#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 RyanGlScott): But that's exactly the definition I had in mind. What I was alluding to is that you //could// remove the `(<$>)` is the case where the wrapped list is empty: {{{#!hs instance Traversable ZipList where traverse (ZipList []) = pure (ZipList []) }}} But what about this case? {{{#!hs traverse (ZipList (x:xs)) = ??? }}} I'm not seeing how you can fill in this case without needing to appeal to `fmap` or `(<*>)` at some point. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13153#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler