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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">Sounds good to me.
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">It would be fantastic if someone could investigate Levent’s suggestion “</span><span style="font-family:"Courier New"">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</span><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">”
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">Otherwise we’ll just end up adding an extra test and everyone’s code will run a little bit slower.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> libraries-bounces@haskell.org [mailto:libraries-bounces@haskell.org]
<b>On Behalf Of </b>Levent Erkok<br>
<b>Sent:</b> 10 April 2013 19:25<br>
<b>To:</b> libraries@haskell.org<br>
<b>Subject:</b> Fix prelude definitions of abs/signum for Floats/Doubles<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">The current definition of "abs" and "signum" for the types Float/Double is not IEEE-754 compliant. To wit:</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> Prelude> abs (-0.0::Float)</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> -0.0</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> Prelude> signum (-0.0::Float)</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> 0.0</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">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.</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">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:</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> <a href="http://www.haskell.org/pipermail/libraries/2011-January/015761.html">http://www.haskell.org/pipermail/libraries/2011-January/015761.html</a></span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> <a href="http://stackoverflow.com/questions/10395761/absolute-value-of-negative-zero-bug-or-a-part-of-the-floating-point-standard">http://stackoverflow.com/questions/10395761/absolute-value-of-negative-zero-bug-or-a-part-of-the-floating-point-standard</a></span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> <a href="http://www.haskell.org/pipermail/haskell-cafe/2013-April/107471.html">http://www.haskell.org/pipermail/haskell-cafe/2013-April/107471.html</a></span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">Proposed fix: Section 6.4.4 of the report <a href="http://www.haskell.org/onlinereport/basic.html#sect6.4">http://www.haskell.org/onlinereport/basic.html#sect6.4</a> 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:</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">instance Num Float where</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> signum x | isNegativeZero x = x</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> | x == 0 = 0</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> | x > 0.0 = 1</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> | otherwise = negate 1</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> abs x | isNegativeZero x = 0</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> | x >= 0 = x</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> | otherwise = negate x</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">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.</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">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.</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">Thanks,</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">-Levent.</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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