Of course unary minus should bind tighter than any infix operator.
I remember suggesting this when the language was designed, but the
Haskell committee was very set against it (mostly Joe Fasel I think).
I think it's too late to change that now, it could really introduce
some subtle bugs with no parse or type errors.
On Mon, Feb 8, 2010 at 5:59 PM, Ross Paterson
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 GHC conforms to the Report; here is the relevant part of the grammar:
exp6 -> exp7 | lexp6 lexp6 -> (lexp6 | exp7) + exp7 | (lexp6 | exp7) - exp7 | - exp7
exp7 -> exp8 | lexp7 lexp7 -> (lexp7 | exp8) * exp8
But I agree they should all be legal, i.e. that unary minus should bind more tightly than any infix operator (as in C). Note that Hugs does not do that:
Hugs> -5 `mod` 2 -1 Hugs> (-5) `mod` 2 1 Hugs> -(5 `mod` 2) -1 _______________________________________________ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime