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►12.5 Syntax Parameters
On this page:
define-syntax-parameter
syntax-parameterize
12.5.1 Syntax Parameter Inspection
syntax-parameter-value
make-parameter-rename-transformer
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12.5 Syntax Parameters

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

syntax

(define-syntax-parameter id expr)

Binds id as syntax to a syntax parameter. The expr is an expression in the transformer environment that serves as the default value for the syntax parameter. The value is typically obtained by a transformer using syntax-parameter-value.

The id can be used with syntax-parameterize or syntax-parameter-value (in a transformer). If expr produces a procedure of one argument or a make-set!-transformer result, then id can be used as a macro. If expr produces a make-rename-transformer result, then id can be used as a macro that expands to a use of the target identifier, but syntax-local-value of id does not produce the target’s value.

Examples:

> (define-syntax-parameter current-class #f)
> (define-syntax-parameter yield (make-rename-transformer #'abort))
> (define-syntax-parameter define/public
    (λ (stx)
      (raise-syntax-error #f "use of a class keyword not in a class" stx)))
> (begin-for-syntax (displayln (syntax-parameter-value #'current-class)))

#f

> (yield 5)

5

syntax

(syntax-parameterize ([id expr] ...) body-expr ...+)

+See also splicing-syntax-parameterize.

Each id must be bound to a syntax parameter using define-syntax-parameter. Each expr is an expression in the transformer environment. During the expansion of the body-exprs, the value of each expr is bound to the corresponding id.

If an expr produces a procedure of one argument or a make-set!-transformer result, then its id can be used as a macro during the expansion of the body-exprs. If expr produces a make-rename-transformer result, then id can be used as a macro that expands to a use of the target identifier, but syntax-local-value of id does not produce the target’s value.

Examples:

> (define-syntax-parameter abort (syntax-rules ()))
> (define-syntax forever
    (syntax-rules ()
      [(forever body ...)
       (call/cc (lambda (abort-k)
         (syntax-parameterize
             ([abort (syntax-rules () [(_) (abort-k)])])
           (let loop () body ... (loop)))))]))
> (define-syntax-parameter it (syntax-rules ()))
> (define-syntax aif
    (syntax-rules ()
      [(aif test then else)
       (let ([t test])
         (syntax-parameterize ([it (syntax-id-rules () [_ t])])
           (if t then else)))]))

12.5.1 Syntax Parameter Inspection

 (require racket/stxparam-exptime) package: base

procedure

(syntax-parameter-value id-stx) → any

  id-stx : syntax?
This procedure is intended for use in a transformer environment, where id-stx is an identifier bound in the normal environment to a syntax parameter. The result is the current value of the syntax parameter, as adjusted by syntax-parameterize form.

This binding is provided for-syntax by racket/stxparam, since it is normally used in a transformer. It is provided normally by racket/stxparam-exptime.

procedure

(make-parameter-rename-transformer id-stx) → any

  id-stx : syntax?
This procedure is intended for use in a transformer, where id-stx is an identifier bound to a syntax parameter. The result is a transformer that behaves as id-stx, but that cannot be used with syntax-parameterize or syntax-parameter-value.

Using make-parameter-rename-transformer is analogous to defining a procedure that calls a parameter. Such a procedure can be exported to others to allow access to the parameter value, but not to change the parameter value. Similarly, make-parameter-rename-transformer allows a syntax parameter to be used as a macro, but not changed.

The result of make-parameter-rename-transformer is not treated specially by syntax-local-value, unlike the result of make-rename-transformer.

This binding is provided for-syntax by racket/stxparam, since it is normally used in a transformer. It is provided normally by racket/stxparam-exptime.

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