Re: [pure-lang-users] Prefix operator and -1

[Albert G. <Dr....@t-...>]

David Baird wrote:
> I have a quite Pure (0.4) question.  This is what I want, but in
> prefix-form not infix-form:
> 
>     > -1*A;
>     -1*A

I don't understand. Do you want "1 multiplied by A negated" or "1 
negated, multiplied by A"? -(1*A) will give you the former, (-1)*A the 
latter (in either Pure or Q). Or you can write it with an explicit 
application of the unary minus operator, which is available as the 'neg' 
function in Pure ('minus' in Q):

   > neg (1*A);
   -1*A
   > (neg 1)*A;
   (-1)*A

Note that in Pure unary minus always has the same precedence as binary 
minus, whereas in Q unary minus has higher precedence than all the infix 
operators. Therefore, -1*A is actually -(1*A) in Pure, whereas it 
denotes (-1)*A in Q.

Usually this difference doesn't really matter, but of course here it 
does. The rationale behind Pure's syntax is that unary minus has the 
right precedence no matter what the precedence of binary minus (which 
you can change freely in the prelude) happens to be. Also, it's 
compatible with mathematical notation, Haskell, Pascal and a bunch of 
other languages (not C though, C has the unary operators at a higher 
precedence level, like Q does).

HTH,
Albert

-- 
Dr. Albert Gr"af
Dept. of Music-Informatics, University of Mainz, Germany
Email:  Dr....@t-..., ag...@mu...
WWW:    http://www.musikinformatik.uni-mainz.de/ag