takeWhile&C. still not fusible

takeWhile is not still fusible, 2 1/2 years after this report:

http://www.haskell.org/pipermail/glasgow-haskell-users/2011-December/021299.html

The discussion suggests making takeWhile a good producer/consumer with foldr/build fusion:

takeWhile' :: (a -> Bool) -> [a] -> [a] takeWhile' p xs = build $ \c n -> foldr (takeWhileF p c n) n xs {-# INLINE takeWhile' #-}

takeWhileF p c n x xs = if p x then x c xs else n

Furthermore, the discussion suggests having rewrite rules to go to this version and then rewrite back (if fusion fails).

The report also mentions concatMap (which is a separate known problem). It also mentions drop and dropWhile, but I don't see when they perform allocations, so I think that's an erroneous request.

I experienced the bug on GHC 7.6.3, but it seems still there in base-4.7.0.0, judging from the source: http://hackage.haskell.org/package/base-4.7.0.0/docs/src/GHC-List.html#takeWhile

Trac metadata

Trac field	Value
Version	7.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
Test case
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CC	ekmett, hvr
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