[commit: ghc] master: Make findIndices fuse (ef2d027)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/ef2d027917ca7e5415ba5e9f3ff439beda89b3ea/ghc

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

commit ef2d027917ca7e5415ba5e9f3ff439beda89b3ea
Author: David Feuer <David.Feuer at gmail.com>
Date:   Tue Oct 21 15:01:26 2014 -0500

    Make findIndices fuse
    
    Summary:
    Steal the findIndices implementation from Data.Sequence, that can
    participate in fold/build fusion
    
    Reviewers: nomeata, austin
    
    Reviewed By: nomeata, austin
    
    Subscribers: thomie, carter, ezyang, simonmar
    
    Differential Revision: https://phabricator.haskell.org/D345


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

ef2d027917ca7e5415ba5e9f3ff439beda89b3ea
 libraries/base/Data/OldList.hs | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/libraries/base/Data/OldList.hs b/libraries/base/Data/OldList.hs
index ff85154..0e6709e 100644
--- a/libraries/base/Data/OldList.hs
+++ b/libraries/base/Data/OldList.hs
@@ -277,12 +277,12 @@ findIndices      :: (a -> Bool) -> [a] -> [Int]
 #ifdef USE_REPORT_PRELUDE
 findIndices p xs = [ i | (x,i) <- zip xs [0..], p x]
 #else
--- Efficient definition
-findIndices p ls = loop 0# ls
-                 where
-                   loop _ [] = []
-                   loop n (x:xs) | p x       = I# n : loop (n +# 1#) xs
-                                 | otherwise = loop (n +# 1#) xs
+-- Efficient definition, adapted from Data.Sequence
+{-# INLINE findIndices #-}
+findIndices p ls = build $ \c n ->
+  let go x r k | p x       = I# k `c` r (k +# 1#)
+               | otherwise = r (k +# 1#)
+  in foldr go (\_ -> n) ls 0#
 #endif  /* USE_REPORT_PRELUDE */
 
 -- | The 'isPrefixOf' function takes two lists and returns 'True'