Negation

[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