12.5 Syntax Parameters

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 begin.html#%28form._%28%28quote._~23~25kernel%29._begin-for-syntax%29%29" data-pltdoc="x">begin-for-syntax (displayln (syntax-parameter-value #'current-class)))

#f
> (yield5)

5

syntax
( syntax-parameterize ([idexpr]...)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 ()
[(foreverbody... )
(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 ()
[(aiftestthenelse )
(let ([ttest])
(syntax-parameterize ([it(syntax-id-rules ()[_ t])])
(if tthenelse )))]))

syntax
( define-rename-transformer-parameter idexpr)

Binds id as syntax to a syntax parameter that must be bound to a make-rename-transformer result and, unlike define-syntax-parameter , syntax-local-value of id does produce the target’s value, including inside of syntax-parameterize .

Examples:

> (define-syntax (teststx)
(syntax-case stx()
[(_ t)
#`#,(syntax-local-value #'t)]))
> (define-syntax one1)
> (define-syntax two2)

> (define-syntax-parameter not-num
(make-rename-transformer #'one))
> (testnot-num)

#<procedure:syntax-parameter>

> (define-rename-transformer-parameter num
(make-rename-transformer #'one))
> (testnum)

1

> (syntax-parameterize ([num(make-rename-transformer #'two)])
(testnum))

2

Added in version 6.3.0.14 of package base.

12.5.1Syntax Parameter Inspection🔗 i

(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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