#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): No, p is not a skolem. Think of the quantified constraint like a local instance decl. It says {{{ local instance forall p. c p => Foldable' f }}} That means, if you want to solve `Foldable' f` (where `f` really is a skolem coming from the instance for `Foldable' (ECC1 c f)`), then * If you can solve `c beta`, where you are free to pick beta * then you can solve `Foldable' f` So we have to guess `beta`. And we can't. Try some top level instances with tyars on the left which don't appear on the right {{{ instance C a => C [b] }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14861#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler