[GHC] #16009: Deprecate `optional` from Text.ParserCombinators.ReadP

[GHC]

#16009: Deprecate `optional` from Text.ParserCombinators.ReadP
-------------------------------------+-------------------------------------
           Reporter:  dbaynard       |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:  8.6.3
          Component:                 |           Version:  8.6.2
  libraries/base                     |
           Keywords:  Applicative,   |  Operating System:  Unknown/Multiple
  Parsers, ReadP                     |
       Architecture:                 |   Type of failure:  Other
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 It seems there is some disagreement on what the type of `optional` should
 be, within `base`.

 `Control.Applicative` defines it as

 {{{#!haskell
 optional :: Alternative f => f a -> f (Maybe a)
 }}}

 By contrast, `Text.ParserCombinators.ReadP` defines it as

 {{{#!haskell
 optional :: ReadP a -> ReadP ()
 }}}

 Worse, ReadP implements `Alternative`. So it entirely possible to
 specialise

 {{{#!haskell
 optional :: ReadP a -> ReadP (Maybe a).
 }}}

 In the broader Haskell ecosystem (and beyond) there is further confusion.
 The `Applicative` definition is used by `parsers`, `megaparsec` and
 purescript's `Data.Maybe`. The `ReadP` definition is used by `Parsec` and
 purescript's `Text.Parsing.StringParser`. `Cabal`, like `base` defines
 both.

 I propose to begin to deprecate `ReadP.optional` ASAP, following
 suggestions in
 https://www.reddit.com/r/haskell/comments/8cqgds/inconsistent_optional_definitions/.
 Code which used the old form may still compile; otherwise `void` should be
 applied.

 There have been some suggestions for new names for the `ReadP` definition,
 such as `optionally` or `optional_`. It may be worth exporting this
 function directly from `Control.Applicative`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16009>
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