#9132: takeWhile&C. still not fusible -------------------------------------+------------------------------------- Reporter: Blaisorblade | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: libraries/base | Version: 7.8.2 Keywords: fusion | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: Runtime Difficulty: Moderate (less | performance bug than a day) | Test Case: Blocked By: | Blocking: Related Tickets: | -------------------------------------+------------------------------------- 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 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9132 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler