Subject: Re: string-append with evaluation order
From: (Rob Warnock)
Date: 1997/08/19
Newsgroups: comp.lang.scheme
Message-ID: <5tb25i$>

David Fox  <> wrote:
| > I'm looking for a function that mimicks the behavior of string-append, with
| > one extension: the order of evaluation of the expressions passed to the
| > function is defined. They should be evaluated from left to right. Any
| > suggestions?
| The order of evaluation of for-each is guaranteed.  Perhaps this works:
| (define-macro (my-string-append . args)
|   `(let ((strings '()))
|      (for-each (lambda (string) (set! strings (cons string strings)))
| 	       (list ,@args))
|      (apply string-append (reverse strings))))

As written, that doesn't help, unfortunately. The args will be evaluated --
in undetermined ordered -- before the call to "list", which occurs before
the call to "for-each". You need to do the sequencing at macro-expand time,
something like this quick&dirty version:

(define-macro (my-string-append . args)
  (if (null? args)
    (let ((tmp (gensym)))
      `(let ((,tmp ,(car args)))
	 (string-append ,tmp (my-string-append ,@(cdr args)))))))

[And with a little more work in the macro, you can reduce this to only
one call to "string-append", and get rid of the final "" as well...]


Rob Warnock, 7L-551
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