[GHC] #15457: (~) and (!) are parsed inconsistently in types (plus documentation warts)

[GHC]

#15457: (~) and (!) are parsed inconsistently in types (plus documentation warts)
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           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.8.1
          Component:  Compiler       |           Version:  8.4.3
  (Parser)                           |
           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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 (Spawned from
 https://ghc.haskell.org/trac/ghc/ticket/10056?replyto=41#comment:41)

 `~` and `!` are slightly special in the parser to allow strict annotations
 and lazy annotations (with `-XStrict`) in data types. As a result, we
 initially parse all uses of `~`/`!` as bangs, and we use a post-parsing
 pass (`mergeOps`/`splitTilde`) to figure out which uses of `~` are
 actually meant to refer to the equality type constructor.

 There's a couple of unsatisfying things here:

 1. `splitTilde` handles `~`, but not `!`. This means that any use of `!`
 as a type operator will not work, as evidenced by this GHCi session:

 {{{
 λ> type a ! b = Either a b

 <interactive>:1:6: error:
     Malformed head of type or class declaration: a !b
 }}}

    We should update `splitTilde` to handle `!` as well. (And perhaps give
 that function a new name to reflect its more ambitious goals.)
 2. `Note [Parsing ~]` is supposed to explain all of this hullabaloo, but
 it does a rather poor job of it. Let's add some more prose to it.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15457>
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