Insidious List Context
By Jeff Boes
September 20, 2012
Recently, I fell into a deep pit. Not literally, but a deep pit of Perl debugging. As a result, I’m here to warn you and yours about “Insidious List Context(TM)”.
(Note: this is a fairly elementary discussion, for people early in their Perl wizardry training.)
Perl has two contexts for evaluating expressions: list and scalar. (All who know this stuff cold can skip down a ways.) “Scalar” context is what non-Perl languages just call “normal reality”, but Perl likes to do things … differently … so we have more than one context.
In scalar context, a scalar is a scalar is a scalar, but a list becomes a scalar that represents the number of items in the list. Thus,
@x = (1, 1, 1); # @x is a list of three 1s
# vs.
$x = (1, 1, 1); # $x is "3", the list size
In list context, a list of things is still a list of things. That’s pretty simple, but when you are expecting a scalar and you get a list, your world can get pretty confused.
Okay, now the know-it-alls have rejoined us. I had a Perl hashref being initialized with code something like this:
my $hr = {
KEY1 => $value1,
KEY2 => $value2,
KEY_TROUBLE => (defined($foo) ? mysub($foo) : 1),
KEY3 => $value3,
};
So here is the issue: if mysub() returns a list, then the hashref will get extra data. Remember, Perl n00bs, “=>” is not a magical operator, it’s just a “fat comma”. So a construction like this:
1 => (2, 3, 4)
is really the same as:
1, 2, 3, 4
Here’s a complete example to illustrate just what size and shape hole I fell into:
use strict;
use Data::Dumper;
my($value1,$value2,$value3,$foo) = qw(value1 value2 value3 foo);
my $hr = {
KEY1 => $value1,
KEY2 => $value2,
KEY_TROUBLE => (defined($foo) ? mysub($foo) : 1),
KEY3 => $value3,
};
print Data::Dumper->Dumper($hr);
sub mysub {
return qw(junk extrajunk);
}
This outputs:
$VAR1 = 'Data::Dumper';
$VAR2 = {
'extrajunk' => 'KEY3',
'KEY2' => 'value2',
'KEY1' => 'value1',
'value3' => undef,
'KEY_TROUBLE' => 'junk'
};
Now, the actual subroutine involved in my little adventure was even more insidious: it returned a list context because it was evaluating a regular expression, in a list context. Its actual source:
sub is_yes {
return( defined($_[0]) && ($_[0] =~ /^[yYtT1]/));
}
So watch those expression-evaluation contexts; they can turn fairly harmless expressions into code-busters.
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