I just came across this fun Perl function that I can't think of a (good) use for, but have to share.
In the Scalar::Util module is the function dualvar:
Returns a scalar that has the value NUM in a numeric context and the value STRING in a string context.
$foo = dualvar 10, "Hello";
$num = $foo + 2; # 12
$str = $foo . " world"; # Hello world
Using that in the right place could lead a future programmer down some fun debugging paths!
3 comments:
interesting factoid (based on empirical experimentation): boolean context must be determined in part based on string context
$ perl -MScalar::Util+dualvar -le 'print dualvar(0,"true") ? q{true} : q{false}'
> true
$ perl -MScalar::Util+dualvar -le 'print dualvar(1,"") ? q{true} : q{false}'
> false
$ perl -MScalar::Util+dualvar -le 'print dualvar(0,"true") + 0 ? q{true} : q{false}'
> false
So yes, interesting, and also very bug-possible. :-)
How about in a dynamic sort routine? Perhaps this is fairly contrived in itself, but if you assigned the same "value" to both parts of the dualvar and then switched operators in a sort you'd get different results between say "cmp" and "<=>".
use Scalar::Util qw( dualvar );
my @list = (
dualvar(10, 10),
dualvar(11, 11),
dualvar(100, 100),
);
print "Numerical\n";
for (sort { $a <=> $b } @list) {
print "\t$_\n";
}
print "\nAlphabetical\n";
for (sort { $a cmp $b } @list) {
print "\t$_\n";
}
Granted it seems a little odd that such a value you'd want to sort both ways.
some cases where I use dualvar:
- a string which is '0' (which usually
casts to false) but nevertheless true,
like in while (read_next_char())
- to implement $! behavior, e.g. have
a useful error message in string context
but an integer for comparison as error
number
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