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

[GHC]

#9344: takeWhile does not participate in list fusion
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              Reporter:  dfeuer      |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:              |          Version:  7.8.3
  libraries/base                     |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:  Runtime     |  Related Tickets:
  performance bug                    |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by dfeuer):

 I messed around with RULES a bit, and the following seem to work okay for
 starters. Some more rules will be needed to make map/takeWhile and
 takeWhile/map and whatever else do the right thing. One thing I'm not too
 clear about: what's the advantage of the explicitly recursive default
 definition over `takeWhile p = foldr (\x r -> if p x then x:r else [])
 []`?

 {{{
 {-# INLINE [0] takeWhileFB #-}
 takeWhileFB :: (elt -> lst -> lst) -> lst -> (elt -> Bool) -> elt -> lst
 -> lst
 takeWhileFB kons knil p = \x rest -> if p x then x `kons` rest else knil

 {-# NOINLINE [1] takeWhile #-}
 takeWhile :: (a -> Bool) -> [a] -> [a]
 takeWhile _ []          =  []
 takeWhile p (x:xs)
             | p x       =  x : takeWhile p xs
             | otherwise =  []


 {-# RULES
 "takeWhile"     [~1] forall p xs. takeWhile p xs = build (\kons knil ->
 foldr (takeWhileFB kons knil p) knil xs)
 "takeWhileList" [1]  forall p.    foldr (takeWhileFB (:) [] p) [] =
 takeWhile p
 "takeWhileFB"        forall kons knil p q. takeWhileFB (takeWhileFB kons
 knil p) knil q =
                         \x rest -> if (q x && p x) then x `kons` rest else
 knil
  #-}
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9344#comment:5>
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