#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 RyanGlScott): Yes, I think I was a bit too bold in declaring comment:23 to be a viable workaround (in fact, I'm now discovering places in the aforementioned large codebase where this technique breaks down). A much uglier (but likelier to work in the long run) hack is to add another parameter to `C`: {{{#!hs class (forall b. (ta ~ T a, Show b) => Show (ta b)) => C a ta where type family T a :: * -> * data D a = MkD (T a Int) deriving instance (ta ~ T a, C a ta) => Show (D a) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14860#comment:25 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler