Another thing about this, if the definition produced by the macro is instead
generated by the `make-transformer' macro then it will have the wrong number of
marks on it when everything is finally expanded. `syntax-local-introduce' fixes
this by essentially removing that extra mark.
#lang racket
(require (for-meta 0 racket/splicing)
(for-meta 1 syntax/parse)
(for-meta 2 syntax/parse
racket/base))
(begin-for-syntax
(define-syntax (make-transformer stx)
(syntax-parse stx
[(_ name (pattern ...) action)
#'(quote-syntax (define-syntax name
(lambda (stx) ;; HERE change to #'(quote-syntax
(syntax-parse stx
[(_ pattern ...) action]))))])))
(define-syntax (macro-generator stx)
(syntax-parse stx
[(_ name pattern action)
#'(splicing-let-syntax
([make (lambda (stx)
(syntax-parse stx
[(_)
(syntax-local-introduce
(make-transformer name pattern action))]))])
(make))]))
(macro-generator foo (x ...) #'(list x ...))
(foo 1 2 3)
The `name' passed as an argument to `make-transformer' will not have the mark
applied to it by the transformer in the splicing-let-syntax, so the resulting
define-syntax form will use a different 'foo' from the one passed to
`macro-generator'. Using `syntax-local-introduce' applies the mark that was
used when `(make)' was expanded thus removing all marks from the final
expansion.
On 06/01/2012 12:16 PM, Jon Rafkind wrote:
> Today's macro PSA is about macro-generating-macro forms. If you use a macro
> to generate a transformer then you must be careful about nesting `syntax'
> forms otherwise you will interpolate templates too many times. Specifically
> #'#'x will interpolate x twice. Given the following bindings:
>
> chicken = (syntax (egg ...))
> pattern = (syntax chicken)
>
> The problem is if an additional `syntax' is wrapped around `pattern'. After
> one interpolation of `syntax' we will have
>
> (syntax (egg ...))
>
> At which point `egg' had better be bound as a pattern variable otherwise
> interpolation will fail.
>
> To work around this problem the `quote-syntax' form should be used:
>
> #'(quote-syntax pattern)
>
> which prevents the (syntax (egg ...)) layer from being interpolated because
> `quote-syntax' does not do interpolation.
>
> The following example demonstrates the issue. On the line 'HERE' the inner
> syntax should be `quote-syntax' otherwise the following error is produced
>
> x.rkt:27:24: syntax: no pattern variables before ellipses in template at:
> ... in: (_ x ...)
>
> To be fair this issue only comes up because I used `make-transformer' in the
> body of a phase 2 function so the output of `make-transformer' had to have
> two levels of syntax, because its a macro that should ultimately evaluate to
> a syntax form. A simpler solution would have been to do
>
> #'(define-syntax name (make-transformer pattern action))
>
> So that `make-transformer' only had to return one level of syntax. But anyway
> I got into this situation for other reasons.
>
> #lang racket
>
> (require (for-meta 0 racket/splicing)
> (for-meta 1 syntax/parse)
> (for-meta 2 syntax/parse
> racket/base))
>
> (begin-for-syntax
> (define-syntax (make-transformer stx)
> (syntax-parse stx
> [(_ (pattern ...) action)
> #'#'(lambda (stx) ;; HERE change to #'(quote-syntax
> (syntax-parse stx
> [(_ pattern ...) action]))])))
>
> (define-syntax (macro-generator stx)
> (syntax-parse stx
> [(_ name pattern action)
> #'(splicing-let-syntax
> ([make (lambda (stx)
> (syntax-parse stx
> [(_ name)
> (with-syntax ([transformer (make-transformer pattern
> action)])
> #'(define-syntax name transformer))]))])
> (make name))]))
>
> (macro-generator foo (x ...) #'(list x ...))
> (foo 1 2 3)
>
>
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