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Implementation of MAP, SOME, EVERY, etc...
- To: Moon%SCRC-STONY-BROOK.ARPA@GODOT, cork%umass-cs.csnet%CSNET-RELAY.ARPA@GODOT
- Subject: Implementation of MAP, SOME, EVERY, etc...
- From: Guy Steele <godot!gls@cca-unix>
- Date: Mon, 01 Apr 1985 17:39:00 -0000
- Cc: common-lisp%SU-AI.ARPA@GODOT, gls@AQUINAS
- In-reply-to: <850327161712.2.MOON@EUPHRATES.SCRC.Symbolics.COM>
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Date: Wed, 27 Mar 85 16:17 EST
From: David A. Moon <cca!Moon%SCRC-STONY-BROOK.ARPA@godot>
Subject: Implementation of MAP, SOME, EVERY, etc...
To: Dan Corkill <cork%umass-cs.csnet%CSNET-RELAY.ARPA@godot>
In-Reply-To: The message of 19 Mar 85 11:58-EST from Dan Corkill <email@example.com>
Comment: Message remailed after delay caused by distribution list error.
Date: Tue, 19 Mar 85 11:58 EST
From: Dan Corkill <firstname.lastname@example.org>
There is an easily made implementation error in MAP, SOME, EVERY, etc.
The published semantics indicate that these functions use the length of
the shortest argument sequence to determine how many iterations to
perform (and in this way parallel the semantics of mapc, mapcar, etc.).
Thus it would appear that:
(map 'list '+ '(1 2 3) (star 2))
where star builds a circular list of its argument should be legal and
return => (3 4 5).
Implementations that use length to check the length of argument sequences
will break on such a call.
The Common Lisp LENGTH function returns NIL when given a circular list.
From the Common Lisp manual, page 265:
"LIST-LENGTH differs from LENGTH when the list is circular;
LENGTH may fail to return, whereas LIST-LENGTH will return NIL."