CODEWITHSUNDEEP
schemeracket
user618815
user618815

Reputation:

Why can't this macro execute?

I have the following dumb test:

(define-syntax a
 (lambda (stx)
   (syntax-case stx ()
     [(k e s) #'(let ((show display)) (e s))])))
(a show "something")

Why can't this work? (The error shows in DrRacket is expand: unbound identifier in module in: show.

However, The following can work:

(define-syntax a
 (lambda (stx)
   (syntax-case stx ()
     [(k e s) 
      (with-syntax ((show (datum->syntax #'k 'show)))
        #'(let ((show display)) (e s)))])))
(a show "something")

Then WHY?

Upvotes: 0

Views: 218

Answers (1)

Eli Barzilay
Eli Barzilay

Reputation: 29546

Um, I assume that you're trying this out after reading the blog post I mentioned in an earlier answer -- but that blog post is explaining exactly this issue. Specifically, your first example has two different show identifiers, one that is bound by the macro, and a different one that is coming from the toplevel use (and is unbound). OTOH, in the second case you're creating a show with the lexical context of the user code.

Upvotes: 2

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