►The Racket Reference
▼2 Syntactic Forms
►2.14 Definitions: define, define-syntax, ...
On this page:
define
define-values
define-syntax
define-syntaxes
define-for-syntax
define-values-for-syntax
2.14.1 require Macros
define-require-syntax
2.14.2 provide Macros
define-provide-syntax
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2.14 Definitions: define, define-syntax, ...

+Definitions: define in The Racket Guide introduces definitions.

(define id expr)
(define (head args) body ...+)
 
head = id
  | (head args)
     
args = arg ...
  | arg ... . rest-id
     
arg = arg-id
  | [arg-id default-expr]
  | keyword arg-id
  | keyword [arg-id default-expr]
The first form binds id to the result of expr, and the second form binds id to a procedure. In the second case, the generated procedure is (CVT (head args) body ...+), using the CVT meta-function defined as follows:

(CVT (id . kw-formals) . datum)   = (lambda kw-formals . datum)
(CVT (head . kw-formals) . datum) = (lambda kw-formals expr)
                                     if (CVT head . datum) = expr

In an internal-definition context (see Internal Definitions), a define form introduces a local binding. At the top level, the top-level binding for id is created after evaluating expr, if it does not exist already, and the top-level mapping of id (in the namespace linked with the compiled definition) is set to the binding at the same time.

In a context that allows liberal expansion of define, id is bound as syntax if expr is an immediate lambda form with keyword arguments or args include keyword arguments.

Examples:

(define x 10)
> x

10

(define (f x)
  (+ x 1))

 

> (f 10)

11

(define ((f x) [y 20])
  (+ x y))

 

> ((f 10) 30)

40

> ((f 10))

30

(define-values (id ...) expr)
Evaluates the expr, and binds the results to the ids, in order, if the number of results matches the number of ids; if expr produces a different number of results, the exn:fail:contract exception is raised.

In an internal-definition context (see Internal Definitions), a define-values form introduces local bindings. At the top level, the top-level binding for each id is created after evaluating expr, if it does not exist already, and the top-level mapping of each id (in the namespace linked with the compiled definition) is set to the binding at the same time.

Examples:

(define-values () (values))
(define-values (x y z) (values 1 2 3))
> z

3

(define-syntax id expr)
(define-syntax (head args) body ...+)
The first form creates a transformer binding (see Transformer Bindings) of id with the value of expr, which is an expression at phase level 1 relative to the surrounding context. (See Identifiers and Binding for information on phase levels.) Evaluation of expr side is parameterized to set current-namespace as in let-syntax.

The second form is a shorthand the same as for define; it expands to a definition of the first form where the expr is a lambda form.

In an internal-definition context (see Internal Definitions), a define-syntax form introduces a local binding.

Examples:

> (define-syntax foo
    (syntax-rules ()
      ((_ a ...)
       (printf "~a\n" (list a ...)))))
> (foo 1 2 3 4)

(1 2 3 4)

> (define-syntax (bar syntax-object)
    (syntax-case syntax-object ()
      ((_ a ...)
       #'(printf "~a\n" (list a ...)))))
> (bar 1 2 3 4)

(1 2 3 4)

(define-syntaxes (id ...) expr)
Like define-syntax, but creates a transformer binding for each id. The expr should produce as many values as ids, and each value is bound to the corresponding id.

When expr produces zero values for a top-level define-syntaxes (i.e., not in a module or internal-definition position), then the ids are effectively declared without binding; see Macro-Introduced Bindings.

In an internal-definition context (see Internal Definitions), a define-syntaxes form introduces local bindings.

Examples:

> (define-syntaxes (foo1 foo2 foo3)
    (let ([transformer1 (lambda (syntax-object)
                          (syntax-case syntax-object ()
                            [(_) #'1]))]
          [transformer2 (lambda (syntax-object)
                          (syntax-case syntax-object ()
                            [(_) #'2]))]
          [transformer3 (lambda (syntax-object)
                          (syntax-case syntax-object ()
                            [(_) #'3]))])
      (values transformer1
              transformer2
              transformer3)))
> (foo1)

1

> (foo2)

2

> (foo3)

3

(define-for-syntax id expr)
(define-for-syntax (head args) body ...+)
Like define, except that the binding is at phase level 1 instead of phase level 0 relative to its context. The expression for the binding is also at phase level 1. (See Identifiers and Binding for information on phase levels.) The form is a shorthand for (begin-for-syntax (define id expr)) or (begin-for-syntax (define (head args) body ...+)).

Within a module, bindings introduced by define-for-syntax must appear before their uses or in the same define-for-syntax form (i.e., the define-for-syntax form must be expanded before the use is expanded). In particular, mutually recursive functions bound by define-for-syntax must be defined by the same define-for-syntax form.

Examples:

> (define-for-syntax helper 2)
> (define-syntax (make-two syntax-object)
    (printf "helper is ~a\n" helper)
    #'2)
> (make-two)

helper is 2

2

; `helper' is not bound in the runtime phase
> helper

reference to undefined identifier: helper

> (define-for-syntax (filter-ids ids)
    (filter identifier? ids))
> (define-syntax (show-variables syntax-object)
    (syntax-case syntax-object ()
      [(_ expr ...)
       (with-syntax ([(only-ids ...)
                      (filter-ids (syntax->list #'(expr ...)))])
         #'(list only-ids ...))]))
> (let ([a 1] [b 2] [c 3])
    (show-variables a 5 2 b c))

'(1 2 3)

(define-values-for-syntax (id ...) expr)
Like define-for-syntax, but expr must produce as many values as supplied ids, and all of the ids are bound (at phase level 1).

Examples:

> (define-values-for-syntax (foo1 foo2) (values 1 2))
> (define-syntax (bar syntax-object)
    (printf "foo1 is ~a foo2 is ~a\n" foo1 foo2)
    #'2)
> (bar)

foo1 is 1 foo2 is 2

2

2.14.1 require Macros

 (require racket/require-syntax)
The bindings documented in this section are provided by the racket/require-syntax library, not racket/base or racket.

(define-require-syntax id expr)
(define-require-syntax (id args ...) body ...+)
The first form is like define-syntax, but for a require sub-form. The proc-expr must produce a procedure that accepts and returns a syntax object representing a require sub-form.

This form expands to define-syntax with a use of make-require-transformer; see require Transformers for more information.

The second form is a shorthand the same as for define-syntax; it expands to a definition of the first form where the expr is a lambda form.

2.14.2 provide Macros

 (require racket/provide-syntax)
The bindings documented in this section are provided by the racket/provide-syntax library, not racket/base or racket.

(define-provide-syntax id expr)
(define-provide-syntax (id args ...) body ...+)
The first form is like define-syntax, but for a provide sub-form. The proc-expr must produce a procedure that accepts and returns a syntax object representing a provide sub-form.

This form expands to define-syntax with a use of make-provide-transformer; see provide Transformers for more information.

The second form is a shorthand the same as for define-syntax; it expands to a definition of the first form where the expr is a lambda form.

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