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advice on how to use syntax transformers

From: Federico Beffa
Subject: advice on how to use syntax transformers
Date: Fri, 6 Mar 2015 15:51:46 +0100


I'm writing to ask for help in understanding syntax
transformers. Specifically, I'm trying to construct a function of the
following form

(define (key->value meta)
  (match meta
    (() '())
    (((("name") value) rest ...)
    (((k value) rest ...)
     (key->value (cdr meta)))
    (_ "key Not fount")))

where I would like the ability to program the actual form of the ("name")
pattern with a second function argument.  I've tried with the following

(define (key->value meta key)
  (define-syntax match-key
    ;;(let ((key '("name")))
    (lambda (x)
      (syntax-case x ()
        ((_ e
            (((k v) r ...) b ...) c ...)
         #`(match e
             (((#,key v) r ...)
             (((k v) r ...) b ...) c ...)))));)

  (match-key meta
             (((k v) r ...)
              (key->value (cdr meta) key))
             (_ "key not fount")))

which however, doesn't work and gives the following error:

ice-9/psyntax.scm:1274:12: In procedure #<procedure 29405a0 (val key m)>:
ice-9/psyntax.scm:1274:12: Syntax error:
unknown location: reference to identifier outside its scope in form key

In ice-9/psyntax.scm:
  1274:12  0 (#<procedure 29405a0 (val key m)> #(syntax-object key
((top) #(# …) …) …) …)

If however I make key a local variable with the commented let expression, then
it works (but I loose the programming feature that I'm trying to implement).

I would greatly appreciate an hint on why the above is incorrect and what is
the correct form.

Thanks you in advance for your help.

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