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
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