#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 RyanGlScott): Replying to [comment:3 simonpj]:
(And BTW it's possible that `Show (Apply f Int)` might reduce to `Show Bool` and we'd be stuck.
So? If we ever use this instance an get `Show Bool`, then reject that use site! Just as you would reject if you tried to use an instance that had a quantified constraint `(forall xx. c (Free c xx))` and you tried to instantiate `c` with, say, `Show`, and there is no `Show (Free Show xx)` instance. You keep muttering about "we don't expect type functions in instance heads". I would qualify that statement to "we don't expect type functions in //top-level// instance heads"—there shouldn't be anything problematic about allowing them in quantified constraints! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14860#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler