#13153: Several Traversable instances have an extra fmap -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: dfeuer Type: bug | Status: new Priority: normal | Milestone: 8.4.1 Component: Core Libraries | Version: 8.1 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 nomeata): The `fmap` in {{{ instance Traversable ZipList where traverse f (ZipList xs) = ZipList <$> traverse f xs }}} is not polymorphic, but rather a concret one (the one from `Functor []`), right? I would expect that to be resolved to actual `map`, for which we have a `map/coerce` rule, which should optimize the whole `ZipLib <$>` stuff away. Did you check the core if there is really an `fmap` or `map` left? If so, then the rules don't work as expected, and that is a bug. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13153#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler