jur at cs.uu.nl
Mon Feb 8 14:04:44 EST 2010
On Feb 8, 2010, at 5:18 PM, Simon Peyton-Jones wrote:
> 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.
Helium accepts them all as well, and also delivers the, for me,
> I believe that the language specifies that all should be rejected. http://haskell.org/onlinereport/syntax-iso.html
> 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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