#13328: Foldable, Functor, and Traversable deriving handle phantom types badly -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.4.1 Component: Compiler | Version: 8.1 Resolution: | Keywords: deriving-perf 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): There's one more scenario that this proposed change would wreak havoc with. It's quite possible for users to define unlawful `Functor` instances and depend on them in other derived `Functor` instances: {{{#!hs newtype Unlawful a = Unlawful Int instance Functor Unlawful where fmap f (Unlawful i) = Unlawful (i + 1) newtype WrapUnlawful a = WrapUnlawful (Unlawful a) deriving Functor }}} But under this proposal, the generated `Functor` instance for `WrapUnlawful` would be: {{{#!hs instance Functor WrapUnlawful where fmap _ = coerce }}} This would be another departure in behavior, as now `fmap`ping over a `WrapUnlawful` wouldn't apply `(+1)` to the field of the `Unlawful` (icky as it may be). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13328#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler