#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: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: Runtime | Related Tickets: performance bug | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by nomeata):
One thing I'm not too clear about: what's the advantage of the explicitly recursive default definition over takeWhile p = foldr (\x r -> if p x then x:r else []) []
Nothing per se, unless benchmarking shows that the explicit version is faster, I guess. It might even be that simply {{{ takeWhile p xs = build (\kons knil -> foldr (takeWhileFB kons knil p) knil xs) {-# INLINEABLE takeWhile #-} }}} is good enough, assuming the non-inlined `takeWhile` gets compiled to good code (i.e. `build`, `foldr` and `takeWhileFB` are inlined). But I don’t have much experiences to predict that, so you’ll have to experiment and look at core to find out -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9344#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler