#8678: Derivin `Functor` complains about existential type -------------------------------------+------------------------------------ Reporter: heisenbug | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.7 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by heisenbug): Replying to [comment:3 monoidal]:
My guess is that the type of `Standard` is rewritten to `m ~ S n => a -> NonStandard m a` (which is existential in `n`). Simpler test case: `data A t a where A :: A Int a deriving Functor`.
Yeah, this is Henry Ford polymorphism ("you can have your car in any color as long as it is black"). But still, `deriving Functor` should only concern itself with the last parameter, which is manifestly not existential. Also, the parameter referred to has kind `Nat`, and thus is irrelevant for endofunctors on `*`. Furthermore even your example establishes a //functional dependency// between `t ~ Int`, so it should not count as existential. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8678#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler