Hi, Am Montag, den 12.12.2011, 15:37 -0500 schrieb wren ng thornton:
I've noticed that take and filter are good producers (and consumers) for list fusion, but takeWhile, drop, and dropWhile are not. Is there any reason for this discrepancy?
If not, would I need to go through the libraries@ process for fixing it, or should I just submit a patch?
while preparing my guest lecture¹ about Haskell Performance recently I also noticed that takeWhile (and concatMap!) are not setup for list fusion, here is what I showed the students; it improved performance considerably in the testcase. 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 Of course, for a proper inclusion one first has to find out if takeWhile' is sufficiently fast even when not fused, or whether one has to do the „replace first, try to fuse, and then try to replace back by original function if not fused“ tick that is used for, e.g., (++). But yes, you are right that all these function ought to be fusable. Greetings, Joachim ¹ http://www.joachim-breitner.de/blog/archives/539-Guest-lecture-on-Haskell-pe... -- Joachim "nomeata" Breitner mail@joachim-breitner.de | nomeata@debian.org | GPG: 0x4743206C xmpp: nomeata@joachim-breitner.de | http://www.joachim-breitner.de/