[GHC] #14896: QuantifiedConstraints: GHC does doesn't discharge constraints if they are quantified

[GHC]

#14896: QuantifiedConstraints: GHC does doesn't discharge constraints if they are
quantified
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.5
           Keywords:                 |  Operating System:  Unknown/Multiple
  QuantifiedConstraints, wipT2893    |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 This came up on a
 [https://www.reddit.com/r/haskell/comments/8257mz/how_quantifiedconstraints_can_let_us_put_join/dv8hfxb/
 reddit thread],

 {{{#!hs
 {-# Language QuantifiedConstraints #-}

 class (forall aa. Functor (bi aa)) => Bifunctor bi where
   first :: (a -> a') -> (bi a b -> bi a' b)

 bimap :: Bifunctor bi => (a -> a') -> (b -> b') -> (bi a b -> bi a' b')
 bimap f g = first f . fmap g
 }}}

 This is the type we want for `bimap` even if we mix & match `Bifunctor`
 and `Functor`. We already know that we can `fmap @(bi xx)` for any `xx`
 but this is not the inferred type.

 Instead GHC infers a type (tidied up) with a superfluous `Functor`
 constraint

 {{{#!hs
 bimap :: (Bifunctor bi, Functor (bi a)) => (a -> a') -> (b -> b') -> (bi a
 b -> bi a' b')
 }}}

 Indeed post-composing with a superfluous `fmap @(bi a')` incurs yet
 another `Functor` constraint

 {{{#!hs
 bimap :: (Bifunctor bi, Functor (bi a), Functor (bi a')) => (a -> a') ->
 (b -> b') -> (bi a b -> bi a' b')
 bimap f g = fmap id . first f . fmap g
 }}}

 A terminology question, I'm not sure how to phrase what GHC isn't doing to
 the `Functor` constraints: ‘discharge’?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14896>
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