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Re: Equality predicates, signed zeroes, R5RS and R6RS

From: Mike Gran
Subject: Re: Equality predicates, signed zeroes, R5RS and R6RS
Date: Mon, 31 Jan 2011 13:36:48 -0800 (PST)

> From:Mark H Weaver <address@hidden>
> To:address@hidden
> Cc:Andy Wingo <address@hidden>; Taylor R Campbell <address@hidden>
> Sent:Monday, January 31, 2011 11:53 AM
> Subject:Equality predicates, signed zeroes, R5RS and R6RS
> An issue has come to my attention that deserves wider discussion.
> Since at least Guile 1.8, (= 0.0 -0.0) has returned #t
> but (eqv? 0.0 -0.0) has returned #f, and this is still the case.
> PLT Scheme agrees with us that (eqv? 0.0 -0.0) is #f, but MIT/GNU Scheme,
> SCM, Chicken, and Gauche all return #t in this case.

As an aside, as of a couple of months ago, it was possible to create
the following seven versions of zero in Guile using the default reader.

0, ±0.0, ±0.0±0.0i 

Note that there is no way to make negative zero integer (-0).  And
there are no zero rationals:  ±0/2, for example.

Each scheme has its quirks.

MzScheme 4.1 would let you create complex numbers with integer representation
real and float representation imaginary parts: 0+0.0i, for example. The integer
parts can only be positive zero and the float parts can be ±0.0.

But as a physicist, I declare that this is all madness. 


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