Fix prelude definitions of abs/signum for Floats/Doubles
Simon Peyton-Jones
simonpj at microsoft.com
Thu Apr 11 10:24:26 CEST 2013
Sounds good to me.
It would be fantastic if someone could investigate Levent's suggestion "Of course, implementations can take advantage of the underlying CPU's native floating-point abs/sign functions if available as well, avoiding explicit tests at the Haskell code; based on the underlying platform"
Otherwise we'll just end up adding an extra test and everyone's code will run a little bit slower.
Simon
From: libraries-bounces at haskell.org [mailto:libraries-bounces at haskell.org] On Behalf Of Levent Erkok
Sent: 10 April 2013 19:25
To: libraries at haskell.org
Subject: Fix prelude definitions of abs/signum for Floats/Doubles
The current definition of "abs" and "signum" for the types Float/Double is not IEEE-754 compliant. To wit:
Prelude> abs (-0.0::Float)
-0.0
Prelude> signum (-0.0::Float)
0.0
The correct result should be the other way around; abs returning 0.0 and signum returning -0.0 when they receive a negative-zero. The same also holds for the type Double.
The issue came up several times in several forums, with general consensus that the behavior should match the IEEE-754 specs. Here're three different discussions on this matter:
http://www.haskell.org/pipermail/libraries/2011-January/015761.html
http://stackoverflow.com/questions/10395761/absolute-value-of-negative-zero-bug-or-a-part-of-the-floating-point-standard
http://www.haskell.org/pipermail/haskell-cafe/2013-April/107471.html
Proposed fix: Section 6.4.4 of the report http://www.haskell.org/onlinereport/basic.html#sect6.4 gives "default" definitions for abs/signum; which fails to take into account of negative-zero values for floating-point types. An easy fix would be to add to the report a note on the status of negative-zero for Real/Float instances; with individual implementations explicitly checking for negative-0 first. For instance GHC's implementation can be changed as follows:
instance Num Float where
signum x | isNegativeZero x = x
| x == 0 = 0
| x > 0.0 = 1
| otherwise = negate 1
abs x | isNegativeZero x = 0
| x >= 0 = x
| otherwise = negate x
A similar change would need to be done for the "Num Double" instance as well. Of course, implementations can take advantage of the underlying CPU's native floating-point abs/sign functions if available as well, avoiding explicit tests at the Haskell code; based on the underlying platform.
Library guidelines suggest a discussion period of 2 weeks on this issue. I'm hoping that we can resolve the issue in a timely manner, and at least GHC's implementation can match the desired semantics in the next release. If there's consensus at the end of 2-weeks; I'll go ahead and create a corresponding ticket for GHC.
Thanks,
-Levent.
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