Aliasing swapped values
Ovid
Apologies if this has been covered. What should this do?
($x,$y) := ($y,$x);
In Perl5:
$x=2; $y=3;
print "x: $x y: $y\n";
(*::x, *::y) = (*::y, *::x);
$y=4;
print "x: $x y: $y\n";
$x=5;
print "x: $x y: $y\n";
This program shows typeglob aliasing. If we try to alias swapped
value, we merely swap what they are pointing at and we haven't aliased
anything. I am not sure this should be appropriate behavior for Perl6.
Aliasing will be much more common in Perl6 so this will be a trap for
many. In fact, consider this:
($c, $d) := ($y, $x);
# later
($x,$y) := ($c, $d); # whoops!
That's going to confuse the heck out of people. If we do as Perl5
does, we'll merely be swapping values since aliasing really doesn't
make much sense here, but that means for this case, ":=" == "=".
Cheers,
Ovid
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Juerd
> Apologies if this has been covered. What should this do?
> ($x,$y) := ($y,$x);
It would let $x be a second name for the variable that is also called
$y, and $y for $x. The old names $x and $y are overwritten, so
essentially the names for the two variables swapped without copying or
moving values around.
This is assuming that bound variables are visible only after the entire
bind expression is handled. Which would have to mean that the
expressions for default values are special.
> ($c, $d) := ($y, $x);
> # later
> ($x,$y) := ($c, $d); # whoops!
I imagine that that has the same effect as
($x, $y) := ($y, $x);
($c, $d) := ($x, $y);
Of which I wonder if it can be written as
($c, $d) := ($x, $y) := ($y, $x);
Juerd
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