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Non-standard macros and special forms

Making extra libraries and extensions available

require-library

[syntax] (require-library ID ...)

This form does all the necessary steps to make the libraries or extensions given in ID ... available. It loads syntactic extensions, if needed and generates code for loading/linking with core library modules or separately installed extensions.

During interpretation/evaluation require-library performs one of the following:

During compilation, one of the following happens instead:

To make long matters short - just use require-library and it will normally figure everything out for dynamically loadable extensions and core library units.

ID should be a pure extension name and should not contain any path prefixes (for example dir/lib...) is illegal).

ID may also be a list that designates an extension-specifier. Currently the following extension specifiers are defined:

See also: set-extension-specifier!

require-extension

[syntax] (require-extension ID ...)

This is equivalent to (require-library ID ...) but performs an implicit import, if necessary. ID may also be an import specification (using rename, only, except or prefix).

This implementation of require-extension is compliant with SRFI-55 (see the SRFI-55 document for more information).

use

[syntax] (use ID ...)

use is just a shorter alias for require-extension.

Binding forms for optional arguments

optional

[syntax] (optional ARGS DEFAULT)

Use this form for procedures that take a single optional argument. If ARGS is the empty list DEFAULT is evaluated and returned, otherwise the first element of the list ARGS. It is an error if ARGS contains more than one value.

(define (incr x . i) (+ x (optional i 1)))
(incr 10)                                   ==> 11
(incr 12 5)                                 ==> 17

case-lambda

[syntax] (case-lambda (LAMBDA-LIST1 EXP1 ...) ...)

Expands into a lambda that invokes the body following the first matching lambda-list.

(define plus
  (case-lambda 
    (() 0)
    ((x) x)
    ((x y) (+ x y))
    ((x y z) (+ (+ x y) z))
    (args (apply + args))))

(plus)                      ==> 0
(plus 1)                    ==> 1
(plus 1 2 3)                ==> 6

For more information see the documentation for SRFI-16

let-optionals

[syntax] (let-optionals ARGS ((VAR1 DEFAULT1) ...) BODY ...)

Binding constructs for optional procedure arguments. ARGS should be a rest-parameter taken from a lambda-list. let-optionals binds VAR1 ... to available arguments in parallel, or to DEFAULT1 ... if not enough arguments were provided. let-optionals* binds VAR1 ... sequentially, so every variable sees the previous ones. it is an error if any excess arguments are provided.

(let-optionals '(one two) ((a 1) (b 2) (c 3))
  (list a b c) )                               ==> (one two 3)

let-optionals*

[syntax] (let-optionals* ARGS ((VAR1 DEFAULT1) ... [RESTVAR]) BODY ...)

Binding constructs for optional procedure arguments. ARGS should be a rest-parameter taken from a lambda-list. let-optionals binds VAR1 ... to available arguments in parallel, or to DEFAULT1 ... if not enough arguments were provided. let-optionals* binds VAR1 ... sequentially, so every variable sees the previous ones. If a single variable RESTVAR is given, then it is bound to any remaining arguments, otherwise it is an error if any excess arguments are provided.

(let-optionals* '(one two) ((a 1) (b 2) (c a))
  (list a b c) )                               ==> (one two one)

Other binding forms

and-let*

[syntax] (and-let* (BINDING ...) EXP1 EXP2 ...)

SRFI-2. Bind sequentially and execute body. BINDING can be a list of a variable and an expression, a list with a single expression, or a single variable. If the value of an expression bound to a variable is #f, the and-let* form evaluates to #f (and the subsequent bindings and the body are not executed). Otherwise the next binding is performed. If all bindings/expressions evaluate to a true result, the body is executed normally and the result of the last expression is the result of the and-let* form. See also the documentation for SRFI-2.

rec

[syntax] (rec NAME EXPRESSION)
[syntax] (rec (NAME VARIABLE ...) BODY ...)

Allows simple definition of recursive definitions. (rec NAME EXPRESSION) is equivalent to (letrec ((NAME EXPRESSION)) NAME) and (rec (NAME VARIABLE ...) BODY ...) is the same as (letrec ((NAME (lambda (VARIABLE ...) BODY ...))) NAME).

cut

[syntax] (cut SLOT ...)
[syntax] (cute SLOT ...)

Syntactic sugar for specializing parameters.

define-values

[syntax] (define-values (NAME ...) EXP)

Defines several variables at once, with the result values of expression EXP.

fluid-let

[syntax] (fluid-let ((VAR1 X1) ...) BODY ...)

Binds the variables VAR1 ... dynamically to the values X1 ... during execution of BODY ....

let-values

[syntax] (let-values (((NAME ...) EXP) ...) BODY ...)

SRFI 11. Binds multiple variables to the result values of EXP .... All variables are bound simultaneously.

let*-values

[syntax] (let*-values (((NAME ...) EXP) ...) BODY ...)

SRFI 11. Binds multiple variables to the result values of EXP .... The variables are bound sequentially.

(let*-values (((a b) (values 2 3))
              ((p) (+ a b)) )
  p)                               ==> 5

letrec-values

[syntax] (letrec-values (((NAME ...) EXP) ...) BODY ...)

Binds the result values of EXP ... to multiple variables at once. All variables are mutually recursive.

(letrec-values (((odd even)
                   (values 
                     (lambda (n) (if (zero? n) #f (even (sub1 n))))
                     (lambda (n) (if (zero? n) #t (odd (sub1 n)))) ) ) )
  (odd 17) )                           ==> #t

parameterize

[syntax] (parameterize ((PARAMETER1 X1) ...) BODY ...)

Binds the parameters PARAMETER1 ... dynamically to the values X1 ... during execution of BODY .... (see also: make-parameter in Parameters). Note that PARAMETER may be any expression that evaluates to a parameter procedure.

receive

[syntax] (receive (NAME ...) VALUEEXP BODY ...)
[syntax] (receive (NAME1 ... NAMEn . NAMEn+1) VALUEEXP BODY ...)
[syntax] (receive VALUEEXP)

SRFI-8. Syntactic sugar for call-with-values. Binds variables to the result values of VALUEEXP and evaluates BODY ....

(receive VALUEEXP) is equivalent to (receive _ VALUEEXP _).

set!-values

[syntax] (set!-values (NAME ...) EXP)

Assigns the result values of expression EXP to multiple variables.

Substitution forms and macros

define-constant

[syntax] (define-constant NAME CONST)

Define a variable with a constant value, evaluated at compile-time. Any reference to such a constant should appear textually after its definition. This construct is equivalent to define when evaluated or interpreted. Constant definitions should only appear at toplevel. Note that constants are local to the current compilation unit and are not available outside of the source file in which they are defined. Names of constants still exist in the Scheme namespace and can be lexically shadowed. If the value is mutable, then the compiler is careful to preserve its identity. CONST may be any constant expression, and may also refer to constants defined via define-constant previously. This for should only be used at top-level.

define-inline

[syntax] (define-inline (NAME VAR ...) BODY ...)
[syntax] (define-inline (NAME VAR1 ... VARn . VARn+1) BODY ...)
[syntax] (define-inline NAME EXP)

Defines an inline procedure. Any occurrence of NAME will be replaced by EXP or (lambda (VAR ... [. VAR]) BODY ...). This is similar to a macro, but variable-names and -scope will be correctly handled. Inline substitutions take place after macro-expansion. EXP should be a lambda-expression. Any reference to NAME should appear textually after its definition. Note that inline procedures are local to the current compilation unit and are not available outside of the source file in which they are defined. Names of inline procedures still exist in the Scheme namespace and can be lexically shadowed. This construct is equivalent to define when evaluated or interpreted. Inline definitions should only appear at toplevel.

define-for-syntax

[syntax] (define-for-syntax (NAME VAR ...) EXP1 ...)
[syntax] (define-for-syntax (NAME VAR1 ... VARn . VARn+1) EXP1 ...)
[syntax] (define-for-syntax NAME [VALUE])

Defines the toplevel variable NAME at macro-expansion time. This can be helpful when you want to define support procedures for use in macro-transformers, for example.

define-compiler-syntax

[syntax] (define-compiler-syntax NAME)
[syntax] (define-compiler-syntax NAME TRANSFORMER)
[syntax] (define-compiler-syntax (NAME VAR ...) BODY ...)

Defines what is usually called a compiler macro in Lisp: NAME should be the name of a globally or locally bound procedure. Any direct call to this procedure will be transformed before compilation, which allows arbitrary rewritings of function calls.

TRANSFORMER can be a syntax-rules expression or an explicit-renaming transformer procedure. Returning the original form in an explicit-renaming macro or simply "falling trough" all patterns in a syntax-rules form will keep the original expression and compile it normally.

In the interpreter this form does nothing and returns an unspecified value.

Compiler-syntax is always local to the current compilation unit and can not be exported.

define-compiler-syntax should only be used at top-level. Local compiler-syntax can be defined with let-compiler-syntax.

(define-compiler-syntax +
  (syntax-rules ()
    ((_) 1)
    ((_ x 0) x) ) )

If no transformer is given, then (define-compiler-syntax NAME) removes any compiler-syntax definitions for NAME.

let-compiler-syntax

[syntax] (let-compiler-syntax ((NAME [TRANSFORMER]) ...) BODY ...)

Allows definition local compiler macros, which are only applicable inside BODY .... By not providing a TRANSFORMER expression, compiler-syntax for specific identifiers can be temporarily disabled.

Conditional forms

select

[syntax] (select EXP ((KEY ...) EXP1 ...) ... [(else EXPn ...)])

This is similar to case, but the keys are evaluated.

unless

[syntax] (unless TEST EXP1 EXP2 ...)

Equivalent to:

(if (not TEST) (begin EXP1 EXP2 ...))

when

[syntax] (when TEST EXP1 EXP2 ...)

Equivalent to:

(if TEST (begin EXP1 EXP2 ...))

Record structures

define-record

[syntax] (define-record NAME SLOTNAME ...)

Defines a record type. Call make-NAME to create an instance of the structure (with one initialization-argument for each slot). (NAME? STRUCT) tests any object for being an instance of this structure. Slots are accessed via (NAME-SLOTNAME STRUCT) and updated using (NAME-SLOTNAME-set! STRUCT VALUE).

(define-record point x y)
(define p1 (make-point 123 456))
(point? p1)                      ==> #t
(point-x p1)                     ==> 123
(point-y-set! p1 99)
(point-y p1)                     ==> 99

define-record-type

[syntax] (define-record-type NAME (CONSTRUCTOR TAG ...) PREDICATE (FIELD ACCESSOR [MODIFIER]) ...)

SRFI-9 record types. For more information see the documentation for SRFI-9.

As an extension the MODIFIER may have the form (setter PROCEDURE), which will define a SRFI-17 setter-procedure for the given PROCEDURE that sets the field value. Usually PROCEDURE has the same name is ACCESSOR (but it doesn't have to).

define-record-printer

[syntax] (define-record-printer (NAME RECORDVAR PORTVAR) BODY ...)
[syntax] (define-record-printer NAME PROCEDURE)

Defines a printing method for record of the type NAME by associating a procedure with the record type. When a record of this type is written using display, write or print, then the procedure is called with two arguments: the record to be printed and an output-port.

(define-record-type foo (make-foo x y z) foo?
  (x foo-x)
  (y foo-y)
  (z foo-z))
(define f (make-foo 1 2 3))
(define-record-printer (foo x out)
  (fprintf out "#,(foo ~S ~S ~S)"
           (foo-x x) (foo-y x) (foo-z x)) )
(define-reader-ctor 'foo make-foo)
(define s (with-output-to-string
              (lambda () (write f))))
s                                   ==> "#,(foo 1 2 3)"
(equal? f (with-input-from-string
              s read)))             ==> #t

Other forms

assert

[syntax] (assert EXP [STRING ARG ...])

Signals an error if EXP evaluates to false. An optional message STRING and arguments ARG ... may be supplied to give a more informative error-message. If compiled in unsafe mode (either by specifying the -unsafe compiler option or by declaring (unsafe)), then this expression expands to an unspecified value. The result is the value of EXP.

begin-for-syntax

[syntax] (begin-for-syntax EXP ...)

Equivalent to (begin EXP ...), but performs the evaluation of the expression during macro-expansion time.

cond-expand

[syntax] (cond-expand FEATURE-CLAUSE ...)

Expands by selecting feature clauses. This form is allowed to appear in non-toplevel expressions.

Predefined feature-identifiers are "situation" specific:

compile
eval, library, match, compiling, srfi-11, srfi-15, srfi-31, srfi-26, srfi-16, utils, regex, srfi-4, match, srfi-1, srfi-69, srfi-28, extras, srfi-8, srfi-6, srfi-2, srfi-0, srfi-10, srfi-9, srfi-55, srfi-61 chicken, srfi-23, srfi-30, srfi-39, srfi-62, srfi-17, srfi-12.
load
srfi-69, srfi-28, extras, srfi-8, srfi-6, srfi-2, srfi-0, srfi-10, srfi-9, srfi-55, srfi-61, chicken, srfi-23, srfi-30, srfi-39, srfi-62, srfi-17, srfi-12. library is implicit.
eval
match, csi, srfi-11, srfi-15, srfi-31, srfi-26, srfi-16, srfi-69, srfi-28, extras, srfi-8, srfi-6, srfi-2, srfi-0, srfi-10, srfi-9, srfi-55, srfi-61, chicken, srfi-23, srfi-30, srfi-39, srfi-62, srfi-17, srfi-12. library is implicit.

The following feature-identifiers are available in all situations: (machine-byte-order), (machine-type), (software-type), (software-version), where the actual feature-identifier is platform dependent.

In addition the following feature-identifiers may exist: applyhook, extraslot, ptables, dload.

For further information, see the documentation for SRFI-0.

ensure

[syntax] (ensure PREDICATE EXP [ARGUMENTS ...])

Evaluates the expression EXP and applies the one-argument procedure PREDICATE to the result. If the predicate returns #f an error is signaled, otherwise the result of EXP is returned. If compiled in unsafe mode (either by specifying the -unsafe compiler option or by declaring (unsafe)), then this expression expands to an unspecified value. If specified, the optional ARGUMENTS are used as arguments to the invocation of the error-signalling code, as in (error ARGUMENTS ...). If no ARGUMENTS are given, a generic error message is displayed with the offending value and PREDICATE expression.

eval-when

[syntax] (eval-when (SITUATION ...) EXP ...)

Controls evaluation/compilation of subforms. SITUATION should be one of the symbols eval, compile or load. When encountered in the evaluator, and the situation specifier eval is not given, then this form is not evaluated and an unspecified value is returned. When encountered while compiling code, and the situation specifier compile is given, then this form is evaluated at compile-time. When encountered while compiling code, and the situation specifier load is not given, then this form is ignored and an expression resulting into an unspecified value is compiled instead.

The following table should make this clearer:

In compiled code In interpreted code
eval ignore evaluate
compile evaluate at compile time ignore
load compile as normal ignore

include

[syntax] (include STRING)

Include toplevel-expressions from the given source file in the currently compiled/interpreted program. If the included file has the extension .scm, then it may be omitted. The file is searched in the current directory and, if not found, in all directories specified in the -include-path option.

nth-value

[syntax] (nth-value N EXP)

Returns the Nth value (counting from zero) of the values returned by expression EXP.

time

[syntax] (time EXP1 ...)

Evaluates EXP1 ... and prints elapsed time and some values about GC use, like time spent in major GCs, number of minor and major GCs.

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