[GHC] #12447: Pretty-printing of equality `~` without parentheses

[GHC]

#12447: Pretty-printing of equality `~` without parentheses
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Based off on
 [https://github.com/ekmett/constraints/blob/master/src/Data/Constraint/Deferrable.hs
 Data.Constraint.Deferrable]

 {{{#!hs
 {-# Language RankNTypes, ConstraintKinds #-}

 import Data.Typeable

 class Deferrable p where
     deferEither :: proxy p -> (p => r) -> Either String r

 instance (Typeable a, Typeable b) => Deferrable (a ~ b) where
   deferEither = undefined
 }}}

 `PolyKinds` aren't enabled so `deferEither @(_ ~ _)` is not enough to
 select the `Deferrable (a ~ b)` instance, but it is displayed without
 parentheses

 {{{
 $ ghci -XTypeApplications -ignore-dot-ghci tyiS.hs
 GHCi, version 8.0.1: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( tyiS.hs, interpreted )
 Ok, modules loaded: Main.
 *Main> :t deferEither @(_ ~ _)
 deferEither @(_ ~ _)
   :: Deferrable t ~ t1 =>
      proxy t ~ t1 -> (t ~ t1 => r) -> Either String r
 }}}

 Instead of a preferable

 {{{#!hs
 deferEither @(_ ~ _) :: Deferrable (t ~ t1) => proxy (t ~ t1) -> (t ~ t1
 => r) -> Either String r
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12447>
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