Path: utzoo!mnetor!uunet!lll-winken!lll-tis!mordor!sri-spam!ames!ll-xn!oberon!sdcrdcf!sdcsmb!sea!eggert
From: eggert@sea.sm.unisys.com (Paul Eggert)
Newsgroups: comp.lang.prolog
Subject: Re: Arithmetic problems with Quintus Prolog
Message-ID: <#a?GP4|0@sea.sm.unisys.com>
Date: 14 Apr 88 21:56:49 GMT
References: <873@cresswell.quintus.UUCP>
Organization: Unisys Santa Monica
Lines: 20

In article <873@cresswell.quintus.UUCP> ok@quintus.UUCP (Richard A. O'Keefe)
argues that a Prolog implementation cannot obey the IEEE floating point
standard while following the proposed rule "If a number can be exactly
represented in both floating point and integer formats, then use integer
format."  His argument has one crucial flaw that invalidates its conclusion.
O'Keefe writes:

	(5) For any integer N for which all expressions are defined, if
		ZI is N-N,
		NF is float(N),
		ZF is NF-NF
	    then ZF is distinguishable from -ZF by IEEE rules, but ZI is not
	    distinguishable from -ZI.

Not so.  Under the proposed rule, "N is -0" unifies N with -0.0, not with 0,
so ZI can be distinguished from -ZI.

I don't argue that the proposed rule should be part of a Prolog standard,
because it would slow many Prologs down.  But I maintain that the rule is
consistent, that the rule conforms to the IEEE standard, and that it is more
intuitive for "1 = 1.0" to succeed than to fail.