Wanted: warning option for usages of unary minus
John Meacham
john at repetae.net
Fri May 18 20:24:14 EDT 2007
On Thu, May 17, 2007 at 06:40:04PM +0200, Twan van Laarhoven wrote:
> Simon Marlow wrote:
> >...
> >
> >Really? I'm beginning to have second thoughts about the proposed change
> >to negation for Haskell'. The main reason, and this isn't pointed out
> >as well as it should be on the wiki, is that "x-1" will cease to be an
> >infix application of (-), it will parse as x applied to the literal
> >(-1). And this is different from "x - 1"
>
> There is one other alternative for parsing:
> "-" is a unary minus if and only if it is
> a) preceded by whitespace or one of "[({;,", and
> b) not followed by whitespace.
>
> So:
> x - 1 == (-) x 1
> x-1 == (-) x 1
> x -1 == x (negate 1)
> x -(1) == x (negate 1)
> x (-1) == x (negate 1)
> x (- 1) == x (\y -> y - 1)
also
(c) has a digit after it.
also note that a big point of this is that we can get rid of 'negate' in
the translation, so the literal
-1 now desugars to
(fromInteger -1)
rather than the current
(negate (fromInteger 1))
(which requires 2 dictionary lookups for a simple constant! sigh.
we also have the option of removing negate from the 'Num' class if so
desired.
John
--
John Meacham - ⑆repetae.net⑆john⑈
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