[GHC] #10056: Inconsistent precedence of ~

[GHC]

#10056: Inconsistent precedence of ~
-------------------------------------+-------------------------------------
        Reporter:  crockeea          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  7.8.4
  (Parser)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #10059, #10431,   |  Differential Rev(s):  Phab:D4876
  #14316                             |
       Wiki Page:                    |
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Comment (by RyanGlScott):

 An update on this ticket. It seems that the only remaining thing here is
 to wire in an appropriately low fixity for `(~)` (and `(~~)`, which GHC
 introduced after the creation of this ticket). As it turns out, this part
 is way, way easier than I originally thought. This patch suffices, in
 fact:

 {{{#!diff
 diff --git a/compiler/rename/RnFixity.hs b/compiler/rename/RnFixity.hs
 index f1bfb38..05d1b89 100644
 --- a/compiler/rename/RnFixity.hs
 +++ b/compiler/rename/RnFixity.hs
 @@ -27,6 +27,8 @@ import Maybes
  import Data.List
  import Data.Function    ( on )
  import RnUnbound
 +import PrelNames        ( eqTyConKey, heqTyConKey )
 +import Unique

  {-
  *********************************************************
 @@ -124,6 +126,9 @@ lookupFixityRn_help' name occ
      --    a>0 `foo` b>0
      -- where 'foo' is not in scope, should not give an error (Trac #7937)

 +  | name `hasKey` eqTyConKey || name `hasKey` heqTyConKey
 +  = pure (True, Fixity NoSourceText (-2) InfixN)
 +
    | otherwise
    = do { local_fix_env <- getFixityEnv
         ; case lookupNameEnv local_fix_env name of {
 diff --git a/testsuite/tests/ghci/scripts/T10059.stdout
 b/testsuite/tests/ghci/scripts/T10059.stdo
 index 92fbb45..854f52a 100644
 --- a/testsuite/tests/ghci/scripts/T10059.stdout
 +++ b/testsuite/tests/ghci/scripts/T10059.stdout
 @@ -1,4 +1,6 @@
  class (a ~ b) => (~) (a :: k0) (b :: k0)       -- Defined in ‘GHC.Types’
 +infix -2 ~
  (~) :: k0 -> k0 -> Constraint
  class (a GHC.Prim.~# b) => (~) (a :: k0) (b :: k0)
         -- Defined in ‘GHC.Types’
 +infix -2 ~
 }}}

 I'm giving this a precedence of -2, since in #15235, we decided to give
 `(->)` a precedence of -1, and the consensus in this ticket is that
 `(~)`/`(~~)` should have a lower precedence than everything else.

 Unfortunately, things are never as simple as they appear. Even with this
 patch, `(->)` will //still// have a lower precedence than `(~)` in
 practice. Why? Because saying that `(->)` has a precedence of -1 is a bit
 of a lie; in reality, it has a precedence closer to -∞, since `(->)` has
 special treatment in the parser, which causes it to bind more tightly than
 it ought to.

 Note that `(~)` is also treated specially in the parser, but there is a
 post-parsing pass which flattens uses of `(~)` to appear as ordinary type
 operators. Perhaps we should extend this treatment to `(->)` as well?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10056#comment:38>
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