#9345: Data.List.inits is extremely slow -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.8.4 Component: | Version: 7.8.3 libraries/base | Keywords: Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Easy (less than 1 Unknown/Multiple | hour) Type of failure: Runtime | Blocked By: performance bug | Related Tickets: Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by dfeuer): I've worked on it a bit, and if I'm not mistaken, `myScanl'` is safe if, whenever `a /= _|_`, and however I choose `evil`, `wrong`, and `f`, the "correct" expression {{{ e1 = a `evil` foldr evil wrong $ foldr (\b g x -> (let b' = f x b in b' `seq` (b' : g b'))) (\b -> b `seq` []) bs a }}} gives the same result as the fused expression {{{ e2 = a `evil` foldr (\b g x -> (let b' = f x b in b' `seq` (b' `evil` g b'))) (\b -> b `seq` wrong) bs a }}} But I'm not sure how to prove this or find a counterexample. Any ideas? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9345#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler