#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): I'm using ghc 8.4.3, and compiling your example exactly with -O2 produces no difference whether i compile with '-fpedantic-bottoms' or '-fno- pedantic-bottoms' produce the same core. {{{ module Foo (foo) where (#) :: (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) }}} the core shows `foo = map`. This is in contrast to @nomeata's comment from 5 years ago, where compilation with -fpedantic-bottoms is necessary. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7542#comment:24 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler