Simon Peyton-Jones simonpj at
Mon Feb 8 11:18:07 EST 2010


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 accepts them all.

I believe that the language specifies that all should be rejected.

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.

I'm inclined to start a Haskell Prime ticket to fix this language definition bug.  But first, can anyone think of a reason *not* to allow all the above?


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