Re: [q-lang-users] This feels like a dumb question, but...

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

Andrew Berg wrote:
> I (think that I) understand the = vs eq difference.  I was surprised
> that when I tried to apply either one to an expression where the "open F
> 0 0" had failed, all I got was a bigger expression.

Well, you're trying to compare a constructor term "open F 0 0" to a
tuple "()" which isn't defined. If there's no definition for an
expression in Q then it's a normal form. So you get what you asked for. ;-)

Of course you could have equations like

	(X=X) = true;
	(_=_) = false otherwise;

in your program, which would make (=) default to syntactic equality, but
 I wouldn't recommend that.

>>-f F = true where () = close (open F 0 0);
>>        = false otherwise;
> 
> Aah, that's probably the version I was looking for.  So, if the
> evaluation fails in a "where" clause, the containing equation fails
> also.

Not quite. It's the pattern match in the local variable definition,
"where () = close (open F 0 0)", which fails (if close fails). Here we
don't actually define a variable, of course, but the left-hand side of
the definition, (), is matched against the result of the right-hand side
"close (open F 0 0)" anyway. If the pattern match in a local variable
definition fails then the entire equation fails as well, see Section 7.4
[http://q-lang.sourceforge.net/qdoc/qdoc_7.html#SEC29]. This neat little
trick is applicable whenever you expect a certain outcome of a
computation and want the equation to fail if you get something else.

> It's kind of amusing, I started out trying to write this way, but the
> only version of stat that I could find in the manual seemed to take a
> FD, which required calling "open" anyhow, so since I only needed "-f"
> just then I settled on a really dumb implementation.

Yes, we have stat, lstat *and* fstat. Actually, most of the POSIX
interface is there. :) I guess that Q can't really compete with Perl on
this ground, but most of the stuff you need for everyday scripting
should be readily available.

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