Path: utzoo!attcan!uunet!snorkelwacker!ai-lab!zurich.ai.mit.edu!markf
From: markf@zurich.ai.mit.edu (Mark Friedman)
Newsgroups: comp.lang.scheme
Subject: Re: Scheme is unnecessarily biased towards lists
Message-ID: <MARKF.90Sep20105627@montreux.ai.mit.edu>
Date: 20 Sep 90 14:56:27 GMT
References: <9009200822.AA02949@vis.>
Sender: news@ai.mit.edu
Reply-To: markf@zurich.ai.mit.edu
Organization: M.I.T. Artificial Intelligence Lab.
Lines: 56
In-reply-to: greg@vis.UUCP's message of 20 Sep 90 08:22:19 GMT

In article <9009200822.AA02949@vis.> greg@vis.UUCP writes:

   [Scheme] should replace the generic arithmetic operators with
   type-specific ones and provide a generic dispatch mechanism (e.g.,
   OO messaging system) to solve the general problem.  BTW, has anyone
   else find that Scheme's generic operators actually make adding new
   numeric types more difficult?

What is difficult or not general about the following:

(set! + (lambda addends
           (if (my-new-numeric-types? addends)
               (apply my-new-numeric-type+ addends)
               (apply + addends))))

With macros you could abstract the mechanism with something like:

(define-macro (add-new-numeric-type type-name)
  (let ((symbol-append (lambda (sym1 sym2)
			 (string->symbol
			  (string-append
			   (symbol->string sym1)
			   (symbol->string sym2))))))
    (let ((type-predicate-name (symbol-append type-name '?))
	  (addition-proc-name (symbol-append type-name '+))
	  (multiplication-proc-name (symbol-append type-name '*))
	  ...)
      `(begin
	 (set! + (lambda addends
		   (if (,type-predicate-name addends)
		       (apply ,addition-proc-name addends)
		       (apply + addends))))
	 (set! * ...)
	 ...))))

He (greg@vis.UUCP) also writes:

   I've always wondered why vectors are not on a more equal footing
   with lists.  It seems like pure prejudice to me.  Why is (+ 2 2)
   O.K.  but #(+ 2 2) is not?

They are both O.K.  It happens that most interpretors use an evaluator
that treats a list as denoting a combination and applies the car of
the list to the cdr.  It doesn't have to be this way and I don't think
that there is anything in the scheme standards that legislates this
behavior.

-Mark
--

Mark Friedman
MIT Artificial Intelligence Lab
545 Technology Sq.
Cambridge, Ma. 02139

markf@zurich.ai.mit.edu