[plt-scheme] Define-syntax and #hash - ugly compiling behavior

You mean the following?

#lang scheme
(define-syntax (my-hash stx)
 (syntax-case stx ()
  ((_ (key . val) ...)
   (let
    ((keys (map syntax->datum (syntax->list #'(key ...))))
     (vals (map syntax->datum (syntax->list #'(val ...)))))
    ; make the hash-table at expansion time!
    (datum->syntax #'here (make-immutable-hash (map cons keys vals)))))))

; tests:
(define h (my-hash (a . 1) (b . 2) (c . 3)))
(hash-map h list) ; --> ((a 1) (b 2) (c 3))
(hash-ref h 'a) ; 1

Best wishes, Jos.
  ----- Original Message ----- 
  From: Patrick Steiger 
  To: plt-scheme at list.cs.brown.edu 
  Sent: Monday, September 14, 2009 5:39 AM
  Subject: [plt-scheme] Define-syntax and #hash - ugly compiling behavior


  Hello, all. This is my first post on the list, I'm a CS student and I've been giving Scheme a shot. (liking it a lot, so far).

  Now, given the following code:

  (define-syntax myhash
    (syntax-rules (->)
      [(_ (x -> y) ...)
       #hash((x . y) ...)]))

  When trying to compile it, I get:

  . read: expected `(' or `[' or `{' to start a hash pair

  I was told that this happens because the compiler recognizes #hash as a hash-table long before (in the lexical analysis?) knowing that it is inside a (define-syntax), and therefore doesn't get the semantics of '...' - it just interprets it as illegal syntax.

  This seems like a major wart with the language. What are your opinions?

  -- 
  Patrick Steiger.

  "Theory is when you know something, but it doesn't work. Practice is
  when something works, but you don't know why.
  Programmers combine theory and practice: Nothing works and they don't
  know why." - Anon 



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