#14860: QuantifiedConstraints: Can't quantify constraint involving type family -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.5 checker) | Keywords: Resolution: | QuantifiedConstraints wipT2893 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 simonpj): But the error is at line 17, where we don't know `f` and cannot reduce `Apply f a`. So we really do have a local instance {{{ local instance forall a. Show (Apply f a) }}} You might argue that we should use it to solve `Show (Apply f Int)` or any other instantiation, but we don't. Because we don't expect type functions in instance heads. (And BTW it's possible that `Show (Apply f Int)` might reduce to `Show Bool` and we'd be stuck. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14860#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler