are replaced in the output by all of the
subforms they match in the input, distributed as indicated. It is an
error if the output cannot be built up as specified.

Identifiers that appear in the template but are not pattern variables
or the identifier ... are inserted into the output as literal
identifiers. If a literal identifier is inserted as a free identifier
then it refers to the binding of that identifier within whose scope the
instance of syntax-rules appears. If a literal identifier is inserted
as a bound identifier then it is in effect renamed to prevent
inadvertent captures of free identifiers.

As an example, if let and cond are defined as in section 7.3 then they
are hygienic (as required) and the following is not an error.

 (let ((=> #f))
   (cond (#t => 'ok)))                   ===> ok

The macro transformer for cond recognizes => as a local variable, and
hence an expression, and not as the top-level identifier =>, which the
macro transformer treats as a syntactic keyword. Thus the example
expands into

 (let ((=> #f))
   (if #t (begin => 'ok)))

instead of

 (let ((=> #f))
   (let ((temp #t))
     (if temp ('ok temp))))

which would result in an invalid procedure call.

== Program structure

== Standard procedures

This chapter describes Scheme's built-in procedures. The initial (or
"top level") Scheme environment starts out with a number of variables
bound to locations containing useful values, most of which are
primitive procedures that manipulate data. For example, the variable
abs is bound to (a location initially containing) a procedure of one
argument that computes the absolute value of a number, and the variable
+ is bound to a procedure that computes sums. Built-in procedures that
can easily be written in terms of other built-in procedures are
identified as "library procedures".

A program may use a top-level definition to bind any variable. It may
subsequently alter any such binding by an assignment (see 4.1.6). These
operations do not modify the behavior of Scheme's built-in procedures.
Altering any top-level binding that has not been introduced by a
definition has an unspecified effect on the behavior of the built-in
procedures.

=== 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?.

<procedure>(eqv? obj[1] obj[2])</procedure><br>

The eqv? procedure defines a useful equivalence relation on objects.
Briefly, it returns #t if obj[1] and obj[2] 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:

*   obj[1] and obj[2] are both #t or both #f.

*   obj[1] and obj[2] are both symbols and

    (string=? (symbol->string obj1)
              (symbol->string obj2))
                ===>  #t

Note:  This assumes that neither obj[1] nor obj[2] is an
"uninterned symbol" as alluded to in section 6.3.3. This
report does not presume to specify the behavior of eqv? on
implementation-dependent extensions.

*   obj[1] and obj[2] are both numbers, are numerically equal (see =,
    section 6.2), and are either both exact or both inexact.

*   obj[1] and obj[2] are both characters and are the same character
    according to the char=? procedure (section 6.3.4).

*   both obj[1] and obj[2] are the empty list.

*   obj[1] and obj[2] are pairs, vectors, or strings that denote the
    same locations in the store (section 3.4).

*   obj[1] and obj[2] are procedures whose location tags are equal
    (section 4.1.4).

The eqv? procedure returns #f if:

*   obj[1] and obj[2] are of different types (section 3.2).

*   one of obj[1] and obj[2] is #t but the other is #f.

*   obj[1] and obj[2] are symbols but

    (string=? (symbol->string obj[1])
              (symbol->string obj[2]))
                ===>  #f

*   one of obj[1] and obj[2] is an exact number but the other is an
    inexact number.

*   obj[1] and obj[2] are numbers for which the = procedure returns #f.

*   obj[1] and obj[2] are characters for which the char=? procedure
    returns #f.

*   one of obj[1] and obj[2] is the empty list but the other is not.

*   obj[1] and obj[2] are pairs, vectors, or strings that denote
    distinct locations.

*   obj[1] and obj[2] are procedures that would behave differently
    (return different value(s) or have different side effects) for some
    arguments.

 (eqv? 'a 'a)                             ===>  #t
 (eqv? 'a 'b)                             ===>  #f
 (eqv? 2 2)                               ===>  #t
 (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

The next set of examples shows the use of eqv? with procedures that
have local state. Gen-counter must return a distinct procedure every
time, since each procedure has its own internal counter. Gen-loser,
however, returns equivalent procedures each time, since the local state
does not affect the value or side effects of the procedures.

 (define gen-counter
   (lambda ()
     (let ((n 0))
       (lambda () (set! n (+ n 1)) n))))
 (let ((g (gen-counter)))
   (eqv? g g))                   ===>  #t
 (eqv? (gen-counter) (gen-counter))
                                 ===>  #f
 (define gen-loser
   (lambda ()
     (let ((n 0))
       (lambda () (set! n (+ n 1)) 27))))
 (let ((g (gen-loser)))
   (eqv? g g))                   ===>  #t
 (eqv? (gen-loser) (gen-loser))
                                 ===>  unspecified
 
 (letrec ((f (lambda () (if (eqv? f g) 'both 'f)))
          (g (lambda () (if (eqv? f g) 'both 'g))))
   (eqv? f g))
                                 ===>  unspecified
 
 (letrec ((f (lambda () (if (eqv? f g) 'f 'both)))
          (g (lambda () (if (eqv? f g) 'g 'both))))
   (eqv? f g))
                                 ===>  #f

Since it is an error to modify constant objects (those returned by
literal expressions), implementations are permitted, though not
required, to share structure between constants where appropriate. Thus
the value of eqv? on constants is sometimes implementation-dependent.

 (eqv? '(a) '(a))                         ===>  unspecified
 (eqv? "a" "a")                           ===>  unspecified
 (eqv? '(b) (cdr '(a b)))                 ===>  unspecified
 (let ((x '(a)))
   (eqv? x x))                            ===>  #t

Rationale:   The above definition of eqv? allows implementations
latitude in their treatment of procedures and literals:
implementations are free either to detect or to fail to detect that
two procedures or two literals are equivalent to each other, and
can decide whether or not to merge representations of equivalent
objects by using the same pointer or bit pattern to represent both.

<procedure>(eq? obj[1] obj[2])</procedure><br>

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,
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.

 (eq? 'a 'a)                             ===>  #t
 (eq? '(a) '(a))                         ===>  unspecified
 (eq? (list 'a) (list 'a))               ===>  #f
 (eq? "a" "a")                           ===>  unspecified
 (eq? "" "")                             ===>  unspecified
 (eq? '() '())                           ===>  #t
 (eq? 2 2)                               ===>  unspecified
 (eq? #\A #\A)                           ===>  unspecified
 (eq? car car)                           ===>  #t
 (let ((n (+ 2 3)))
   (eq? n n))              ===>  unspecified
 (let ((x '(a)))
   (eq? x x))              ===>  #t
 (let ((x '#()))
   (eq? x x))              ===>  #t
 (let ((p (lambda (x) x)))
   (eq? p p))              ===>  #t

Rationale:   It will usually be possible to implement eq? much more
efficiently than eqv?, for example, as a simple pointer comparison
instead of as some more complicated operation. One reason is that
it may not be possible to compute eqv? of two numbers in constant
time, whereas eq? implemented as pointer comparison will always
finish in constant time. Eq? may be used like eqv? in applications
using procedures to implement objects with state since it obeys the
same constraints as eqv?.

<procedure>(equal? obj[1] obj[2])</procedure><br>

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
 (equal? (lambda (x) x)
         (lambda (y) y))          ===>  unspecified

=== Numbers

Numerical computation has traditionally been neglected by the Lisp
community. Until Common Lisp there was no carefully thought out
strategy for organizing numerical computation, and with the exception
of the MacLisp system [20] little effort was made to execute numerical
code efficiently. This report recognizes the excellent work of the
Common Lisp committee and accepts many of their recommendations. In
some ways this report simplifies and generalizes their proposals in a
manner consistent with the purposes of Scheme.

It is important to distinguish between the mathematical numbers, the
Scheme numbers that attempt to model them, the machine representations
used to implement the Scheme numbers, and notations used to write
numbers. This report uses the types number, complex, real, rational,
and integer to refer to both mathematical numbers and Scheme numbers.
Machine representations such as fixed point and floating point are
referred to by names such as fixnum and flonum.

==== Numerical types

Mathematically, numbers may be arranged into a tower of subtypes in
which each level is a subset of the level above it:

    number
    complex
    real
    rational
    integer

For example, 3 is an integer. Therefore 3 is also a rational, a real,
and a complex. The same is true of the Scheme numbers that model 3. For
Scheme numbers, these types are defined by the predicates number?,
complex?, real?, rational?, and integer?.

There is no simple relationship between a number's type and its
representation inside a computer. Although most implementations of
Scheme will offer at least two different representations of 3, these
different representations denote the same integer.

Scheme's numerical operations treat numbers as abstract data, as
independent of their representation as possible. Although an
implementation of Scheme may use fixnum, flonum, and perhaps other
representations for numbers, this should not be apparent to a casual
programmer writing simple programs.

It is necessary, however, to distinguish between numbers that are
represented exactly and those that may not be. For example, indexes
into data structures must be known exactly, as must some polynomial
coefficients in a symbolic algebra system. On the other hand, the
results of measurements are inherently inexact, and irrational numbers
may be approximated by rational and therefore inexact approximations.
In order to catch uses of inexact numbers where exact numbers are
required, Scheme explicitly distinguishes exact from inexact numbers.
This distinction is orthogonal to the dimension of type.

==== Exactness

Scheme numbers are either exact or inexact. A number is exact if it was
written as an exact constant or was derived from exact numbers using
only exact operations. A number is inexact if it was written as an
inexact constant, if it was derived using inexact ingredients, or if it
was derived using inexact operations. Thus inexactness is a contagious
property of a number. If two implementations produce exact results for
a computation that did not involve inexact intermediate results, the
two ultimate results will be mathematically equivalent. This is
generally not true of computations involving inexact numbers since
approximate methods such as floating point arithmetic may be used, but
it is the duty of each implementation to make the result as close as
practical to the mathematically ideal result.

Rational operations such as + should always produce exact results when
given exact arguments. If the operation is unable to produce an exact
result, then it may either report the violation of an implementation
restriction or it may silently coerce its result to an inexact value.
See section 6.2.3.

With the exception of inexact->exact, the operations described in this
section must generally return inexact results when given any inexact
arguments. An operation may, however, return an exact result if it can
prove that the value of the result is unaffected by the inexactness of
its arguments. For example, multiplication of any number by an exact
zero may produce an exact zero result, even if the other argument is
inexact.

==== Implementation restrictions

Implementations of Scheme are not required to implement the whole tower
of subtypes given in section 6.2.1, but they must implement a coherent
subset consistent with both the purposes of the implementation and the
spirit of the Scheme language. For example, an implementation in which
all numbers are real may still be quite useful.

Implementations may also support only a limited range of numbers of any
type, subject to the requirements of this section. The supported range
for exact numbers of any type may be different from the supported range
for inexact numbers of that type. For example, an implementation that
uses flonums to represent all its inexact real numbers may support a
practically unbounded range of exact integers and rationals while
limiting the range of inexact reals (and therefore the range of inexact
integers and rationals) to the dynamic range of the flonum format.
Furthermore the gaps between the representable inexact integers and
rationals are likely to be very large in such an implementation as the
limits of this range are approached.

An implementation of Scheme must support exact integers throughout the
range of numbers that may be used for indexes of lists, vectors, and
strings or that may result from computing the length of a list, vector,
or string. The length, vector-length, and string-length procedures must
return an exact integer, and it is an error to use anything but an
exact integer as an index. Furthermore any integer constant within the
index range, if expressed by an exact integer syntax, will indeed be
read as an exact integer, regardless of any implementation restrictions
that may apply outside this range. Finally, the procedures listed below
will always return an exact integer result provided all their arguments
are exact integers and the mathematically expected result is
representable as an exact integer within the implementation:

 +            -             *
 quotient     remainder     modulo
 max          min           abs
 numerator    denominator   gcd
 lcm          floor         ceiling
 truncate     round         rationalize
 expt

Implementations are encouraged, but not required, to support exact
integers and exact rationals of practically unlimited size and
precision, and to implement the above procedures and the / procedure in
such a way that they always return exact results when given exact
arguments. If one of these procedures is unable to deliver an exact
result when given exact arguments, then it may either report a
violation of an implementation restriction or it may silently coerce
its result to an inexact number. Such a coercion may cause an error
later.

An implementation may use floating point and other approximate
representation strategies for inexact numbers. This report recommends,
but does not require, that the IEEE 32-bit and 64-bit floating point
standards be followed by implementations that use flonum
representations, and that implementations using other representations
should match or exceed the precision achievable using these floating
point standards [12].

In particular, implementations that use flonum representations must
follow these rules: A flonum result must be represented with at least
as much precision as is used to express any of the inexact arguments to
that operation. It is desirable (but not required) for potentially
inexact operations such as sqrt, when applied to exact arguments, to
produce exact answers whenever possible (for example the square root of
an exact 4 ought to be an exact 2). If, however, an exact number is
operated upon so as to produce an inexact result (as by sqrt), and if
the result is represented as a flonum, then the most precise flonum
format available must be used; but if the result is represented in some
other way then the representation must have at least as much precision
as the most precise flonum format available.

Although Scheme allows a variety of written notations for numbers, any
particular implementation may support only some of them. For example,
an implementation in which all numbers are real need not support the
rectangular and polar notations for complex numbers. If an
implementation encounters an exact numerical constant that it cannot
represent as an exact number, then it may either report a violation of
an implementation restriction or it may silently represent the constant
by an inexact number.

==== Syntax of numerical constants

The syntax of the written representations for numbers is described
formally in section 7.1.1. Note that case is not significant in
numerical constants.

A number may be written in binary, octal, decimal, or hexadecimal by
the use of a radix prefix. The radix prefixes are #b (binary), #o
(octal), #d (decimal), and #x (hexadecimal). With no radix prefix, a
number is assumed to be expressed in decimal.

A numerical constant may be specified to be either exact or inexact by
a prefix. The prefixes are #e for exact, and #i for inexact. An
exactness prefix may appear before or after any radix prefix that is
used. If the written representation of a number has no exactness
prefix, the constant may be either inexact or exact. It is inexact if
it contains a decimal point, an exponent, or a "#" character in the
place of a digit, otherwise it is exact. In systems with inexact
numbers of varying precisions it may be useful to specify the precision
of a constant. For this purpose, numerical constants may be written
with an exponent marker that indicates the desired precision of the
inexact representation. The letters s, f, d, and l specify the use of
short, single, double, and long precision, respectively. (When fewer
than four internal inexact representations exist, the four size
specifications are mapped onto those available. For example, an
implementation with two internal representations may map short and
single together and long and double together.) In addition, the
exponent marker e specifies the default precision for the
implementation. The default precision has at least as much precision as
double, but implementations may wish to allow this default to be set by
the user.

 3.14159265358979F0
         Round to single --- 3.141593
 0.6L0
         Extend to long --- .600000000000000

==== Numerical operations

The reader is referred to section 1.3.3 for a summary of the naming
conventions used to specify restrictions on the types of arguments to
numerical routines. The examples used in this section assume that any
numerical constant written using an exact notation is indeed
represented as an exact number. Some examples also assume that certain
numerical constants written using an inexact notation can be
represented without loss of accuracy; the inexact constants were chosen
so that this is likely to be true in implementations that use flonums
to represent inexact numbers.

<procedure>(number? obj)</procedure><br>
<procedure>(complex? obj)</procedure><br>
<procedure>(real? obj)</procedure><br>
<procedure>(rational? obj)</procedure><br>
<procedure>(integer? obj)</procedure><br>

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 (= 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? 8/4)                  ===>  #t

Note:   The behavior of these type predicates on inexact numbers is
unreliable, since any inaccuracy may affect the result.

Note:   In many implementations the rational? procedure will be the
same as real?, and the complex? procedure will be the same as
number?, but unusual implementations may be able to represent some
irrational numbers exactly or may extend the number system to
support some kind of non-complex numbers.

<procedure>(exact? z)</procedure><br>
<procedure>(inexact? z)</procedure><br>

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

<procedure>(= z[1] z[2] z[3] ...)</procedure><br>
<procedure>(< x[1] x[2] x[3] ...)</procedure><br>
<procedure>(> x[1] x[2] x[3] ...)</procedure><br>
<procedure>(<= x[1] x[2] x[3] ...)</procedure><br>
<procedure>(>= x[1] x[2] x[3] ...)</procedure><br>

These procedures return #t if their arguments are (respectively):
equal, monotonically increasing, monotonically decreasing,
monotonically nondecreasing, or monotonically nonincreasing.

These predicates are required to be transitive.

Note:   The traditional implementations of these predicates in
Lisp-like languages are not transitive.

Note:   While it is not an error to compare inexact numbers using
these predicates, the results may be unreliable because a small
inaccuracy may affect the result; this is especially true of = and
zero?. When in doubt, consult a numerical analyst.

<procedure>(zero? z)</procedure><br>
<procedure>(positive? x)</procedure><br>
<procedure>(negative? x)</procedure><br>
<procedure>(odd? n)</procedure><br>
<procedure>(even? n)</procedure><br>

These numerical predicates test a number for a particular property,
returning #t or #f. See note above.

<procedure>(max x[1] x[2] ...)</procedure><br>
<procedure>(min x[1] x[2] ...)</procedure><br>

These procedures return the maximum or minimum of their arguments.

 (max 3 4)                      ===>  4    ; exact
 (max 3.9 4)                    ===>  4.0  ; inexact

Note:   If any argument is inexact, then the result will also be
inexact (unless the procedure can prove that the inaccuracy is not
large enough to affect the result, which is possible only in
unusual implementations). If min or max is used to compare numbers
of mixed exactness, and the numerical value of the result cannot be
represented as an inexact number without loss of accuracy, then the
procedure may report a violation of an implementation restriction.

<procedure>(+ z[1] ...)</procedure><br>
<procedure>(* z[1] ...)</procedure><br>

These procedures return the sum or product of their arguments.

 (+ 3 4)                         ===>  7
 (+ 3)                           ===>  3
 (+)                             ===>  0
 (* 4)                           ===>  4
 (*)                             ===>  1

<procedure>(- z[1] z[2])</procedure><br>
<procedure>(- z)</procedure><br>
<procedure>(- z[1] z[2] ...)</procedure><br>
<procedure>(/ z[1] z[2])</procedure><br>
<procedure>(/ z)</procedure><br>
<procedure>(/ z[1] z[2] ...)</procedure><br>

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
 (/ 3 4 5)                       ===>  3/20
 (/ 3)                           ===>  1/3

<procedure>(abs x)</procedure><br>

Abs returns the absolute value of its argument.

 (abs -7)                        ===>  7

<procedure>(quotient n[1] n[2])</procedure><br>
<procedure>(remainder n[1] n[2])</procedure><br>
<procedure>(modulo n[1] n[2])</procedure><br>

These procedures implement number-theoretic (integer) division. n[2]
should be non-zero. All three procedures return integers. If n[1]/n[2]
is an integer:

    (quotient n[1] n[2])           ===> n[1]/n[2]
    (remainder n[1] n[2])          ===> 0
    (modulo n[1] n[2])             ===> 0

If n[1]/n[2] is not an integer:

    (quotient n[1] n[2])           ===> n[q]
    (remainder n[1] n[2])          ===> n[r]
    (modulo n[1] n[2])             ===> n[m]

where n[q] is n[1]/n[2] rounded towards zero, 0 < |n[r]| < |n[2]|, 0 <
|n[m]| < |n[2]|, n[r] and n[m] differ from n[1] by a multiple of n[2],
n[r] has the same sign as n[1], and n[m] has the same sign as n[2].

From this we can conclude that for integers n[1] and n[2] with n[2] not
equal to 0,

     (= n[1] (+ (* n[2] (quotient n[1] n[2]))
           (remainder n[1] n[2])))
                                         ===>  #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

<procedure>(gcd n[1] ...)</procedure><br>
<procedure>(lcm n[1] ...)</procedure><br>

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

<procedure>(numerator q)</procedure><br>
<procedure>(denominator q)</procedure><br>

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

<procedure>(floor x)</procedure><br>
<procedure>(ceiling x)</procedure><br>
<procedure>(truncate x)</procedure><br>
<procedure>(round x)</procedure><br>

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

<procedure>(rationalize x y)</procedure><br>

Rationalize returns the simplest rational number differing from x by no
more than y. A rational number r[1] is simpler than another rational
number r[2] if r[1] = p[1]/q[1] and r[2] = p[2]/q[2] (in lowest terms)
and |p[1]| < |p[2]| and |q[1]| < |q[2]|. 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

<procedure>(exp z)</procedure><br>
<procedure>(log z)</procedure><br>
<procedure>(sin z)</procedure><br>
<procedure>(cos z)</procedure><br>
<procedure>(tan z)</procedure><br>
<procedure>(asin z)</procedure><br>
<procedure>(acos z)</procedure><br>
<procedure>(atan z)</procedure><br>
<procedure>(atan y x)</procedure><br>

These procedures are part of every implementation that supports general
real numbers; they compute the usual transcendental functions. Log
computes the natural logarithm of z (not the base ten logarithm). Asin,
acos, and atan compute arcsine (sin^-1), arccosine (cos^-1), and
arctangent (tan^-1), respectively. The two-argument variant of atan
computes (angle (make-rectangular x y)) (see below), even in
implementations that don't support general complex numbers.

In general, the mathematical functions log, arcsine, arccosine, and
arctangent are multiply defined. The value of log z is defined to be
the one whose imaginary part lies in the range from -pi
(exclusive) to pi (inclusive). log 0 is undefined. With log
defined this way, the values of sin^-1 z, cos^-1 z, and tan^-1 z are
according to the following formulae:

 sin^-1 z = - i log (i z + (1 - z^2)^1/2)
 
 cos^-1 z = pi / 2 - sin^-1 z
 
 tan^-1 z = (log (1 + i z) - log (1 - i z)) / (2 i)

The above specification follows [27], which in turn cites [19]; refer
to these sources for more detailed discussion of branch cuts, boundary
conditions, and implementation of these functions. When it is possible
these procedures produce a real result from a real argument.

<procedure>(sqrt z)</procedure><br>

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

<procedure>(expt z[1] z[2])</procedure><br>

Returns z[1] raised to the power z[2]. For z[1] != 0

 z[1]^z[2] = e^z[2] log z[1]

0^z is 1 if z = 0 and 0 otherwise.

<procedure>(make-rectangular x[1] x[2])</procedure><br>
<procedure>(make-polar x[3] x[4])</procedure><br>
<procedure>(real-part z)</procedure><br>
<procedure>(imag-part z)</procedure><br>
<procedure>(magnitude z)</procedure><br>
<procedure>(angle z)</procedure><br>

These procedures are part of every implementation that supports general
complex numbers. Suppose x[1], x[2], x[3], and x[4] are real numbers
and z is a complex number such that

 z = x[1] + x[2]i = x[3] . e^i x[4]

Then

 (make-rectangular x[1] x[2])         ===> z
 (make-polar x[3] x[4])             ===> z
 (real-part z)                          ===> x[1]
 (imag-part z)                          ===> x[2]
 (magnitude z)                          ===> |x[3]|
 (angle z)                              ===> x[angle]

where - pi < x[angle] < pi with x[angle] = x[4] + 2 pi n
for some integer n.

Rationale:   Magnitude is the same as abs for a real argument, but
abs must be present in all implementations, whereas magnitude need
only be present in implementations that support general complex
numbers.

<procedure>(exact->inexact z)</procedure><br>
<procedure>(inexact->exact z)</procedure><br>

Exact->inexact returns an inexact representation of z. The value
returned is the inexact number that is numerically closest to the
argument. If an exact argument has no reasonably close inexact
equivalent, then a violation of an implementation restriction may be
reported.

Inexact->exact returns an exact representation of z. The value returned
is the exact number that is numerically closest to the argument. If an
inexact argument has no reasonably close exact equivalent, then a
violation of an implementation restriction may be reported.

These procedures implement the natural one-to-one correspondence
between exact and inexact integers throughout an
implementation-dependent range. See section 6.2.3.

==== Numerical input and output

<procedure>(number->string z)</procedure><br>
<procedure>(number->string z radix)</procedure><br>

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 [3, 5]; 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.

<procedure>(string->number string)</procedure><br>
<procedure>(string->number string radix)</procedure><br>

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

Note:   The domain of string->number may be restricted by
implementations in the following ways. String->number is permitted
to return #f whenever string contains an explicit radix prefix. If
all numbers supported by an implementation are real, then string->
number is permitted to return #f whenever string uses the polar or
rectangular notations for complex numbers. If all numbers are
integers, then string->number may return #f whenever the fractional
notation is used. If all numbers are exact, then string->number may
return #f whenever an exponent marker or explicit exactness prefix
is used, or if a # appears in place of a digit. If all inexact
numbers are integers, then string->number may return #f whenever a
decimal point is used.

=== Other data types

This section describes operations on some of Scheme's non-numeric data
types: booleans, pairs, lists, symbols, characters, strings and
vectors.

==== Booleans

The standard boolean objects for true and false are written as #t and #f.
What really matters, though, are the objects that the Scheme
conditional expressions (if, cond, and, or, do) treat as true or false.
The phrase "a true value" (or sometimes just "true") means any
object treated as true by the conditional expressions, and the phrase
"a false value" (or "false") means any object treated as false by
the conditional expressions.

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.

Note:   Programmers accustomed to other dialects of Lisp should be
aware that Scheme distinguishes both #f and the empty list from the
symbol nil.

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

 #t                ===>  #t
 #f                ===>  #f
 '#f               ===>  #f

<procedure>(not obj)</procedure><br>

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

<procedure>(boolean? obj)</procedure><br>

Boolean? returns #t if obj is either #t or #f and returns #f otherwise.

 (boolean? #f)                 ===>  #t
 (boolean? 0)                  ===>  #f
 (boolean? '())                ===>  #f

==== Pairs and lists

A pair (sometimes called a dotted pair) is a record structure with two
fields called the car and cdr fields (for historical reasons). Pairs
are created by the procedure cons. The car and cdr fields are accessed
by the procedures car and cdr. The car and cdr fields are assigned by
the procedures set-car! and set-cdr!.

Pairs are used primarily to represent lists. A list can be defined
recursively as either the empty list or a pair whose cdr is a list.
More precisely, the set of lists is defined as the smallest set X such
that

*   The empty list is in X.
*   If list is in X, then any pair whose cdr field contains list is
    also in X.

The objects in the car fields of successive pairs of a list are the
elements of the list. For example, a two-element list is a pair whose
car is the first element and whose cdr is a pair whose car is the
second element and whose cdr is the empty list. The length of a list is
the number of elements, which is the same as the number of pairs.

The empty list is a special object of its own type (it is not a pair);
it has no elements and its length is zero.

Note:   The above definitions imply that all lists have finite
length and are terminated by the empty list.

The most general notation (external representation) for Scheme pairs is
the "dotted" notation (c[1] . c[2]) where c[1] is the value of the
car field and c[2] is the value of the cdr field. For example (4 . 5)
is a pair whose car is 4 and whose cdr is 5. Note that (4 . 5) is the
external representation of a pair, not an expression that evaluates to
a pair.

A more streamlined notation can be used for lists: the elements of the
list are simply enclosed in parentheses and separated by spaces. The
empty list is written () . For example,

 (a b c d e)

and

 (a . (b . (c . (d . (e . ())))))

are equivalent notations for a list of symbols.

A chain of pairs not ending in the empty list is called an improper
list. Note that an improper list is not a list. The list and dotted
notations can be combined to represent improper lists:

 (a b c . d)

is equivalent to

 (a . (b . (c . d)))

Whether a given pair is a list depends upon what is stored in the cdr
field. When the set-cdr! procedure is used, an object can be a list one
moment and not the next:

 (define x (list 'a 'b 'c))
 (define y x)
 y                               ===>  (a b c)
 (list? y)                       ===>  #t
 (set-cdr! x 4)                  ===>  unspecified
 x                               ===>  (a . 4)
 (eqv? x y)                      ===>  #t
 y                               ===>  (a . 4)
 (list? y)                       ===>  #f
 (set-cdr! x x)                  ===>  unspecified
 (list? x)                       ===>  #f

Within literal expressions and representations of objects read by the
read procedure, the forms '<datum>, `<datum>, ,<datum>, and ,@<datum>
denote two-element lists whose first elements are the symbols quote,
quasiquote, unquote, and unquote-splicing, respectively. The second
element in each case is <datum>. This convention is supported so that
arbitrary Scheme programs may be represented as lists. That is,
according to Scheme's grammar, every <expression> is also a <datum>
(see section 7.1.2). Among other things, this permits the use of the
read procedure to parse Scheme programs. See section 3.3.

<procedure>(pair? obj)</procedure><br>

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

 (pair? '(a . b))                ===>  #t
 (pair? '(a b c))                ===>  #t
 (pair? '())                     ===>  #f
 (pair? '#(a b))                 ===>  #f

<procedure>(cons obj[1] obj[2])</procedure><br>

Returns a newly allocated pair whose car is obj[1] and whose cdr is
obj[2]. 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)

<procedure>(car pair)</procedure><br>

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

<procedure>(cdr pair)</procedure><br>

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

<procedure>(set-car! pair obj)</procedure><br>

Stores obj in the car field of pair. The value returned by set-car! is
unspecified.

 (define (f) (list 'not-a-constant-list))
 (define (g) '(constant-list))
 (set-car! (f) 3)                     ===>  unspecified
 (set-car! (g) 3)                     ===>  error

<procedure>(set-cdr! pair obj)</procedure><br>

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

<procedure>(caar pair)</procedure><br>
<procedure>(cadr pair)</procedure><br>
<procedure>(cdddar pair)</procedure><br>
<procedure>(cddddr pair)</procedure><br>

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.

<procedure>(null? obj)</procedure><br>

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

<procedure>(list? obj)</procedure><br>

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

<procedure>(list obj ...)</procedure><br>

Returns a newly allocated list of its arguments.

 (list 'a (+ 3 4) 'c)                    ===>  (a 7 c)
 (list)                                  ===>  ()

<procedure>(length list)</procedure><br>

Returns the length of list.

 (length '(a b c))                       ===>  3
 (length '(a (b) (c d e)))               ===>  3
 (length '())                            ===>  0

<procedure>(append list ...)</procedure><br>

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

<procedure>(reverse list)</procedure><br>

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)

<procedure>(list-tail list k)</procedure><br>

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)))))

<procedure>(list-ref list k)</procedure><br>

Returns the kth 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

<procedure>(memq obj list)</procedure><br>
<procedure>(memv obj list)</procedure><br>
<procedure>(member obj list)</procedure><br>

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)
 (memq 101 '(100 101 102))               ===>  unspecified
 (memv 101 '(100 101 102))               ===>  (101 102)

<procedure>(assq obj alist)</procedure><br>
<procedure>(assv obj alist)</procedure><br>
<procedure>(assoc obj alist)</procedure><br>

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))
 (assq 5 '((2 3) (5 7) (11 13)))    
                                    ===>  unspecified
 (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.

==== Symbols

Symbols are objects whose usefulness rests on the fact that two symbols
are identical (in the sense of eqv?) if and only if their names are
spelled the same way. This is exactly the property needed to represent
identifiers in programs, and so most implementations of Scheme use them
internally for that purpose. Symbols are useful for many other
applications; for instance, they may be used the way enumerated values
are used in Pascal.

The rules for writing a symbol are exactly the same as the rules for
writing an identifier; see sections 2.1 and 7.1.1.

It is guaranteed that any symbol that has been returned as part of a
literal expression, or read using the read procedure, and subsequently
written out using the write procedure, will read back in as the
identical symbol (in the sense of eqv?). The string->symbol procedure,
however, can create symbols for which this write/read invariance may
not hold because their names contain special characters or letters in
the non-standard case.

Note:   Some implementations of Scheme have a feature known as
"slashification" in order to guarantee write/read invariance for
all symbols, but historically the most important use of this
feature has been to compensate for the lack of a string data type.

Some implementations also have "uninterned symbols", which defeat
write/read invariance even in implementations with slashification,
and also generate exceptions to the rule that two symbols are the
same if and only if their names are spelled the same.

<procedure>(symbol? obj)</procedure><br>

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

<procedure>(symbol->string symbol)</procedure><br>

Returns the name of symbol as a string. If the symbol was part of an
object returned as the value of a literal expression (section 4.1.2) 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 -- some implementations will
prefer upper case, others 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.

The following examples assume that the implementation's standard case
is lower case:

 (symbol->string 'flying-fish)     
                                           ===>  "flying-fish"
 (symbol->string 'Martin)                  ===>  "martin"
 (symbol->string
    (string->symbol "Malvina"))     
                                           ===>  "Malvina"

<procedure>(string->symbol string)</procedure><br>

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. See symbol->string.

The following examples assume that the implementation's standard case
is lower case:

 (eq? 'mISSISSIppi 'mississippi)  
                 ===>  #t
 (string->symbol "mISSISSIppi")  
                 ===>  the symbol with name "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

==== Characters

Characters are objects that represent printed characters such as
letters and digits. Characters are written using the notation #\
<character> or #\<character name>. For example:

 #\a       ; lower case letter
 #\A       ; upper case letter
 #\(       ; left parenthesis
 #\        ; the space character
 #\space   ; the preferred way to write a space
 #\newline ; the newline character

Case is significant in #\<character>, but not in #\<character name>. If
<character> in #\<character> is alphabetic, then the character
following <character> must be a delimiter character such as a space or
parenthesis. This rule resolves the ambiguous case where, for example,
the sequence of characters "#\space" could be taken to be either a
representation of the space character or a representation of the
character "#\s" followed by a representation of the symbol "pace."

Characters written in the #\ notation are self-evaluating. That is,
they do not have to be quoted in programs. Some of the procedures that
operate on characters ignore the difference between upper case and
lower case. The procedures that ignore case have "-ci" (for "case
insensitive") embedded in their names.

<procedure>(char? obj)</procedure><br>

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

<procedure>(char=? char[1] char[2])</procedure><br>
<procedure>(char<? char[1] char[2])</procedure><br>
<procedure>(char>? char[1] char[2])</procedure><br>
<procedure>(char<=? char[1] char[2])</procedure><br>
<procedure>(char>=? char[1] char[2])</procedure><br>

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. For example, (char<? #\A #\
    B) returns #t.
*   The lower case characters are in order. For example, (char<? #\a #\
    b) returns #t.
*   The digits are in order. For example, (char<? #\0 #\9) returns #t.
*   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.

Some implementations may generalize these procedures to take more than
two arguments, as with the corresponding numerical predicates.

<procedure>(char-ci=? char[1] char[2])</procedure><br>
<procedure>(char-ci<? char[1] char[2])</procedure><br>
<procedure>(char-ci>? char[1] char[2])</procedure><br>
<procedure>(char-ci<=? char[1] char[2])</procedure><br>
<procedure>(char-ci>=? char[1] char[2])</procedure><br>

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. Some implementations may generalize these procedures to
take more than two arguments, as with the corresponding numerical
predicates.

<procedure>(char-alphabetic? char)</procedure><br>
<procedure>(char-numeric? char)</procedure><br>
<procedure>(char-whitespace? char)</procedure><br>
<procedure>(char-upper-case? letter)</procedure><br>
<procedure>(char-lower-case? letter)</procedure><br>

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.

<procedure>(char->integer char)</procedure><br>
<procedure>(integer->char n)</procedure><br>

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

<procedure>(char-upcase char)</procedure><br>
<procedure>(char-downcase char)</procedure><br>

These procedures return a character char[2] such that (char-ci=? char
char[2]). In addition, if char is alphabetic, then the result of
char-upcase is upper case and the result of char-downcase is lower
case.

==== Strings

Strings are sequences of characters. Strings are written as sequences
of characters enclosed within doublequotes ("). A doublequote can be
written inside a string only by escaping it with a backslash (\), as in

"The word \"recursion\" has many meanings."

A backslash can be written inside a string only by escaping it with
another backslash. Scheme does not specify the effect of a backslash
within a string that is not followed by a doublequote or backslash.

A string constant may continue from one line to the next, but the exact
contents of such a string are unspecified. The length of a string is
the number of characters that it contains. This number is an exact,
non-negative integer that is fixed when the string is created. The 
valid indexes of a string are the exact non-negative integers less than
the length of the string. The first character of a string has index 0,
the second has index 1, and so on.

In phrases such as "the characters of string beginning with index
start and ending with index end," it is understood that the index
start is inclusive and the index end is exclusive. Thus if start and
end are the same index, a null substring is referred to, and if start
is zero and end is the length of string, then the entire string is
referred to.

Some of the procedures that operate on strings ignore the difference
between upper and lower case. The versions that ignore case have
"-ci" (for "case insensitive") embedded in their names.

<procedure>(string? obj)</procedure><br>

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

<procedure>(make-string k)</procedure><br>
<procedure>(make-string k char)</procedure><br>

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.

<procedure>(string char ...)</procedure><br>

Returns a newly allocated string composed of the arguments.

<procedure>(string-length string)</procedure><br>

Returns the number of characters in the given string.

<procedure>(string-ref string k)</procedure><br>

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

<procedure>(string-set! string k char)</procedure><br>

k must be a valid index of string. String-set! stores char in element k
of string and returns an unspecified value.

 (define (f) (make-string 3 #\*))
 (define (g) "***")
 (string-set! (f) 0 #\?)          ===>  unspecified
 (string-set! (g) 0 #\?)          ===>  error
 (string-set! (symbol->string 'immutable)
              0
              #\?)          ===>  error

<procedure>(string=? string[1] string[2])</procedure><br>
<procedure>(string-ci=? string[1] string[2])</procedure><br>

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.

<procedure>(string<? string[1] string[2])</procedure><br>
<procedure>(string>? string[1] string[2])</procedure><br>
<procedure>(string<=? string[1] string[2])</procedure><br>
<procedure>(string>=? string[1] string[2])</procedure><br>
<procedure>(string-ci<? string[1] string[2])</procedure><br>
<procedure>(string-ci>? string[1] string[2])</procedure><br>
<procedure>(string-ci<=? string[1] string[2])</procedure><br>
<procedure>(string-ci>=? string[1] string[2])</procedure><br>

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.

Implementations may generalize these and the string=? and string-ci=?
procedures to take more than two arguments, as with the corresponding
numerical predicates.

<procedure>(substring string start end)</procedure><br>

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).

<procedure>(string-append string ...)</procedure><br>

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

<procedure>(string->list string)</procedure><br>
<procedure>(list->string list)</procedure><br>

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.

<procedure>(string-copy string)</procedure><br>

Returns a newly allocated copy of the given string.

<procedure>(string-fill! string char)</procedure><br>

Stores char in every element of the given string and returns an
unspecified value.

==== 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 that this is the external representation of a vector, not an
expression evaluating to a vector. Like list constants, vector
constants must be quoted:

 '#(0 (2 2 2 2) "Anna")  
                 ===>  #(0 (2 2 2 2) "Anna")

<procedure>(vector? obj)</procedure><br>

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

<procedure>(make-vector k)</procedure><br>
<procedure>(make-vector k fill)</procedure><br>

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.

<procedure>(vector obj ...)</procedure><br>

Returns a newly allocated vector whose elements contain the given
arguments. Analogous to list.

 (vector 'a 'b 'c)                       ===>  #(a b c)

<procedure>(vector-length vector)</procedure><br>

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

<procedure>(vector-ref vector k)</procedure><br>

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

<procedure>(vector-set! vector k obj)</procedure><br>

k must be a valid index of vector. Vector-set! stores obj in element k
of vector. The value returned by vector-set! is unspecified.

 (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

<procedure>(vector->list vector)</procedure><br>
<procedure>(list->vector list)</procedure><br>

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)

<procedure>(vector-fill! vector fill)</procedure><br>

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

=== Control features

This chapter describes various primitive procedures which control the
flow of program execution in special ways. The procedure? predicate is
also described here.

<procedure>(procedure? obj)</procedure><br>

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

<procedure>(apply proc arg[1] ... args)</procedure><br>

Proc must be a procedure and args must be a list. Calls proc with the
elements of the list (append (list arg[1] ...) 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

<procedure>(map proc list[1] list[2] ...)</procedure><br>

The lists 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 lists 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)

<procedure>(for-each proc list[1] list[2] ...)</procedure><br>

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 unspecified.

 (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)

<procedure>(force promise)</procedure><br>

Forces the value of promise (see delay, section 4.2.5). 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

Here is a possible implementation of delay and force. Promises are
implemented here as procedures of no arguments, and force simply calls
its argument:

 (define force
   (lambda (object)
     (object)))

We define the expression

 (delay <expression>)

to have the same meaning as the procedure call

 (make-promise (lambda () <expression>))

as follows

 (define-syntax delay
   (syntax-rules ()
     ((delay expression)
      (make-promise (lambda () expression))))),

where make-promise is defined as follows:

 (define make-promise
   (lambda (proc)
     (let ((result-ready? #f)
           (result #f))
       (lambda ()
         (if result-ready?
             result
             (let ((x (proc)))
               (if result-ready?
                   result
                   (begin (set! result-ready? #t)
                          (set! result x)
                          result))))))))

Rationale:   A promise may refer to its own value, as in the last
example above. Forcing such a promise may cause the promise to be
forced a second time before the value of the first force has been
computed. This complicates the definition of make-promise.

Various extensions to this semantics of delay and force are supported
in some implementations:

*   Calling force on an object that is not a promise may simply return
    the object.

*   It may be the case that there is no means by which a promise can be
    operationally distinguished from its forced value. That is,
    expressions like the following may evaluate to either #t or to #f,
    depending on the implementation:

    (eqv? (delay 1) 1)                  ===>  unspecified
    (pair? (delay (cons 1 2)))          ===>  unspecified

*   Some implementations may implement "implicit forcing," where the
    value of a promise is forced by primitive procedures like cdr and
    +:

    (+ (delay (* 3 7)) 13)          ===>  34

<procedure>(call-with-current-continuation proc)</procedure><br>

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 effect of passing no value or
more than one value to continuations that were not created by
call-with-values is unspecified.

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.

Most programming languages incorporate one or more special-purpose
escape constructs with names like exit, return, or even goto. In
1965, however, Peter Landin [16] invented a general purpose escape
operator called the J-operator. John Reynolds [24] described a
simpler but equally powerful construct in 1972. The catch special
form described by Sussman and Steele in the 1975 report on Scheme
is exactly the same as Reynolds's construct, though its name came
from a less general construct in MacLisp. Several Scheme
implementors noticed that the full power of the catch construct
could be provided by a procedure instead of by a special syntactic
construct, and the name call-with-current-continuation was coined
in 1982. This name is descriptive, but opinions differ on the
merits of such a long name, and some people use the name call/cc
instead.

<procedure>(values obj ...)</procedure><br>

Delivers all of its arguments to its continuation. Except for
continuations created by the call-with-values procedure, all
continuations take exactly one value. Values might be defined as
follows:

 (define (values . things)
   (call-with-current-continuation 
     (lambda (cont) (apply cont things))))

<procedure>(call-with-values producer consumer)</procedure><br>

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

<procedure>(dynamic-wind before thunk after)</procedure><br>

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

<procedure>(eval expression environment-specifier)</procedure><br>

Evaluates expression in the specified environment and returns its
value. Expression must be a valid Scheme expression represented as
data, and environment-specifier must be a value returned by one of the
three procedures described below. Implementations may extend eval to
allow non-expression programs (definitions) as the first argument and
to allow other values as environments, with the restriction that eval
is not allowed to create new bindings in the environments associated
with null-environment or scheme-report-environment.

 (eval '(* 7 3) (scheme-report-environment 5))
                                                            ===>  21
 
 (let ((f (eval '(lambda (f x) (f x x))
                (null-environment 5))))
   (f + 10))
                                                            ===>  20

<procedure>(scheme-report-environment version)</procedure><br>
<procedure>(null-environment version)</procedure><br>

Version must be the exact integer 5, corresponding to this revision of
the Scheme report (the Revised^5 Report on Scheme).
Scheme-report-environment returns a specifier for an environment that
is empty except for all bindings defined in this report that are either
required or both optional and supported by the implementation.
Null-environment returns a specifier for an environment that is empty
except for the (syntactic) bindings for all syntactic keywords defined
in this report that are either required or both optional and supported
by the implementation.

Other values of version can be used to specify environments matching
past revisions of this report, but their support is not required. An
implementation will signal an error if version is neither 5 nor another
value supported by the implementation.

The effect of assigning (through the use of eval) a variable bound in a
scheme-report-environment (for example car) is unspecified. Thus the
environments specified by scheme-report-environment may be immutable.

<procedure>(interaction-environment)</procedure><br>

This procedure returns a specifier for the environment that contains
implementation-defined bindings, typically a superset of those listed
in the report. The intent is that this procedure will return the
environment in which the implementation would evaluate expressions
dynamically typed by the user.

=== Input and output

==== Ports

Ports represent input and output devices. To Scheme, an input port is a
Scheme object that can deliver characters upon command, while an output
port is a Scheme object that can accept characters.

<procedure>(call-with-input-file string proc)</procedure><br>
<procedure>(call-with-output-file string proc)</procedure><br>

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; for call-with-output-file, the effect is unspecified if
the file already exists. 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 signalled. 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 unless it is possible to prove that the port will
never again be used for a read or write operation.

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.

<procedure>(input-port? obj)</procedure><br>
<procedure>(output-port? obj)</procedure><br>

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

<procedure>(current-input-port)</procedure><br>
<procedure>(current-output-port)</procedure><br>

Returns the current default input or output port.

<procedure>(with-input-from-file string thunk)</procedure><br>
<procedure>(with-output-to-file string thunk)</procedure><br>

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; for with-output-to-file, the effect is unspecified if the file
already exists. 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. If an escape procedure is used to escape
from the continuation of these procedures, their behavior is
implementation dependent.

<procedure>(open-input-file filename)</procedure><br>

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.

<procedure>(open-output-file filename)</procedure><br>

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, the effect is unspecified.

<procedure>(close-input-port port)</procedure><br>
<procedure>(close-output-port port)</procedure><br>

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

==== Input

<procedure>(read)</procedure><br>
<procedure>(read port)</procedure><br>

Read converts external representations of Scheme objects into the
objects themselves. That is, it is a parser for the nonterminal <datum>
(see sections 7.1.2 and 6.3.2). 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.

<procedure>(read-char)</procedure><br>
<procedure>(read-char port)</procedure><br>

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.

<procedure>(peek-char)</procedure><br>
<procedure>(peek-char port)</procedure><br>

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.

<procedure>(eof-object? obj)</procedure><br>

Returns #t if obj is an end of file object, otherwise returns #f. The
precise set of end of file objects will vary among implementations, but
in any case no end of file object will ever be an object that can be
read in using read.

<procedure>(char-ready?)</procedure><br>
<procedure>(char-ready? port)</procedure><br>

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.

Rationale:   Char-ready? exists to make it possible for a program
to accept characters from interactive ports without getting stuck
waiting for input. Any input editors associated with such ports
must ensure that characters whose existence has been asserted by
char-ready? cannot be rubbed out. If char-ready? were to return #f
at end of file, a port at end of file would be indistinguishable
from an interactive port that has no ready characters.

==== Output

<procedure>(write obj)</procedure><br>
<procedure>(write obj port)</procedure><br>

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.

<procedure>(display obj)</procedure><br>
<procedure>(display obj port)</procedure><br>

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.
Implementations that allow "slashification" within symbols will
probably want write but not display to slashify funny characters in
symbols.

<procedure>(newline)</procedure><br>
<procedure>(newline port)</procedure><br>

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.

<procedure>(write-char char)</procedure><br>
<procedure>(write-char char port)</procedure><br>

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.

==== System interface

Questions of system interface generally fall outside of the domain of
this report. However, the following operations are important enough to
deserve description here.

<procedure>(load filename)</procedure><br>

Filename should be a string naming an existing file containing Scheme
source code. The load procedure reads expressions and definitions from
the file and evaluates them sequentially. It is unspecified whether the
results of the expressions are printed. The load procedure does not
affect the values returned by current-input-port and
current-output-port. Load returns an unspecified value.

Rationale:   For portability, load must operate on source files.
Its operation on other kinds of files necessarily varies among
implementations.

<procedure>(transcript-on filename)</procedure><br>
<procedure>(transcript-off)</procedure><br>

(These procedures are not implemented in Chicken.)

Filename must be a string naming an output file to be created. The
effect of transcript-on is to open the named file for output, and to
cause a transcript of subsequent interaction between the user and the
Scheme system to be written to the file. The transcript is ended by a
call to transcript-off, which closes the transcript file. Only one
transcript may be in progress at any time, though some implementations
may relax this restriction. The values returned by these procedures are
unspecified.


---
Previous: [[Supported language]]

Next: [[Deviations from the standard]]
