[commit: ghc] master: base: Enable listToMaybe to fuse via foldr/build (4c06ccb)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/4c06ccb71737b77a8165e888ad75417a425549dd/ghc

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

commit 4c06ccb71737b77a8165e888ad75417a425549dd
Author: Douglas Wilson <douglas.wilson at gmail.com>
Date:   Wed Oct 25 19:05:30 2017 -0400

    base: Enable listToMaybe to fuse via foldr/build
    
    Test Plan: Consider whether this is a good idea.
    
    Reviewers: austin, hvr, bgamari, nomeata
    
    Reviewed By: bgamari, nomeata
    
    Subscribers: nomeata, rwbarton, thomie
    
    GHC Trac Issues: #14387
    
    Differential Revision: https://phabricator.haskell.org/D4126


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

4c06ccb71737b77a8165e888ad75417a425549dd
 libraries/base/Data/Maybe.hs | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/libraries/base/Data/Maybe.hs b/libraries/base/Data/Maybe.hs
index d8aad53..5f5d5ac 100644
--- a/libraries/base/Data/Maybe.hs
+++ b/libraries/base/Data/Maybe.hs
@@ -228,9 +228,12 @@ maybeToList  (Just x)  = [x]
 -- >>> maybeToList $ listToMaybe [1,2,3]
 -- [1]
 --
-listToMaybe           :: [a] -> Maybe a
-listToMaybe []        =  Nothing
-listToMaybe (a:_)     =  Just a
+listToMaybe :: [a] -> Maybe a
+listToMaybe = foldr (const . Just) Nothing
+{-# INLINE listToMaybe #-}
+-- We define listToMaybe using foldr so that it can fuse via the foldr/build
+-- rule. See #14387
+
 
 -- | The 'catMaybes' function takes a list of 'Maybe's and returns
 -- a list of all the 'Just' values.