takeWhile does not participate in list fusion

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.

{-# 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

Trac metadata

Trac field	Value
Version	7.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
Test case
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CC	ekmett, hvr
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