#9344: takeWhile does not participate in list fusion -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: | Version: 7.8.3 libraries/base | Keywords: Resolution: | Operating System: Unknown/Multiple Differential Revisions: | Type of failure: Runtime Architecture: | performance bug Unknown/Multiple | Test Case: Difficulty: Unknown | Blocking: Blocked By: | Related Tickets: | -------------------------------------+------------------------------------- Comment (by dfeuer): I wouldn't call it "failing" so much as "incomplete success". I haven't investigated in detail, and may not be good enough with Core to figure it out anyway. The simple example {{{ #!haskell print $ length $ takeWhileFB (<10000000) [1..20000000] }}} allocates virtually nothing. The more complex example {{{ #!haskell print $ length $ filter (\x->x `rem` 3 == 0) $ takeWhileFB (<10000000) $ filter even [1..20000000] }}} allocates a substantial amount, but only something like a third of what `Prelude.takeWhile` does. I haven't attempted any benchmarking, and I'm too tired right now to look into those test cases. As for speed, if I'm not very much mistaken it takes a lot of slowness and/or a good number of mispredicted branches to make up for the cache effects that excessive allocation will have when combined with non-trivial code, so I believe that should probably be a secondary concern. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9344#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler