[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
      Resolution:                    |             Keywords:
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):

 Same with

 {{{#!hs
 {-# Language KindSignatures, TypeOperators #-}

 import Data.Kind
 import Data.Type.Equality

 class    Foo (p :: Constraint)
 instance Foo (a ~ b)
 instance Foo (a ~~ b)
 }}}

 {{{
 $ ghci -ignore-dot-ghci /tmp/tOrO.hs
 GHCi, version 8.0.1: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( /tmp/tOrO.hs, interpreted )
 Ok, modules loaded: Main.
 *Main> :i Foo
 class Foo (p :: Constraint)     -- Defined at /tmp/tOrO.hs:6:1
 instance [safe] Foo a ~ b -- Defined at /tmp/tOrO.hs:8:10
 }}}

 {{{
 *Main> :set -fprint-equality-relations
 *Main> :i Foo
 class Foo (p :: Constraint)     -- Defined at /tmp/tOrO.hs:6:1
 instance [safe] Foo (a :: *) ~~ (b :: *)
   -- Defined at /tmp/tOrO.hs:8:10
 }}}

 Should be `
 {{{
 *Main> :i Foo
 class Foo (p :: Constraint)     -- Defined at /tmp/tOrO.hs:6:1
 instance [safe] Foo (a ~ b) -- Defined at /tmp/tOrO.hs:8:10
 *Main> :set -fprint-equality-relations
 *Main> :i Foo
 class Foo (p :: Constraint)     -- Defined at /tmp/tOrO.hs:6:1
 instance [safe] Foo ((a :: *) ~~ (b :: *))
   -- Defined at /tmp/tOrO.hs:8:10
 }}}

 ----

 Is this the same as #12005, two instances are defined:

 {{{#!hs
 instance Foo (a ~ b)
 instance Foo (a ~~ b)
 }}}

 but only one displayed in `:info`: is the first subsumed by the latter?
 There is some funny business going on in the
 [https://hackage.haskell.org/package/base-4.9.0.0/docs/Data-Type-
 Equality.html#t:-126--126- definition of ‘~~’] so I don't know if those
 are separate instances. For example what is the difference between these
 two instances `↓`?
 {{{#!hs
 instance Foo (a ~ b)
 instance Foo ((a::Type) ~~ (b::Type))
 }}}

 Again, only the latter is displayed:

 {{{#!hs
 *Main> :i Foo
 class Foo (p :: Constraint)     -- Defined at /tmp/tOrO.hs:6:1
 instance [safe] Foo a ~ b -- Defined at /tmp/tOrO.hs:8:10

 *Main> :set -fprint-equality-relations
 *Main> :i Foo
 class Foo (p :: Constraint)     -- Defined at /tmp/tOrO.hs:6:1
 instance [safe] Foo (a :: *) ~~ (b :: *)
   -- Defined at /tmp/tOrO.hs:8:10
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12447#comment:1>
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