#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): Remember, a quantified constraint is a bit like a local instance declaration. We don't allow {{{ instance C (F a) where ... }}} where `F` is a type function; and no more should we allow {{{ forall a. Show (Apply f a) }}} as a quantified constraint (`Apply` is a type function). It's a bug that I do not reject it. This works {{{ type family Apply (f :: * -> *) :: * -> * type instance Apply Proxy = Proxy instance (t ~ Apply f, forall a. Show (t a)) => Show (ExTyFam f) where }}} It's tiresome to lift the family application out, but it forces you to ensure that the instance head is ok. Do you agree? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14860#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler