[Haskell-cafe] Re: Unary Minus

On Mon, 6 Apr 2009 12:13:09 +0200, Roel van Dijk wrote:

On Mon, Apr 6, 2009 at 12:04 PM, Benjamin L.Russell wrote:

...

Interesting. ?How is this hack implemented?

This seems to be the relevant grammar: lexp6 -> - exp7 lpat6 -> - (integer | float) (negative literal)

The '6's and the '7' are superscripts. Perhaps the hack is in the precedence of the expression in which an unary minus is allowed.

What's interesting are the following definitions of the functions '-' (binary minus) and "negate" given in "8 Standard Prelude" (see http://www.haskell.org/onlinereport/standard-prelude.html#$tNum):

class (Eq a, Show a) => Num a where (+), (-), (*) :: a -> a -> a negate :: a -> a abs, signum :: a -> a fromInteger :: Integer -> a

-- Minimal complete definition: -- All, except negate or (-) x - y = x + negate y negate x = 0 - x

The type of "negate," "a -> a", where a is a Num, is precisely what is needed for a unary minus. -- Benjamin L. Russell -- Benjamin L. Russell / DekuDekuplex at Yahoo dot com http://dekudekuplex.wordpress.com/ Translator/Interpreter / Mobile: +011 81 80-3603-6725 "Furuike ya, kawazu tobikomu mizu no oto." -- Matsuo Basho^