#14860: QuantifiedConstraints: Can't quantify constraint involving type family -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler (Type | Version: 8.5 checker) | Keywords: Resolution: wontfix | 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 Iceland_jack): These problems were my original motivation for #14822, if `f` has a singleton we can witness `Show (Apply f a)` without changing the definition of `Apply` {{{#!hs wit :: forall f a. SingI f :- Show (Apply f a) wit = Sub (case sing :: Sing f of SingProxy -> Dict) }}} If we had a way of turning `(a :- b)` into `(a => b)` I think this would solve Ryan's problem? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14860#comment:14 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler