[GHC] #11028: Refactor ConDecl

[GHC]

#11028: Refactor ConDecl
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        Reporter:  simonpj           |                Owner:  alanz
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by alanz):

 The key factor is the work currently done in `rnConResult`,

 {{{#!hs
 rnConResult doc _con details (ResTyGADT ls ty)
   = do { (ty', fvs) <- rnLHsType doc ty
        ; let (arg_tys, res_ty) = splitHsFunType ty'
                 -- We can finally split it up,
                 -- now the renamer has dealt with fixities
                 -- See Note [Sorting out the result type] in RdrHsSyn

        ; case details of
            InfixCon {}  -> pprPanic "rnConResult" (ppr ty)
            -- See Note [Sorting out the result type] in RdrHsSyn

            RecCon {}    -> do { unless (null arg_tys)
                                        (addErr (badRecResTy doc))
                               ; return (details, ResTyGADT ls res_ty, fvs)
 }

            PrefixCon {} -> return (PrefixCon arg_tys, ResTyGADT ls res_ty,
 fvs)}
 }}}

 `splitHsFunTy` on `t -> MaybeO a b` will return `([t],MaybeO a b)`, and
 the latter is then returned as the new `res_ty`

 My understanding is that we are doing this so that the ParsedSource more
 accurately reflects what was seen, which can simplify the API Annotations
 work.

 Perhaps we should add a `PostRn` field to the `ConDeclGADT` to capture the
 new `details` and `res_ty`.

 Alternatively, everything except the `rnLHsType` in `rnConResult` can move
 to the type checking.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11028#comment:24>
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