#13814: Unable to resolve instance for polykinded superclass constraint on associated-type-family. -------------------------------------+------------------------------------- Reporter: isovector | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: polykinds, | type families Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): Replying to [comment:3 simonpj]:
The real problem is that `Back` is too polymorphic. If it had kind `k -> k` we'd probably be fine.
Ah, you're right. This program //does// typecheck, as expected: {{{#!hs {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE PolyKinds #-} {-# LANGUAGE TypeFamilies #-} module Test where class Back t class Back (FrontBack t) => Front t where type FrontBack (t :: k) :: k instance Back Bool instance Front Int where type FrontBack Int = Bool }}}
Maybe we should complain about the unresolved kind variables in the class decl?
That sounds like a good approach, yes. The error message might be confusing since it mentions an inferred kind variable, but I'd argue that that error would be far clearer than the current one, which is rather enigmatic. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13814#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler