On Feb 8, 2010, at 5:18 PM, Simon Peyton-Jones wrote:
Folks
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 Foo.hs:4:7: 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, expected results.
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?
Simon
Jurriaan