#7542: GHC doesn't optimize (strict) composition with id --------------------------------------------+------------------------------ Reporter: shachaf | Owner: simonpj Type: bug | Status: new Priority: normal | Milestone: 7.8.1 Component: Compiler | Version: 7.6.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime performance bug | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: --------------------------------------------+------------------------------ Comment (by nomeata): The simple example from the ticket: {{{ (#) :: (b -> c) -> (a -> b) -> a -> c (#) f g = f `seq` g `seq` \x -> f (g x) foo :: (a -> b) -> [a] -> [b] foo f = map (id # f) }}} produces {{{ T7542.foo = \ (@ a) (@ b) (f :: a -> b) (eta :: [a]) -> GHC.Base.map @ a @ b (\ (eta1 :: a) -> f eta1) eta }}} without `-fpedantic-bottoms` and {{{ T7542.foo = GHC.Base.map }}} with, which looks fine. The same happens to the more elaborate example in comment:2 (middle section), `-fpedantic-bottoms` preserves the semantics and with that we get `map2 = map`. So it seems that situation has improved and we can close the ticket (which should not discourage anyone who finds a situation that needs fixing to open it again). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7542#comment:11 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler