[GHC] #7627: Space in nullary unboxed tuples

[GHC]

#7627: Space in nullary unboxed tuples
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    Reporter:  monoidal          |       Owner:                  
        Type:  bug               |      Status:  patch           
    Priority:  normal            |   Milestone:                  
   Component:  Compiler          |     Version:  7.6.1           
    Keywords:                    |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown    
  Difficulty:  Unknown           |    Testcase:                  
   Blockedby:                    |    Blocking:                  
     Related:                    |  
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Changes (by simonpj):

  * difficulty:  => Unknown


Comment:

 I agree that this is a tiresome issue, but I'm not convinced that adding
 spaces is the right way to go.  What is the name of the type constructor?
 Presumably "`(##)`".  We don't have ANY type constructors that have spaces
 in the middle of their names.

 An alternative is to permit "`(##)`" to parse as a type and a term, just
 as `()` does.  The only reason it does not at the moment is becuase if you
 had an operator "`##`", then "`(##)`" would usually be the parenthesised
 version of it.  The lexer has a special check:
 {{{
 <0> {
   "(#" / { ifExtension unboxedTuplesEnabled `alexAndPred`
 notFollowedBySymbol }
          { token IToubxparen }
   "#)" / { ifExtension unboxedTuplesEnabled }
          { token ITcubxparen }
 }
 }}}
 Notice that `notFollowedBySymbol` thing.  That's what stops "`(##)`"
 lexing as "`(#`" followed by "`#)`".  I think.

 So we could just back off this and say that if you have `-XUboxedTuples`
 then "`(##)`" means a unit unboxed tuple, not the operator "`##`".  I
 think that'd be better.

 What about "`(#-)`"?  Maybe that should parse as the parenthesised
 operator "`#-`". So the lexer would essentialy have to recognise "`(##)`"
 and do the right thing (two lexemes).

 Simon M, or anyone else, any opinions?

 Simon

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7627#comment:3>
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