A Real Use for Closures

I think I read about closures first a few years ago in the Camel book (first edition). At the time they seemed to be a neat toy, but not something I would ever need. Now, I've finally come across a scenario where closures provide the best solution.

I'm using XML::Parser (no doubt that's old school but I need something that's solid and portable). With that, I get to define a bunch of callbacks. I want the callbacks to be methods in an object I've created. Problem: XML::Parser wants references to subroutines, not objects. Solution: pass references to anonymous subroutines created on the spot. There's a $self reference in lexical scope, so I can just refer to that in the anonymous subroutine. Thanks to the magic of closures, it's available, even when the sub is being called back later.

    my $parser = new XML::Parser(ErrorContext => 2);
    $parser->setHandlers(
        Start => sub { $self->handle_start(@_) },
        Char  => sub { $self->handle_characters(@_) },
        End   => sub { $self->handle_end(@_) }
    );

I find this surprisingly readable. Even if I didn't know about closures, I would be able to read and understand this code: DWIM at its best. (By the way, syntactic closures seem to have been invented as recently as 1988.)

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