12.5 Syntax Parameters
| (require racket/stxparam) | package: base |
syntax
(define-syntax-parameter id expr)
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.
> (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.
> (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)))]))
syntax
(define-rename-transformer-parameter id expr)
> (define-syntax (test stx) (syntax-case stx () [(_ t) #`#,(syntax-local-value #'t)])) > (define-syntax one 1) > (define-syntax two 2)
> (define-syntax-parameter not-num (make-rename-transformer #'one)) > (test not-num) #<set!-transformer>
> (define-rename-transformer-parameter num (make-rename-transformer #'one)) > (test num) 1
> (syntax-parameterize ([num (make-rename-transformer #'two)]) (test num)) 2
Added in version 6.3.0.14 of package base.
12.5.1 Syntax Parameter Inspection
| (require racket/stxparam-exptime) | package: base |
procedure
(syntax-parameter-value id-stx) → any
id-stx : syntax?
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?
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.