Fix prelude definitions of abs/signum for Floats/Doubles

[David Luposchainsky]

+1 from me. Code breaks are unlikely, because people who stumbled over
this probably patched it up rock solid.

On 2013-04-10 21:13, Carter Schonwald wrote:
> +1, would be good for me
> 
> 
> On Wed, Apr 10, 2013 at 2:27 PM, Felipe Almeida Lessa
> <felipe.lessa at gmail.com <mailto:felipe.lessa at gmail.com>> wrote:
> 
>     +1
> 
>     On Wed, Apr 10, 2013 at 3:24 PM, Levent Erkok <erkokl at gmail.com
>     <mailto:erkokl at gmail.com>> wrote:
>     > 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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> 
> 
> 
>     --
>     Felipe.
> 
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