[GHC] #12514: Can't write unboxed sum type constructors in prefix form

[GHC]

#12514: Can't write unboxed sum type constructors in prefix form
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
  (Parser)                           |
      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:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 Well, it's not //quite// that simple, unfortunately. Vertical bars are a
 bit more finicky to parse than commas, so simply adding a new case to
 `ntgtycon` in `Parser.y` like so:

 {{{#!diff
 diff --git a/compiler/parser/Parser.y b/compiler/parser/Parser.y
 index b9479d9..fa0d0af 100644
 --- a/compiler/parser/Parser.y
 +++ b/compiler/parser/Parser.y
 @@ -75,7 +75,8 @@ import TcEvidence       ( emptyTcEvBinds )
  import ForeignCall
  import TysPrim          ( eqPrimTyCon )
  import PrelNames        ( eqTyCon_RDR )
 -import TysWiredIn       ( unitTyCon, unitDataCon, tupleTyCon,
 tupleDataCon, nilDataCon,
 +import TysWiredIn       ( unitTyCon, unitDataCon, tupleTyCon, sumTyCon,
 +                          tupleDataCon, nilDataCon,
                            unboxedUnitTyCon, unboxedUnitDataCon,
                            listTyCon_RDR, parrTyCon_RDR, consDataCon_RDR )

 @@ -2861,6 +2862,9 @@ ntgtycon :: { Located RdrName }  -- A "general"
 qualified tycon, exc
          | '(#' commas '#)'      {% ams (sLL $1 $> $ getRdrName
 (tupleTyCon Unboxed
                                                          (snd $2 + 1)))
                                         (mo $1:mc $3:(mcommas (fst $2))) }
 +        | '(#' bars '#)'        {% ams (sLL $1 $> $ getRdrName (sumTyCon
 +                                                        (snd $2 + 1)))
 +                                       (mo $1:mc $3:(mbars (fst $2))) }
          | '(' '->' ')'          {% ams (sLL $1 $> $ getRdrName funTyCon)
                                         [mop $1,mu AnnRarrow $2,mcp $3] }
          | '[' ']'               {% ams (sLL $1 $> $ listTyCon_RDR) [mos
 $1,mcs $2] }
 @@ -3468,6 +3472,11 @@ mcs ll = mj AnnCloseS ll
  mcommas :: [SrcSpan] -> [AddAnn]
  mcommas ss = map (\s -> mj AnnCommaTuple (L s ())) ss

 +-- | Given a list of the locations of vertical bars, provide a [AddAnn]
 with an
 +-- AnnVbar entry for each SrcSpan
 +mbars :: [SrcSpan] -> [AddAnn]
 +mbars ss = map (\s -> mj AnnVbar (L s ())) ss
 +
  -- |Get the location of the last element of a OrdList, or noSrcSpan
  oll :: OrdList (Located a) -> SrcSpan
  oll l =
 }}}

 doesn't quite make the cut:

 Things that will parse successfully:
 * `(#| #)`
 * `(# | #)`
 * `(#| | #)`
 * `(# | | #)`
    That is, all sum type constructors such that (1) there's a space
 between the last bar and the `#)`, and (2) all bars are separated with at
 least one character of whitespace.

 Things that fail to parse:
 * `(#|#)`
 * `(# |#)`
 * `(#||#)`
 * `(#| |#)`
 * `(# | |#)`
 * `(# ||#)`
 * `(# || #)` (interestingly, GHC will parse this as the type operator `||`
 surrounded by hash-parens)

 Perhaps we require that bars must be separated by spaces as a prefix type
 constructor? Or perhaps we can finagle with the parser more to fix this
 above issues?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12514#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler