Proposal: NegationBindsTightly

Ticket #135
Dependencies names of other proposals on which this one depends
Related NegativeSyntax

Compiler support

None at present


Which of these definitions are correct Haskell?

  x1 = 4 + -5
  x2 = -4 + 5
  x3 = 4 - -5
  x4 = -4 - 5
  x5 = 4 * -5
  x6 = -4 * 5

Ghc accepts x2, x4, x6 and rejects the others with a message like

    Precedence parsing error
        cannot mix `+' [infixl 6] and prefix `-' [infixl 6] in the same infix expression

Hugs and Helium accept them all. I think that Hugs is right here. After all, there is no ambiguity in any of these expressions. And an application-domain user found this behaviour very surprising.


Make unary minus bind more tightly than any other operator. But not more tightly than function application.


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Something like deleting

    lexp6    ->      - exp7

and adding the production

    exp10    ->      - fexp

but this relates to Haskell 98 rather than Haskell 2010, so the exact change will differ.

Last modified 8 years ago Last modified on Feb 24, 2010 1:21:38 PM