▼Unstable: May Change Without Warning
►8 Definitions
On this page:
8.1 Deferred Evaluation in Modules
at-end
8.2 Conditional Binding
define-if-unbound
define-values-if-unbound
define-syntax-if-unbound
define-syntaxes-if-unbound
8.3 Renaming Definitions
define-renaming
define-renamings
8.4 Forward Declarations
declare-names
8.5 Definition Shorthands
define-with-parameter
define-single-definition
8.6 Macro Definitions
define-syntax-block
8.7 Effectful Transformation
in-phase1
in-phase1/ pass2
Version: 5.2
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8 Definitions

Carl Eastlund <cce@racket-lang.org>

 (require unstable/define)

This library is unstable; compatibility will not be maintained. See Unstable: May Change Without Warning for more information.

Provides macros for creating and manipulating definitions.

8.1 Deferred Evaluation in Modules

(at-end expr)
When used at the top level of a module, evaluates expr at the end of the module. This can be useful for calling functions before their definitions.

Examples:

> (module Failure scheme
    (f 5)
    (define (f x) x))
> (require 'Failure)

reference to an identifier before its definition: f in

module: 'Failure

> (module Success scheme
    (require unstable/define)
    (at-end (f 5))
    (define (f x) x))
> (require 'Success)

8.2 Conditional Binding

(define-if-unbound x e)
(define-if-unbound (f . args) body ...)
(define-values-if-unbound [x ...] e)
(define-syntax-if-unbound x e)
(define-syntax-if-unbound (f . args) body ...)
(define-syntaxes-if-unbound [x ...] e)
Define each x (or f) if no such binding exists, or do nothing if the name(s) is(are) already bound. The define-values-if-unbound and define-syntaxes-if-unbound forms raise a syntax error if some of the given names are bound and some are not.

These are useful for writing programs that are portable across versions of Racket with different bindings, to provide an implementation of a binding for versions that do not have it but use the built-in one in versions that do.

Examples:

> (define-if-unbound x 1)
> x

1

(define y 2)
> (define-if-unbound y 3)
> y

3

8.3 Renaming Definitions

(define-renaming new old)
(define-renamings [new old] ...)
Establishes a rename transformer for each new identifier, redirecting it to the corresponding old identifier.

Examples:

> (define-renaming use #%app)
> (define-renamings [def define] [lam lambda])
> (def plus (lam (x y) (use + x y)))
> (use plus 1 2)

3

8.4 Forward Declarations

(declare-names x ...)
Provides forward declarations of identifiers to be defined later. It is useful for macros which expand to mutually recursive definitions, including forward references, that may be used at the Racket top level.

8.5 Definition Shorthands

(define-with-parameter name parameter)
Defines the form name as a shorthand for setting the parameter parameter. Specifically, (name value body ...) is equivalent to (parameterize ([parameter value]) body ...).

Examples:

> (define-with-parameter with-input current-input-port)
> (with-input (open-input-string "Tom Dick Harry") (read))

'Tom

(define-single-definition define-one-name define-many-name)
Defines a marco define-one-name as a single identifier definition form with function shorthand like define and define-syntax, based on an existing macro define-many-name which works like define-values or define-syntaxes.

Examples:

> (define-single-definition define-like define-values)
> (define-like x 0)
> x

0

> (define-like (f a b c) (printf "~s, ~s\n" a b) c)
> (f 1 2 3)

1, 2

3

8.6 Macro Definitions

(define-syntax-block (macro-decl ...) body ...)
 
macro-decl = macro-id
  | [macro-id expander-id]
Defines a syntax transformer for each macro-id based on the local definition of each expander-id (defaulting to macro-id/proc) in body .... Especially useful for mutually recursive expander functions and phase 1 macro definitions. Subsumes the behavior of define-syntax-set.

Examples:

> (define-syntax-block
      ([implies expand-implies]
       nand)
  
    (define-syntax-rule (==> pattern template)
      (syntax-rules () [pattern template]))
  
    (define expand-implies (==> (_ a b) (or (not a) b)))
    (define nand/proc (==> (_ a ...) (not (and a ...)))))
> (implies #t (printf "True!\n"))

True!

> (implies #f (printf "False!\n"))

#t

> (nand #t #t (printf "All True!\n"))

All True!

#f

> (nand #t #f (printf "Some False!\n"))

#t

> (define-syntax-block (undefined-macro)
    (define irrelevant "Whoops!"))

reference to undefined identifier: undefined-macro/proc

8.7 Effectful Transformation

(in-phase1 e)
Executes e during phase 1 (the syntax transformation phase) relative to its context, during pass 1 if it occurs in a head expansion position.

(in-phase1/pass2 e)
Executes e during phase 1 (the syntax transformation phase) relative to its context, during pass 2 (after head expansion).

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