#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): Update: With [https://ghc.haskell.org/trac/ghc/ticket/2893#comment:34 latest changes] the original code compiles AND we can build this hierarchy for our own `Semigroup`-`Monoid`-`Group` hierarchy {{{#!hs instance semi ~=> Semigroup => Semigroup (Free semi a) where (<>) = liftFree2 (<>) sconcat = undefined stimes = undefined instance mon ~=> Monoid => Monoid (Free mon a) where mempty = liftFree0 mempty mappend = liftFree2 (<>) mconcat = undefined instance grp ~=> Group => Group (Free grp a) where inv = liftFree1 inv }}} but leaving out or inlining default methods `mconcat = foldr mappend mempty` fails. ---- {{{#!hs class Monoid a => Group a where inv :: a -> a type cls ~=> cls' = (forall xx. cls xx => cls' xx :: Constraint) liftFree0 :: (forall xx. cls xx => xx) -> Free cls a liftFree0 a = Free (pure a) liftFree1 :: (forall xx. cls xx => xx -> xx) -> (Free cls a -> Free cls a) liftFree1 f (Free xs) = Free (fmap f xs) liftFree2 :: (forall xx. cls xx => xx -> xx -> xx) -> (Free cls a -> Free cls a -> Free cls a) liftFree2 f (Free xs) (Free ys) = Free (liftA2 f xs ys) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14831#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler