## Non-standard macros and special forms

### Making extra libraries and extensions available

#### require-extension

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

  This is equivalent to `(require-library ID ...)` but performs an implicit `import`, if necessary. Since version 4.4.0, `ID` may also be an import specification (using `rename`, `only`, `except` or `prefix`).

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

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

#### require-library

`(require-library ID ...)` syntax

  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:

  - If `ID` names a built-in feature `chicken srfi-0 srfi-2 srfi-6 srfi-8 srfi-9 srfi-10 srfi-17 srfi-23 srfi-30 srfi-39 srfi-55`, then nothing is done.
  - If `ID` names one of the syntactic extensions `chicken-syntax chicken-ffi-syntax`, then this extension will be loaded.
  - If `ID` names one of the core library units shipped with CHICKEN, then a `(load-library 'ID)` will be performed.
  - If `ID` names an installed extension with the `syntax` or `require-at-runtime` attribute, then the extensions is loaded at compile-time, probably doing a run-time `(require ...)` for any run-time requirements.
  - Otherwise, `(require-library ID)` is equivalent to `(require 'ID)`.

  During compilation, one of the following happens instead:

  - If `ID` names a built-in feature `chicken srfi-0 srfi-2 srfi-6 srfi-8 srfi-9 srfi-10 srfi-17 srfi-23 srfi-30 srfi-39 srfi-55`, then nothing is done.
  - If `ID` names one of the syntactic extensions `chicken-syntax chicken-ffi-syntax`, then this extension will be loaded at compile-time, making the syntactic extensions available in compiled code.
  - If `ID` names one of the core library units shipped with CHICKEN, or if the option `-uses ID` has been passed to the compiler, then a `(declare (uses ID))` is generated.
  - If `ID` names an installed extension with the `syntax` or `require-at-runtime` attribute, then the extension is loaded at compile-time, and code is emitted to `(require ...)` any needed run-time requirements.
  - Otherwise `(require-library ID)` is equivalent to `(require 'ID)`.

  `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:

  - `(srfi NUMBER ...)` is required for SRFI-55 compatibility and is fully implemented
  - `(version ID NUMBER)` is equivalent to `ID`, but checks at compile-time whether the extension named `ID` is installed and whether its version is equal or higher than `NUMBER`. `NUMBER` may be a string or a number, the comparison is done lexicographically (using `string>=?`).

#### use

`(use ID ...)` syntax

  `use` is just a shorter alias for `require-extension`.

#### require-extension-for-syntax

`(require-extension-for-syntax ID ...)` syntax

  An abbreviation for the idiom:

      (begin-for-syntax (require-library ID ...))  ; load extension at expansion-time
      (import-for-syntax ID ...)                   ; import extension for use in syntax-transformers

#### use-for-syntax

`(use-for-syntax ID ...)` syntax

  `use` is just a shorter alias for `require-extension-for-syntax` (which is quite a mouthful).

### Binding forms for optional arguments

#### optional

`(optional ARGS DEFAULT)` syntax

  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

`(case-lambda (LAMBDA-LIST1 EXP1 ...) ...)` syntax

  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

`(let-optionals ARGS ((VAR1 DEFAULT1) ...) BODY ...)` syntax

  Binding constructs for optional procedure arguments. `ARGS` is normally 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*

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

  Binding constructs for optional procedure arguments. `ARGS` is normally 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*

`(and-let* (BINDING ...) EXP1 EXP2 ...)` syntax

  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.

#### letrec*

`(letrec* ((VARIABLE EXPRESSION) ...) BODY ...)` syntax

  Implements R6RS/R7RS `letrec*`. `letrec*` is similar to `letrec` but binds the variables sequentially and is to `letrec` what `let*` is to `let`.

#### rec

`(rec NAME EXPRESSION)` syntax
`(rec (NAME VARIABLE ...) BODY ...)` syntax

  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

`(cut SLOT ...)` syntax
`(cute SLOT ...)` syntax

  Syntactic sugar for specializing parameters.

#### define-values

`(define-values (NAME ...) VALUEEXP)` syntax
`(define-values (NAME1 ... NAMEn . NAMEn+1) VALUEEXP)` syntax
`(define-values NAME VALUEEXP)` syntax

  Defines several variables at once, with the result values of expression `VALUEEXP`, similar to `set!-values`.

#### fluid-let

`(fluid-let ((VAR1 X1) ...) BODY ...)` syntax

  Binds the variables `VAR1 ...` dynamically to the values `X1 ...` during execution of `BODY ...`. This implements SRFI-15.

#### let-values

`(let-values (((NAME ...) VALUEEXP) ...) BODY ...)` syntax

  Binds multiple variables to the result values of `VALUEEXP ...`. All variables are bound simultaneously. Like `define-values`, the `(NAME ...)` expression can be any basic lambda list (dotted tail notation is supported). This implements SRFI-11.

#### let*-values

`(let*-values (((NAME ...) VALUEEXP) ...) BODY ...)` syntax

  Binds multiple variables to the result values of `VALUEEXP ...`. The variables are bound sequentially. Like `let-values`, the `(NAME ...)` expression can be any basic lambda list (dotted tail notation is supported). This is also part of SRFI-11.

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

#### letrec-values

`(letrec-values (((NAME ...) VALUEEXP) ...) BODY ...)` syntax

  Binds the result values of `VALUEEXP ...` to multiple variables at once. All variables are mutually recursive. Like `let-values`, the `(NAME ...)` expression can be any basic lambda list (dotted tail notation is supported).

      (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

`(parameterize ((PARAMETER1 X1) ...) BODY ...)` syntax

  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

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

  SRFI-8. Syntactic sugar for `call-with-values`. Binds variables to the result values of `VALUEEXP` and evaluates `BODY ...`, similar `define-values` but lexically scoped.

  `(receive VALUEEXP)` is equivalent to `(receive _ VALUEEXP _)`. This shortened form is not described by SRFI-8.

#### set!-values

`(set!-values (NAME ...) VALUEEXP)` syntax
`(set!-values (NAME1 ... NAMEn . NAMEn+1) VALUEEXP)` syntax
`(set!-values NAME VALUEEXP)` syntax

  Assigns the result values of expression `VALUEEXP` to multiple variables, similar to `define-values`.

### Substitution forms and macros

#### define-constant

`(define-constant NAME CONST)` syntax

  Defines 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, but it must be possible to evaluate the expression at compile-time.

#### define-inline

`(define-inline (NAME VAR ...) BODY ...)` syntax
`(define-inline (NAME VAR ... . VAR) BODY ...)` syntax
`(define-inline NAME EXP)` syntax

  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 are handled correctly.

  Inline substitutions take place after macro-expansion, and any reference to `NAME` should appear textually after its definition. 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. Inline definitions should only appear at the toplevel.

  Note that the `inline-limit` compiler option does not affect inline procedure expansion, and self-referential inline procedures may cause the compiler to enter an infinite loop.

  In the third form, `EXP` must be a lambda expression.

  This construct is equivalent to `define` when evaluated or interpreted.

#### define-for-syntax

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

  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.

  Note that `define-for-syntax` definitions within a module are implicitly added to that module's import library. Refer to the documentation on import libraries for more information.

#### define-compiler-syntax

`(define-compiler-syntax NAME)` syntax
`(define-compiler-syntax NAME TRANSFORMER)` syntax

  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 a transformer procedure (as returned by `er-macro-transformer` or `ir-macro-transformer`). Returning the original form in an explicit/implicit-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. Compiler-syntax defined inside a module is not visible outside of that module.

  `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

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

  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

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

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

#### unless

`(unless TEST EXP1 EXP2 ...)` syntax

  Equivalent to:

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

#### when

`(when TEST EXP1 EXP2 ...)` syntax

  Equivalent to:

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

### Record structures

#### define-record

`(define-record NAME SLOTNAME ...)` syntax

  Defines a record type. This defines a number of procedures for creating, accessing, and modifying record members.

  Call `make-NAME` to create an instance of the structure (with one initialization-argument for each slot, in the listed order).

  `(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

##### SRFI-17 setters

`SLOTNAME` may alternatively also be of the form

     (setter SLOTNAME)

In this case the slot can be read with `(NAME-SLOTNAME STRUCT)` as usual, and modified with `(set! (NAME-SLOTNAME STRUCT) VALUE)` (the slot-accessor has an associated SRFI-17 "setter" procedure) instead of the usual `(NAME-SLOTNAME-set!` `STRUCT` `VALUE)`.

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

#### define-record-type

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

  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

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

  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

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

  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

`(begin-for-syntax EXP ...)` syntax

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

#### cond-expand

`(cond-expand FEATURE-CLAUSE ...)` syntax

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

  Predefined feature-identifiers are "situation" specific:

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

  The following feature-identifier classes 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: `cross-chicken`, `dload`, `manyargs`, `ptables`.

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

### delay-force

CHICKEN supports the R7RS `delay-force` syntax which allows for iterative lazy algorithms to be expressed in bounded space.

For more information regarding this behaviour, see the SRFI-45 rationale.

#### ensure

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

  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

`(eval-when (SITUATION ...) EXP ...)` syntax

  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

`(include STRING)` syntax

  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

`(nth-value N EXP)` syntax

  Returns the `N`th value (counting from zero) of the values returned by expression `EXP`.

#### time

`(time EXP1 ...)` syntax

  Performs a major garbage collection, evaluates `EXP1 ...` and prints elapsed CPU time and some values about GC use, like time spent in major GCs, number of minor and major GCs. The output is sent to the port that is the current value of `(current-error-port)`. Nested invocations of this form will give incorrect results for all output but the innermost.

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