[GHC] #4180: do not consider associativity for unary minus for fixity resolution

[GHC]

#4180: do not consider associativity for unary minus for fixity resolution
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    Reporter:  maeder            |       Owner:                   
        Type:  proposal          |      Status:  new              
    Priority:  normal            |   Component:  Compiler         
     Version:  6.12.3            |    Keywords:  fixity resolution
          Os:  Unknown/Multiple  |    Testcase:                   
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown     
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 1.  currently an expression "1 + - 1" is rejected, because "1 + (-1)"
 looks as being bracketed to the right, whereas + and - are left
 associative. However, no other bracketing is possible, so "1 + - 1" is
 unambiguous and should not be subject to further fixity resolution.

 2.  if an infix expressions starts with an unary minus, the associativity
 should not matter for the unary minus. Why should "- 1 ## 1" be rejected
 for a right- or non-assoc operator "##"? Precedence alone is sufficient to
 decide between "(- 1) ## 1" and "- (1 ## 1)". The latter choice is taken
 for a higher precedence of the infix operator and the former choice should
 always be taken
 for an equal or lower precedence as is done for "- 1 + 1", but without
 looking at associativity!

 I'll attach an alternative fixity resolution in the spirit of
 10.6 of http://www.haskell.org/~simonmar/haskell-2010-draft-
 report-2/haskellch10.html

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4180>
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