#7542: GHC doesn't optimize (strict) composition with id -------------------------------------+------------------------------------- Reporter: shachaf | Owner: simonpj Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by chessai): Ah, you're right. look at this: {{{ module Foo (foo) where data List a = Nil | Cons a (List a) mapList :: (a -> b) -> List a -> List b mapList f Nil = Nil mapList f (Cons a l) = Cons (f a) (mapList f l) (#) :: (b -> c) -> (a -> b) -> a -> c (#) f g = f `seq` g `seq` \x -> f (g x) foo :: (a -> b) -> List a -> List b foo f = mapList (id # f) xs }}} The core i get looks like: {{{ mapList mapList = \ @ a_avr @ b_avs f_atJ ds_d10t -> case ds_d10t of { Nil -> Nil; Cons a1_atL l_atM -> Cons (f_atJ a1_atL) (mapList f_atJ l_atM) foo = \ @ a_avy @ b_avz f_atQ xs_atR -> mapList f_atQ xs_atR }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7542#comment:27 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler