[r6rs-discuss] [Formal] It is not clear what negating 0.0 produces

[Felix Klock]

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Name: Felix Klock
Email: pfr6rs at pnkfx.org
type of issue: Enhancement
priority: Trivial
R6RS component: Arithmetic
version of the report: 5.91
summary: It is not clear what negating -0.0 produces (if it is even  
specified)
description:

(FYI this whole comment is modulo the assumption that we're talking  
about an implementation of R6RS that uses IEEE binary floating point  
numbers.)

The specification of - says on page 40 that when - is applied to a  
single argument, it returns the additive inverse of its argument.   
Likewise, the specification of fl- says on page 101 that when fl- is  
applied to a single argument, it returns the additive flonum inverse  
of its argument.

I am not sure that these descriptions uniquely specify the value of  
(- 0.0) or (fl- 0.0).  In particular, aren't 0.0 and -0.0 both  
additive inverses of 0.0?  That is, for any number x, (= x (+ x 0.0  
-0.0) (+ x 0.0 0.0)), right?

I do not believe there is an obviously correct choice between 0.0 and  
-0.0.  In support of this statement, here is an informal tally of  
Scheme system behaviors (a * means that particular scheme  
implementation does not yield negative infinity from (/ 1.0 (* -1.0  
0.0)), and therefore there may be no way to actually observe a -0.0):

(- 0.0) is -0.0
MzScheme
Chicken*
Gambit
Chez
Kawa

(- 0.0) is 0.0
Larceny
Scheme48*
Bigloo
SCM*
MIT Scheme*

(Corrections to the above tally are welcome.)

If R6RS does specify whether (- 0.0) produces -0.0 or 0.0, there is  
some support for choosing -0.0 given in Goldberg [1], where he says  
that the identity "-x = 0 - x fails for x = +0.".

Perhaps the choice between the two possible results is meant to be  
left unspecified.  But even then I think it should be explicitly  
stated as being unspecified, in the same manner that (fllog -0.0) is  
given as an example producing an unspecified

Proposals for how to fix the issue:
Either:
1. Specify that (fl- 0.0) produces -0.0 and (fl- -0.0) produces 0.0,  
and likewise for -.
2. Specify that (fl- 0.0) produces -0.0 and (fl- -0.0) produces 0.0.   
For (- 0.0) and (- -0.0), explicitly state that the result is either  
-0.0 or 0.0, but the choice is otherwise unspecified.
3. For both fl- and -, explicitly state that the result is either  
-0.0 or 0.0, but the choice is otherwise unspecified.

-Felix

[1] "What Every Computer Scientist Should Know About Floating-Point  
Arithmetic"
     http://docs.sun.com/source/806-3568/ncg_goldberg.html