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From: tchrist@convex.com (Tom Christiansen)
Newsgroups: comp.lang.perl
Subject: bit vectors, lvalues, and magical relational operators
Message-ID: <108948@convex.convex.com>
Date: 19 Nov 90 16:47:05 GMT
Sender: news@convex.com
Reply-To: tchrist@convex.com (Tom Christiansen)
Organization: Convex Computer Corp, Richardson, TX
Lines: 90


Here's three postings in one:

1. BIT VECTORS

I've been playing with bit vectors (which are really pretty neat as long
as you're careful not to == them), and I'm not entirely clear on what the
third value is the in the vec() call.  I thought it was a length analogous
to substr, but empirical evidence suggests otherwise.  I looked briefly at
do_vec(), and it looks like you're using it as some sort of multiplier.
Could you shed any light here?  The only one of these that gives results I
understand is the first one:

    vec($a, 40, 1) = 1; &pvec($a);
    vec($b, 40, 4) = 1; &pvec($b);
    vec($c, 40, 1) = 4; &pvec($c);
    vec($d, 40, 4) = 4; &pvec($d);

    sub pvec {
	local($v) = @_;
	local($i);
	local($len) = length($v);
	printf "%5d: ", $len;
	print "\n"; return;
	for ($i = 0; $i < ($len*8); $i++) {
	    print vec($v,$i,1);
	    print ' ' if ($i + 1) % 8 == 0;
	}
	print "\n";
    }

And how come I can do |, &, ^, and ~ on bits, but not << or >>?


2. LVALUEs

On lvalues, I thought assign returned an lvalue, but that's not quite
so in all contexts, is it?  None of these work:

    1. ($a = $b) = $c;
    2. chop(@list = <>);
    3. ($x ? $a : $b) = $c;

Even though these are just fine:

    4. ($a = $b) =~ tr/a-z/A-Z/;
    5. chop($a = $b);
    6. print ++($a = 'red');  # "ree"

I don't see why 5 should work and 2 not, nor why 6 works but 1 and 3
do not.  I'm sure the answer is buried deep inside perl.y somewhere.


3. MAGICAL RELATIONAL OPERATORS

Here's another idea: the in the spirit of || and && being a bit
magical by allowing you do say:

    $a = $b || $c;

instead of 

    ($a = $b) || ($a = $c);

how about having relational operators like < and > being defined to return
one of their operands in such a way as to make this possible:

    if ($a < $b < $c)
    if ($a > $b > $c)

They're already leftly-associative, so that's ok.  All they'd
need to do is return return their left-hand operand if true
instead of 1.  How many scripts would this break?  I think
people usually do 

    if ($a < $b) 

which would still work fine, instead of :

    $a = 1 + ($b > $c);

which would probably not.

Of course, once you've done this, you'd want the same treatment
on "<=", ">=", "lt", "le", "gt", and "ge" as well.

How does that sound?


--tom