#14861: QuantifiedConstraints: Can't use forall'd variable in context -------------------------------------+------------------------------------- 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): If I have {{{ instance forall p. Show p => Show T }}} then when I invoke the instance declaration I get a `Show beta` constraint with absolutely nothing telling the compiler what `beta` should be. Yes, it could be solved with `beta := Int` or a variety of other things. But GHC never guesses. Same here. You have {{{ instance (forall p. c p => Foldable' f) -- Yikes: nothing constrains p => Foldable' (ECC1 c f) where }}} When the quantified constraint fires, to solve `Foldable' f`, it generates `c beta` with no constraints whatsoever on `beta`. The fact that we have in scope `c p` does not help, because it requires quessing that `beta := p`. Sorry! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14861#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler