[GHC] #9344: takeWhile does not participate in list fusion

22 Jul 2014


      #9344: takeWhile does not participate in list fusion
-------------------------------------+-------------------------------------
        Reporter:  dfeuer            |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  libraries/base    |                 Version:  7.8.3
        Keywords:                    |  Differential Revisions:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |              Difficulty:  Unknown
       Test Case:                    |              Blocked By:
        Blocking:                    |         Related Tickets:
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 `takeWhile` doesn't do the list fusion thing. This alternative definition
 seems to fix that, at least to a great extent. It fused completely in a
 simple test, and incompletely but still usefully in a more complex one. I
 don't know how to write the appropriate translate/untranslate RULES for it
 yet.

 {{{
 #!haskell
 {-# LANGUAGE ScopedTypeVariables #-}

 takeWhileFB :: forall a . (a -> Bool) -> [a] -> [a]
 takeWhileFB p xs = build tw'
   where
     tw' :: forall b . (a -> b -> b) -> b -> b
     tw' kons knil = foldr go knil xs
       where
         go x rest | p x       = x `kons` rest
                   | otherwise = knil

 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9344
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