Pretty-printing of equality `~` without parentheses

Based off on Data.Constraint.Deferrable

{-# 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

deferEither @(_ ~ _) :: Deferrable (t ~ t1) => proxy (t ~ t1) -> (t ~ t1 => r) -> Either String r

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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