Thursday, July 3, 2008

Perl: Handy, but Ugly

In what will probably be a many-part series, here's an oddity of Perl that had me tearing out my hair for a couple of hours...

If you know Perl well, feel free to skip this paragraph. Perl has a handy but ugly notion of context. Specifically, code execute in either a scalar or a list context: if a single value is expected, the code executes in scalar context; if a list of values is expected, it executes in list context (that's vague, but good enough for now). Then, code behaves differently depending on the context.

One example of context is getting the length of a list. Given a list @foo = ('a', 'b', 'c'), then @foo in scalar context is the length of @foo. Thus, $x = @foo sets the single value $x to 3 (the code executes in scalar context because $x is a single value, so Perl expects a single value assigned to it).

Now for a pop quiz. If @foo = ('a', 'b', 'c'); $x = @foo sets $x to 3, what does $x = ('a', 'b', 'c') do? Turns out it sets $x to c. Fascinating, isn't it?

The reason is that the comma does different things in list and scalar contexts. In a list context, comma is the list building operator. Thus, ('a', 'b', 'c') in list context (such as when assigned to the list variable @foo) returns a list with three items. However, in scalar context, comma is like C's comma: it executes both its left and right operands, then returns the result of the right. For instance, 'a', 'b' returns b, and 'a', 'b', 'c' returns c. Thus, when we assign ('a', 'b', 'c') to a single value, the code executes in scalar context, returning c.

Of course, I wasn't lucky enough to have this bite me in such a simple form. Instead, consider this (still heavily simplified) example:

sub foo {
    $a = "hello";
    $b = "world";
    return ($a, $b);

print join(" ", foo()) . "\n";
print scalar(foo()) . "\n";
I naively thought this would print hello world then 2. Instead, we get hello world then world. Today's lesson, then: when returning lists from functions, assign them to a list variable first.

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