#14961: QuantifiedConstraints: introducing classes through equality constraints fails -------------------------------------+------------------------------------- Reporter: mrkgnao | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.5 Resolution: | Keywords: | QuantifiedConstraints wipT2893 Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: #14860 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): I get different results for `wip/T2893` branch. First I need `FlexibleContexts`. Second, all the definitions fail. For the first one I get {{{ T14961.hs:16:8: error: • Could not deduce: c0 a from the context: (c ~=> Monoid, c a) bound by the type signature for: foo :: forall (c :: * -> Constraint) a. (c ~=> Monoid, c a) => a at T14961.hs:16:8-45 • In the ambiguity check for ‘foo’ To defer the ambiguity check to use sites, enable AllowAmbiguousTypes In the type signature: foo :: forall c a. c ~=> Monoid => (c a => a) | 16 | foo :: forall c a. c ~=> Monoid => (c a => a) -- ok | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ }}} And the same happens if I simplify the type a bit more to {{{ foo :: forall c a. (forall x. c x => Monoid x) => (c a => a) -- ok }}} Sure enough, this is an ambiguous type! In a call, how do you expect `c` to be instantiated?? I'm at a loss for what you are trying to achieve here. Before we can look at substituting equalities, we need to work out these simpler questions. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14961#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler