#14831: QuantifiedConstraints: Odd superclass constraint -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.5 Resolution: | Keywords: | QuantifiedConstraints wipT2893 Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Ah. Maybe not a bug. Consider {{{ instance (forall xx. cls xx => Semigroup xx) => Monoid (Free cls a) where mempty = undefined }}} We need the superclass {{{ [W] Semigroup (Free cls a) }}} What you were hoping was that GHC would use the instance {{{ instance (forall xx. cls xx => Semigroup xx) => Semigroup (Free cls a) where }}} to reduce the wanted superclass to {{{ [W] (forall xx. cls xx => Semigroup xx) }}} which we can readily solve from the context of the `Monoid` instance. But ALASf, the `Monoid` instace claims to have a way to solve `(Semigroup xx)` for any `xx`. Currently GHC tries the local, quantified constraints ''first'', so from {{{ [W] Semigroup (Free cls a) }}} using the quantified constraint, we get {{{ [W] cls (Free cls a) }}} which we have no way to solve. The quantified constraint does look far too general. I'll declare this not-a-bug for now. I have not looked at comment:2 yet; let's sort this part out first. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14831#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler