3.17 Procedures

Applies proc using the content of (list* v ... lst) as the (by-position) arguments. The #:<kw> kw-arg sequence is also supplied as keyword arguments to proc, where #:<kw> stands for any keyword.

The given proc must accept as many arguments as the number of vs plus length of lst, it must accept the supplied keyword arguments, and it must not require any other keyword arguments; otherwise, the exn:fail:contract exception is raised. The given proc is called in tail position with respect to the apply call.

Examples:

> (apply + '(1 2 3))

6

> (apply + 1 2 '(3))

6

> (apply + '())

0

> (apply sort (list (list '(2) '(1)) <) #:key car)

'((1) (2))

Examples:

> ((compose - sqrt) 10)

-3.1622776601683795

> ((compose sqrt -) 10)

0+3.1622776601683795i

> ((compose list split-path) (bytes->path #"/a" 'unix))

'(#<path:/> #<path:a> #f)

The given name is used for printing an error message if the resulting procedure is applied to the wrong number of arguments. In addition, if proc is an accessor or mutator produced by struct, make-struct-field-accessor, or make-struct-field-mutator, the resulting procedure also uses name when its (first) argument has the wrong type. More typically, however, name is not used for reporting errors, since the procedure name is typically hard-wired into an internal check.

3.17.1 Keywords and Arity

Like apply, but kw-lst and kw-val-lst supply by-keyword arguments in addition to the by-position arguments of the vs and lst, and in addition to the directly supplied keyword arguments in the #:<kw> kw-arg sequence, where #:<kw> stands for any keyword.

The given kw-lst must be sorted using keyword<?. No keyword can appear twice in kw-lst or in both kw-list and as a #:<kw>, otherwise, the exn:fail:contract exception is raised. The given kw-val-lst must have the same length as kw-lst, otherwise, the exn:fail:contract exception is raised. The given proc must accept all of the keywords in kw-lst plus the #:<kw>s, it must not require any other keywords, and it must accept as many by-position arguments as supplied via the vs and lst; otherwise, the exn:fail:contract exception is raised.

Examples:

(define (f x #:y y #:z [z 10])     (list x y z))

> (keyword-apply f '(#:y) '(2) '(1))

'(1 2 10)

> (keyword-apply f '(#:y #:z) '(2 3) '(1))

'(1 2 3)

> (keyword-apply f #:z 7 '(#:y) '(2) '(1))

'(1 2 7)

• An exact non-negative integer, which means that the procedure accepts a arguments, only.

• A arity-at-least instance, which means that the procedure accepts (arity-at-least-value a) or more arguments.

• A list containing integers and arity-at-least instances, which means that the procedure accepts any number of arguments that can match one of the elements of a.

Generally, procedure-arity always produces an arity that is normalized. Specifically, it is either the empty list (corresponding to the procedure (case-lambda)), one of the first two cases above, or a list that contains at least two elements. If it is a list, there is at most one arity-at-least instance that appears as the last element of the list, all of the other elements are sorted in ascending order, and there are no duplicate elements.

Examples:

> (procedure-arity cons)

2

> (procedure-arity list)

(arity-at-least 0)

> (arity-at-least? (procedure-arity list))

#t

> (arity-at-least-value (procedure-arity list))

0

> (arity-at-least-value (procedure-arity (lambda (x . y) x)))

1

> (procedure-arity (case-lambda [(x) 0] [(x y) 1]))

'(1 2)

Examples:

> (procedure-arity-includes? cons 2)

#t

> (procedure-arity-includes? display 3)

#f

If the arity specification allows arguments that are not in (procedure-arity proc), the exn:fail:contract exception is raised. If proc accepts keyword argument, either the keyword arguments must be all optional (and they are not accepted in by the arity-reduced procedure) or arity must be the empty list (which makes a procedure that cannot be called); otherwise, the exn:fail:contract exception is raised.

Examples:

> (define my+ (procedure-reduce-arity + 2))

> (my+ 1 2)

3

> (my+ 1 2 3)

+: expects 2 arguments, given 3: 1 2 3

Examples:

> (procedure-keywords +)

'()

'()

> (procedure-keywords (lambda (#:tag t #:mode m) t))

'(#:mode #:tag)

'(#:mode #:tag)

> (procedure-keywords (lambda (#:tag t #:mode [m #f]) t))

'(#:tag)

'(#:mode #:tag)

When the result is called with keyword arguments, then proc is called; the first argument is a list of keywords sorted by keyword<?, the second argument is a parallel list containing a value for each keyword, and the remaining arguments are the by-position arguments.

When the result is called without keyword arguments, then plain-proc is called. Furthermore, procedure-arity obtains its result from plain-proc.

Examples:

(define show     (make-keyword-procedure (lambda (kws kw-args . rest)                               (list kws kw-args rest))))

> (show 1)

'(() () (1))

> (show #:init 0 1 2 3 #:extra 4)

'((#:extra #:init) (4 0) (1 2 3))

Examples:

(define orig-show     (make-keyword-procedure (lambda (kws kw-args . rest)                               (list kws kw-args rest))))

(define show (procedure-reduce-keyword-arity                 orig-show 3 '(#:init) '(#:extra #:init)))

> (show #:init 0 1 2 3 #:extra 4)

'((#:extra #:init) (4 0) (1 2 3))

> (show 1)

struct ...t/private/kw.rkt:186:14: expects 3 arguments plus

an argument with keyword #:init plus an optional argument

with keyword #:extra, given 1: 1

> (show #:init 0 1 2 3 #:extra 4 #:more 7)

...t/private/kw.rkt:186:14: does not expect an argument

with keyword #:more; arguments were: 1 2 3 #:extra 4 #:init

0 #:more 7

If the prop:procedure property value is an exact non-negative integer, it designates a field within the structure that should contain a procedure. The integer must be between 0 (inclusive) and the number of non-automatic fields in the structure type (exclusive, not counting supertype fields). The designated field must also be specified as immutable, so that after an instance of the structure is created, its procedure cannot be changed. (Otherwise, the arity and name of the instance could change, and such mutations are generally not allowed for procedures.) When the instance is used as the procedure in an application expression, the value of the designated field in the instance is used to complete the procedure call. (This procedure can be another structure that acts as a procedure; the immutability of procedure fields disallows cycles in the procedure graph, so that the procedure call will eventually continue with a non-structure procedure.) That procedure receives all of the arguments from the application expression. The procedure’s name (see object-name), arity (see procedure-arity), and keyword protocol (see procedure-keywords) are also used for the name, arity, and keyword protocol of the structure. If the value in the designated field is not a procedure, then the instance behaves like (case-lambda) (i.e., a procedure which does not accept any number of arguments). See also procedure-extract-target.

Providing an integer proc-spec argument to make-struct-type is the same as both supplying the value with the prop:procedure property and designating the field as immutable (so that a property binding or immutable designation is redundant and disallowed).

Examples:

> (struct annotated-proc (base note)       #:property prop:procedure                  (struct-field-index base))

> (define plus1 (annotated-proc                     (lambda (x) (+ x 1))                     "adds 1 to its argument"))

> (procedure? plus1)

#t

> (annotated-proc? plus1)

#t

> (plus1 10)

11

> (annotated-proc-note plus1)

"adds 1 to its argument"

When the prop:procedure value is a procedure, it should accept at least one non-keyword argument. When an instance of the structure is used in an application expression, the property-value procedure is called with the instance as the first argument. The remaining arguments to the property-value procedure are the arguments from the application expression (including keyword arguments). Thus, if the application expression provides five non-keyword arguments, the property-value procedure is called with six non-keyword arguments. The name of the instance (see object-name) and its keyword protocol (see procedure-keywords) are unaffected by the property-value procedure, but the instance’s arity is determined by subtracting one from every possible non-keyword argument count of the property-value procedure. If the property-value procedure cannot accept at least one argument, then the instance behaves like (case-lambda).

Providing a procedure proc-spec argument to make-struct-type is the same as supplying the value with the prop:procedure property (so that a specific property binding is disallowed).

Examples:

> (struct fish (weight color)       #:mutable       #:property       prop:procedure       (lambda (f n)         (let ([w (fish-weight f)])           (set-fish-weight! f (+ n w)))))

> (define wanda (fish 12 'red))

> (fish? wanda)

#t

> (procedure? wanda)

#t

> (fish-weight wanda)

12

> (for-each wanda '(1 2 3))

> (fish-weight wanda)

18

If the value supplied for the prop:procedure property is not an exact non-negative integer or a procedure, the exn:fail:contract exception is raised.

When a prop:procedure property value is a procedure, the procedure is not returned by procedure-extract-target. Such a procedure is different from one accessed through a structure field, because it consumes an extra argument, which is always the structure that was applied as a procedure. Keeping the procedure private ensures that is it always called with a suitable first argument.

Arity-mismatch reporting automatically uses procedure-extract-target when the prop:arity-string property is not associated with a procedure structure type.

Examples:

> (struct evens (proc)       #:property prop:procedure (struct-field-index proc)       #:property prop:arity-string       (lambda (p)         "an even number of arguments"))

> (define pairs       (evens        (case-lambda         [() null]         [(a b . more)          (cons (cons a b)                (apply pairs more))])))

> (pairs 1 2 3 4)

'((1 . 2) (3 . 4))

> (pairs 5)

#<procedure>: expects an even number of arguments, given 1:

5

If v is not an instance of type, or if the first field of v applied to v1 and v2 produces #f, then proc is applied to v, v1, and v2, and its result is returned by checked-procedure-check-and-extract.

3.17.2 Reflecting on Primitives

A primitive procedure is a built-in procedure that is implemented in low-level language. Not all procedures of racket/base are primitives, but many are. The distinction is mainly useful to other low-level code.

3.17.3 Additional Procedure Functions

Examples:

> ((const 'foo) 1 2 3)

'foo

> ((const 'foo))

'foo

Examples:

> (filter (negate symbol?) '(1 a 2 b 3 c))

'(1 2 3)

> (map (negate =) '(1 2 3) '(1 1 1))

'(#f #t #t)

Examples:

> ((curry list) 1 2)

#<procedure:curried>

> ((curry cons) 1)

#<procedure:curried>

> ((curry cons) 1 2)

'(1 . 2)

After the first application of the result of curry, each further application accumulates arguments until an acceptable number of arguments have been accumulated, at which point the original proc is called.

Examples:

> (((curry list) 1 2) 3)

'(1 2 3)

> (((curry list) 1) 3)

'(1 3)

> ((((curry foldl) +) 0) '(1 2 3))

6

A function call (curry proc v ...) is equivalent to ((curry proc) v ...). In other words, curry itself is curried.

The curry function provides limited support for keyworded functions: only the curry call itself can receive keyworded arguments to be propagated eventually to proc.

Examples:

> (map ((curry +) 10) '(1 2 3))

'(11 12 13)

> (map (curry + 10) '(1 2 3))

'(11 12 13)

> (map (compose (curry * 2) (curry + 10)) '(1 2 3))

'(22 24 26)

> (define foo (curry (lambda (x y z) (list x y z))))

> (foo 1 2 3)

'(1 2 3)

> (((((foo) 1) 2)) 3)

'(1 2 3)

Example:

> (map (curryr list 'foo) '(1 2 3))

'((1 foo) (2 foo) (3 foo))