Path: utzoo!attcan!uunet!husc6!rice!leto!matthias
From: matthias@leto.rice.edu (Matthias Felleisen)
Newsgroups: comp.lang.scheme
Subject: Re: collect special form for streams
Message-ID: <748@thalia.rice.edu>
Date: 1 Jun 88 19:09:26 GMT
References: <294@gt-eedsp.UUCP> <50472@ti-csl.CSNET>
Sender: usenet@rice.edu
Reply-To: matthias@rice.edu (Matthias Felleisen)
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Organization: Rice University, Houston
Lines: 43

In article <50472@ti-csl.CSNET> gateley@mips.UUCP (John Gateley) writes:
>In article <294@gt-eedsp.UUCP> schw@gt-eedsp.UUCP (Dave Schwartz) writes:
>>How can the "collect" special form for streams (from Chap. 3 of
>>Structure and Interpretation of Computer Programs) be implemented for
>>TI PC-Scheme? I have Kent Dybvig's "extend-syntax," but this does not
>>appear to be powerful enough for this special form.
>
>Extend-syntax's pattern matching is not powerful enough to do this.
>You have to use the "with" feature to generate the v functions. (by calling
>a function which builds them, i.e. you do them by hand). I also used a
>help macro to make the flatmap parts of the expression (watch out for
>keywords). If this is not enough help, email me and I will send you my code.
>
>John Gateley
>gateley@tilde.csc.ti.com

John, what do you think of this one?  

(syntax
 (collect <result> ((<v1> <set1>) ...) <restriction>)
 (map (lambda (tuple)
	(help-res (<v1> ...) <result>))
      (filter (lambda (tuple)
		(help-res (<v1> ...) <restirction>))
	      (flat-help ((<v1> <set1>) ...) (list <v1> ...)))))

(extend-syntax (help-res)
  [(help-res (<v1> <v2> ...) <res>)
   (let ((<v1> (car tuple)) (tuple (cdr tuple)))
     (help-res (<v2> ...) <res>))]
  [(help-res () <res>) <res>])

(extend-syntax (flat-help)
  [(flat-help ((<v1> <set1>) (<v2> <set2>) (<v3> <set3>) ...) last)
   (flatmap (lambda (<v1>)
	      (flat-help ((<v2> <set2>) (<v3> <set3>) ...) last))
	    <set1>)]
  [(flat-help ((<v1> <set1>)) last)
   (map (lambda (<v1>) last) <set1>)])

-- Matthias 

P.S. Eugene Kohlbecker invented extend-syntax.