[GHC] #16185: Add an AnonArgFlag to FunTy

[GHC]

#16185: Add an AnonArgFlag to FunTy
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           Reporter:  simonpj        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.3
           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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 In Trac #15952 Richard and I agreed that it makes sense to add a flag to
 `FunTy` to distinguish `(->)` from `(=>)`.  This ticket tracks progress.

 See [ticket:15952#comment:7] and [ticket:15952#comment:8].

 Main changes:

 * Define `AnonArgFlag` in `Var`, alongside `ArgFlag`
 {{{
 data AnonArgFlag
   = VisArg    -- Used for (->): an ordinary non-dependent arrow
               -- The argument is visible in source code
   | InvisArg  -- Used for (=>): a non-dependent predicate arrow
               -- The argument is invisible in source code
 }}}

 * `Type` looks like this
 {{{
 data Type
   = ...

   | FFunTy      -- ^ t1 -> t2   Very common, so an important special case
                 -- FFunTy for "full function type"; see pattern synonym
 for FunTy
      { ft_af  :: AnonArgFlag  -- Is this (->) or (=>)?
      , ft_arg :: Type           -- Argument type
      , ft_res :: Type }         -- Resuult type
 }}}

 * Add a uni-directional pattern synonym for the old `FunTy`
 {{{
 pattern FunTy :: Type -> Type -> Type
 pattern FunTy arg res <- FFunTy { ft_arg = arg, ft_res = res }
 }}}

 Everything else follows routinely.

 A huge win is that `isPredTy` vanishes almost completely.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16185>
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