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@haskell.org [mailto:libraries-bounces@haskell.org]
On Behalf Of Levent Erkok
Sent: 10 April 2013 19:25
To: libraries@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:
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.