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Ovid (2709)

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Stuff with the Perl Foundation. A couple of patches in the Perl core. A few CPAN modules. That about sums it up.

Journal of Ovid (2709)

Friday June 06, 2008
05:50 AM

Closures to Ease Procedural Pain

[ #36603 ]

We have some legacy procedural code (xml builders) which really needs to be refactored into proper classes. This is a perfect case where classes are preferred to procedural code because we have several of these modules which have identical functions with identical arguments. They share a lot of behavior but have to pass their arguments everywhere. There are plenty of ad hoc functions to add XML elements, but I find myself routinely needing a simple, easy to use 'single element builder'. For example, if I just have <foo version="2">bar</foo>, I'm in a lot of pain because of how difficult it can be to build XML with this system. So the first thing I did was create this function:

sub add_simple_node {
    my ( $doc, $node, $name, $value, $attribs ) = @_;
    $attribs ||= {};

    my $element = $doc->createElement($name);

    foreach my $attrname (keys %{$attribs}) {
        my $attrvalue = $attribs->{$attrname};
        $element->setAttribute($attrname, $attrvalue);

To use it, I have to do something like the following. This is really ugly, but it's easier than doing this manually every time (the examples below don't show the optional attribute creation).


That might build and attach three simple elements like this:

<title>first title</title>

However, I still don't like that duplication. What I want is a proper instance where I can store the document and node, but I don't have that. Closures to the rescue!

sub make_node_builder {
    my ( $document, $node ) = @_;
    return sub { add_simple_node( $document, $node, @_ ) };

With that, my above code simplifies to the following much more readable code:

my $add_simple_node = make_node_builder( $document, $segment_event_node );

$add_simple_node( 'offset', $segment_event->seconds_offset );
foreach my $property (qw/position title/) {
    $add_simple_node->($property, $segment_event->$property);

If you find yourself wanting objects but you don't have classes, think about closures.

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