#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 Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: #16123 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Hmm. Well, please note that this quantified constraint will fire on a constraint like `Show (Maybe Int)` Why? Because we can instantiate `Show (ta b)` to give `(Show (Maybe Int)`. So the quantified constraint overlaps with the top-level instance; but quantified constraints are tried (currently) before top-level instances, so the quantified constraint will "win". That might lead you to try to solve `(Maybe ~ T a)` which will probably fail. Perhaps in the applications you have in mind this will be fine. And if so, that's great. But I just wanted to point out that there might be unintended consequence. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14860#comment:24 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler