#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 Iceland_jack): Similarly {{{#!hs newtype Free :: (Type -> Constraint) -> (Type -> Type) where Free :: () -> Free cls a instance (forall xx. cls xx => Semigroup xx) => Semigroup (Free cls a) where (<>) = undefined }}} fails because of the default methods `sconcat` and `stimes` {{{#!hs class Semigroup a where (<>) :: a -> a -> a sconcat :: NonEmpty a -> a stimes :: Integral b => b -> a -> a }}} {{{ GHCi, version 8.5.20180128: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( I.hs, interpreted ) I.hs:10:10: error: • Could not deduce: cls (Free cls a) arising from a use of ‘GHC.Base.$dmsconcat’ from the context: forall xx. cls xx => Semigroup xx bound by the instance declaration at I.hs:10:10-70 • In the expression: GHC.Base.$dmsconcat @(Free cls a) In an equation for ‘sconcat’: sconcat = GHC.Base.$dmsconcat @(Free cls a) In the instance declaration for ‘Semigroup (Free cls a)’ • Relevant bindings include sconcat :: GHC.Base.NonEmpty (Free cls a) -> Free cls a (bound at I.hs:10:10) | 10 | instance (forall xx. cls xx => Semigroup xx) => Semigroup (Free cls a) where | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ I.hs:10:10: error: • Could not deduce: cls (Free cls a) arising from a use of ‘GHC.Base.$dmstimes’ from the context: forall xx. cls xx => Semigroup xx bound by the instance declaration at I.hs:10:10-70 or from: Integral b bound by the type signature for: stimes :: forall b. Integral b => b -> Free cls a -> Free cls a at I.hs:10:10-70 • In the expression: GHC.Base.$dmstimes @(Free cls a) In an equation for ‘stimes’: stimes = GHC.Base.$dmstimes @(Free cls a) In the instance declaration for ‘Semigroup (Free cls a)’ • Relevant bindings include stimes :: b -> Free cls a -> Free cls a (bound at I.hs:10:10) | 10 | instance (forall xx. cls xx => Semigroup xx) => Semigroup (Free cls a) where | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Failed, no modules loaded. Prelude> }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14831#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#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

#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 RyanGlScott): You didn't post the full code that you're alluding to in comment:5, but my hunch is that you're experiencing the same bug as in https://ghc.haskell.org/trac/ghc/ticket/5927#comment:32. This probably deserves its own ticket regardless. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14831#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#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 Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * keywords: QuantifiedConstraints wipT2893 => QuantifiedConstraints -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14831#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler