[commit: ghc] master: Make Prelude.abs handle -0.0 correctly (#7858) (6f6ee6e)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/6f6ee6eaa348b1a4815190c4d526d5c81c264fa7/ghc

>---------------------------------------------------------------

commit 6f6ee6eaa348b1a4815190c4d526d5c81c264fa7
Author: Alexander Berntsen <alexander at plaimi.net>
Date:   Mon Aug 18 21:42:12 2014 -0500

    Make Prelude.abs handle -0.0 correctly (#7858)
    
    Summary:
    Make the `Float` and `Double` implementations of `abs` handle -0.0
    correctly per IEEE-754.
    
    abs (-0.0::Float) and abs (-0.0::Double) previously returned -0.0, when
    they should return 0.0. This patch fixes this.
    
    Signed-off-by: Alexander Berntsen <alexander at plaimi.net>
    
    Test Plan: abs (-0.0::Double) should = 0.0 instead of (-0.0)
    
    Reviewers: ekmett, hvr, austin, rwbarton
    
    Reviewed By: austin, rwbarton
    
    Subscribers: phaskell, trofi, simonmar, relrod, ezyang, carter
    
    Differential Revision: https://phabricator.haskell.org/D145
    
    GHC Trac Issues: #7858


>---------------------------------------------------------------

6f6ee6eaa348b1a4815190c4d526d5c81c264fa7
 libraries/base/GHC/Float.lhs | 10 ++++++----
 libraries/base/changelog.md  |  2 ++
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/libraries/base/GHC/Float.lhs b/libraries/base/GHC/Float.lhs
index e0c4f4a..52fc9a9 100644
--- a/libraries/base/GHC/Float.lhs
+++ b/libraries/base/GHC/Float.lhs
@@ -205,8 +205,9 @@ instance  Num Float  where
     (-)         x y     =  minusFloat x y
     negate      x       =  negateFloat x
     (*)         x y     =  timesFloat x y
-    abs x | x >= 0.0    =  x
-          | otherwise   =  negateFloat x
+    abs x    | x == 0    = 0 -- handles (-0.0)
+             | x >  0    = x
+             | otherwise = negateFloat x
     signum x | x == 0.0  = 0
              | x > 0.0   = 1
              | otherwise = negate 1
@@ -370,8 +371,9 @@ instance  Num Double  where
     (-)         x y     =  minusDouble x y
     negate      x       =  negateDouble x
     (*)         x y     =  timesDouble x y
-    abs x | x >= 0.0    =  x
-          | otherwise   =  negateDouble x
+    abs x    | x == 0    = 0 -- handles (-0.0)
+             | x >  0    = x
+             | otherwise = negateDouble x
     signum x | x == 0.0  = 0
              | x > 0.0   = 1
              | otherwise = negate 1
diff --git a/libraries/base/changelog.md b/libraries/base/changelog.md
index 06c9fa5..251ca88 100644
--- a/libraries/base/changelog.md
+++ b/libraries/base/changelog.md
@@ -18,6 +18,8 @@
     enabled, so that the `Monoid` instance for `Proxy` are polykinded
     like `Proxy` itself is.
 
+  * Make `abs` handle (-0.0) correctly per IEEE-754.
+
 ## 4.7.0.1  *Jul 2014*
 
   * Bundled with GHC 7.8.3