[GHC] #8822: Allow -- ^ Haddock syntax on record constructors

[GHC]

#8822: Allow -- ^ Haddock syntax on record constructors
-------------------------------------+-------------------------------------
        Reporter:  Fuuzetsu          |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.9
  (Parser)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #9770, #12050     |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by harpocrates):

 I have a patch that implements both the initial idea and the the
 suggestion in comment:4. I'll link to it shortly. The only unpleasantness
 here is that we probably want to maintain backwards compatibility.

 For that, I decided to lift doc comments on the last field of the
 constructor to the constructor, provided there are no other doc comments
 either on the constructor or any of the other fields. This ensures that
 existing docs will all still parse the same way.

 {{{
  data Foo
    = Bar Int String  -- ^ doc on `Bar` constructor

    | Baz             -- ^ doc on the `Baz` constructor
        Int           -- ^ doc on the `Int` field of `Baz`
        String        -- ^ doc on the `String` field of `Baz`

    | Int :+ String   -- ^ doc on the `:+` constructor

    | Int             -- ^ doc on the `Int` field of the `:*` constructor
        :*            -- ^ doc on the `:*` constructor
      String          -- ^ doc on the `String` field of the `:*`
 constructor

    | Boo { x :: () } -- ^ doc on the `Boo` record constructor

    | Boa             -- ^ doc on the `Boa` record constructor
        { y :: () }
 }}}

 The patch also adds support for doc comments on GADT constructor
 arguments.

 {{{
 data Foo where
   Foo :: Int    -- ^ `Int` field of `Foo`
       -> String -- ^ `String` field of `Foo`
       -> Foo
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8822#comment:5>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler