[GHC] #14831: QuantifiedConstraints: Odd superclass constraint

[GHC]

#14831: QuantifiedConstraints: Odd superclass constraint
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        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:                    |
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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>
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