On Mon, Feb 08, 2010 at 04:18:07PM +0000, 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 Foo.hs:4:7: 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. 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?
What would be the actual change proposed? If it is something concrete and not something like "negatives should be interpreted as unary minus when otherwise it would lead to a parse error" then that wouldn't be good. I have enough issues with the layout rule as is :) John -- John Meacham - ⑆repetae.net⑆john⑈ - http://notanumber.net/