[racket] quote-syntax in macro-generator

From: Jon Rafkind (rafkind at cs.utah.edu)
Date: Sat Jun 2 00:29:16 EDT 2012

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

  (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)
         ([make (lambda (stx)
                  (syntax-parse stx
                       (make-transformer name pattern action))]))])

(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)
> ____________________
>   Racket Users list:
>   http://lists.racket-lang.org/users

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.racket-lang.org/users/archive/attachments/20120601/daa65233/attachment-0001.html>

Posted on the users mailing list.