[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
      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:                    |
-------------------------------------+-------------------------------------
Description changed by Iceland_jack:

Old description:

> 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’?

New description:

 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') id` 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#comment:1>
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