This post originated from an RSS feed registered with Perl Buzz by Andy Lester. Original Post: Template Toolkit's DEFAULT directive does not short-circuit Feed Title: Perlbuzz Feed URL: http://perlbuzz.com/atom.xml Feed Description: What's happening in the world of Perl programming, including Perl 5, Perl 6, the CPAN and Parrot? Latest Perl Buzz Posts Latest Perl Buzz Posts by Andy Lester Latest Posts From Perlbuzz

Template Toolkit's DEFAULT does not do short-circuit evaluation like you might think it would.

If you have

[% DEFAULT foo = user.calculate_foo; %]

Then the results of the method call user.calculate_foo are assigned to foo, unless foo already has a true value.

However, whether or not foo already has a true value, user.calculcate_foo is always invoked.

You might expect the code to look like this:

$foo = $user->calculate_foo() unless $foo;

But it's really like this;

$temp = $user->calculate_foo();
$foo = $temp unless $foo;

So, if user.calculate_foo is an expensive function that you don't want to invoke unless you have to, you'll have to resort to an explicit IF block:

IF !foo;
    foo = user.calculate_foo;
END

It's also worth noting that DEFAULT is not for checking if a variable is set, but for checking if it is true.