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From: lgm@cbnewsc.ATT.COM (lawrence.g.mayka)
Newsgroups: comp.object
Subject: Re: Heterogeneous lists (once more, with feeling!)
Message-ID: <LGM.91Apr17101021@cbnewsc.ATT.COM>
Date: 17 Apr 91 14:10:21 GMT
References: <1594@optima.cs.arizona.edu> <3810@ssc-bee.ssc-vax.UUCP>
	<1989:Apr1206:29:5291@kramden.acf.nyu.edu>
	<1991Apr14.211215.1818@visix.com>
Sender: lgm@cbnewsc.att.com (lawrence.g.mayka)
Organization: AT&T Bell Laboratories
Lines: 30
In-Reply-To: adam@visix.com's message of 14 Apr 91 21:12:15 GMT

In article <1991Apr14.211215.1818@visix.com> adam@visix.com writes:
   The MAPCAR example is from the more heterogeneous side, in that
   the only thing the objects necessarily have in common is
   "they all have the operation identified by parameter X, which
   takes as argument the object itself and returns another object"
   (not even that if you want MAPCAR to catch failures!)

   You could, of course, build a solution in C around that fact,
   creating a list of anonymous monadic functions for each application
   of MAPCAR.

Actually, Common Lisp MAPCAR is more flexible than this.  It takes one
*or more* lists as arguments, and applies the given operation to
corresponding elements of those lists.  Moreover, the operation itself
does not necessarily take a fixed number of arguments, either.  Thus:

(MAPCAR #'LIST '(A B)) => ((A) (B))

(MAPCAR #'LIST '(A B) '(C D)) => ((A C) (B D))

(MAPCAR #'LIST '(A B) '(C D) '(E F)) => ((A C E) (B D F))

MAPCAR is indeed a reusable abstraction.


	Lawrence G. Mayka
	AT&T Bell Laboratories
	lgm@iexist.att.com

Standard disclaimer.