Paul Evans > Config-XPath > Config::XPath::Manual

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NAME ^

Config::XPath::Manual - Manual to accompany Config::XPath class

INTRODUCTION ^

This module provides easy access to configuration data stored in an XML file. Configuration is retrieved using XPath keys; various methods exist to convert the result to a variety of convenient forms.

Because the operation of this module centres around XPath expressions, the reader is assumed to be familiar with them. For more information on XPath in general, see http://www.w3.org/TR/xpath.

EXAMPLE DATA FILE ^

The methods are best explained by example. The following examples will all use the following data file

 <elements>
   <el name="hydrogen" symbol="H">
     <isotope number="1" NA="0.99985" name="protium"></isotope>
     <isotope number="2" NA="0.00115" name="deuterium"></isotope>
     <isotope number="3" name="tritium">
       <halflife unit="years">12.32</halflife>
     </isotope>
   </el>
 
   <el name="carbon" symbol="C" state="solid">
     <isotope number="12" NA="0.989"></isotope>
     <isotope number="13" NA="0.011"></isotope>
     <isotope number="14">
       <halflife unit="years">5730</halflife>
     </isotope>
   </el>
 
   <el name="oxygen" symbol="O">
     <isotope number="16" NA="0.9978"></isotope>
     <isotope number="17" NA="0.00038"></isotope>
     <isotope number="18" NA="0.0021"></isotope>
   </el>
 </elements>

The following examples will presume this file has been loaded with:

 use Config::XPath;

 my $config = Config::XPath->new( filename => "example.xml" );

DATA ACCESS METHODS ^

Strings

XPath expressions that yield strings, integers or booleans can be evaluated using the get_string() method.

 my $hydrogen_symbol = 
     $config->get_string( '//el[@name="hydrogen"]/@symbol' );

 my $C_name = $config->get_string( '//el[@symbol="C"]/@name' );

 my $number_of_elements = $config->get_string( 'count(//el)' );

A default value can be provided which will be returned if the specified node does not exist in the XML data.

 my $oxygen_state =
     $config->get_string( '//el[@name="oxygen"]/@state',
                          default => "gas" );

If no XML node is found and a default value is not supplied, an exception is thrown. A value of undef may be passed as the default, which can be used instead of catching an exception, to tell if the requested node was absent from the XML data because no real XPath string result will ever yield undef.

Structures

Beyond simple strings, entire structures can be queried at once, with the get() method. These consist of XPath expressions as the leaves of a tree, formed out of anonymous array and hash references. The result will be returned in a structure of identical layout to the input.

 my $hydrogen = $config->get( { name   => '//el[1]/@name',
                                symbol => '//el[1]/@symbol' } );

 print "$hydrogen->{name}'s symbol is $hydrogen->{symbol}\n";

As with get_string(), default values can be supplied. These are given in a tree of similar structure to the paths.

 $hydrogen = $config->get(
    [ '//el[1]/@oranges', '//el[1]/@name' ],
    default => [ undef, "name" ]
 );

 # $hydrogen is now [ undef, 'hydrogen' ]

If arrays are used (as in this example), the undef values used to fill "holes" in the array will cause undef to be returned rather than exceptions being thrown.

If this distinction is important to your program, make sure to use hashes instead of arrays when dealing with structures that have default values.

 my $tritium = $config->get(
    {
       name     => '//el[1]/isotope[3]/@name',
       halflife => '//el[1]/isotope[3]/halflife'
    },
    default => { halflife => "none" }
 );

Lists

To obtain a list of values from a nodeset expression use the get_list() method.

 my @element_names = $config->get_list( '//el/@name' );

The one-argument form of get_list() will obtain the string value of each node in the nodeset directly. A second argument can specify an alternate expression to obtain.

 my @element_names = $config->get_list( '//el', '@name' );
 # Equivalent to the previous example

 my @number_of_isotopes = $config->get_list( '//el', 'count(isotope)' );

As with the get() method, a structure of XPath expressions can be given which will cause structures of results to be returned, and default values can be provided.

 my @iso = $config->get_list( '//el',
                              { name          => '@name',
                                first_isotope => 'isotope[1]/@number' } );

 my @hydrogen_halflives = $config->get_list(
    '//el[@name="hydrogen"]/isotope',
    { number => '@number', value => 'halflife', unit => 'halflife/@unit' },
    default => { unit => 'forever', value => '0' }
 );

Mappings

An association from keys to values can be obtained with the get_map() method.

 my $symbols = $config->get_map( '//el', '@name', '@symbol' );
 print "The symbol for carbon is $symbols->{carbon}\n";

A structure of XPath expressions and default values work the same way as get_list().

SUBCONFIGURATIONS ^

In a newly-constructed Config::XPath object, the XPath context is at the root node. In order to make several related lookups easier to perform on other nodes in the data, a subconfiguration object can be constructed with the get_sub. This is an object where the context is at another node in the data.

 my $oxygen_config = $config->get_sub( '//el[@name="oxygen"]' );

 my $oxygen_symbol = $oxygen_config->get_string( '@symbol' );
 my $n_isotopes    = $oxygen_config->get_string( 'count(isotope)' );

 my @isotopes = $oxygen_config->get_list( 'isotope/@number' );

 my $abundances = $oxygen_config->get_map(
                     'isotope',
                     '@number', '@NA',
                     default => 'trace' );

A list of subconfigurations can be generated with get_sub_list

 foreach my $element_config ( $config->get_sub_list( '//el' ) ) {
    my $name  = $element_config->get_string( '@name' );
    my $state = $element_config->get_string( '@state',
                                             default => "gas" );
    print "$name is a $state at STP\n";
 }

SEE ALSO ^

AUTHOR ^

Paul Evans <leonerd@leonerd.org.uk>

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