XML and Scripting Languages

XML::Parser provides a method for invoking functions for each tag in the XML document. For each tag, the parsing module calls a function with the tag's name. Thus we can define a set of functions that perform the transformations, connect to databases, and implement our business logic.

To enable the function callbacks based on tag names, we need to invoke the parser with the "Subs" style. We also need to specify which namespace the function callbacks reside in, via the "Pkg" option:

 my $parser = new XML::Parser(Style=>'Subs', 

                              Pkg=>'SubHandlers', 

 			     ErrorContext => 2);

 

 $parser->setHandlers(Char  => \&char_handler); 

This will cause a series of function callbacks based on the tags and the contents of the XML file. The start of a tag will invoke a function with the same name as the tag in the SubHandlers namespace. The contents of the tag will be handled by the char_handler function, and the end of the tag will invoke a function with the same name as the tag, with an "_" appended (for example, for the end of the tag symbol, the function SubHandlers::symbol_() will be called).

Our XML file will cause the following sequence of function calls:

 SubHandlers::stock_quotes();

 SubHandlers::stock_quote();

 SubHandlers::symbol();

 char_handler();

 SubHandlers::symbol_();

 SubHandlers::when();

 .... 

Now, it is a simple matter to write the transformation functions. We can still perform simple substitutions:

 sub symbol {

   print "<img src=images/";

 }

 

 sub symbol_ {

   print ".gif>\n";

 } 

which use the stock symbol, contained in the contents of the symbol tag, to insert an image of the same name in the resulting HTML page. We can also implement more complicated logic:

 sub price {

   my $expat = shift; my $element = shift;

 

   # Read the attributes

   while (@_) {

     my $att = shift;

     my $val = shift;

     $attr{$att} = $val;

   }

 

   my $type  = $attr{'type'};

   my $price = $attr{'value'};

 

   if ($type eq 'ask') {

     $label="Ask Price";

   } elsif ($type eq 'open') {

     $label="Opening Price";

   }

 

   print "<td align=left>\n<b>$label</b></td>\n";

   print "<td align=right>$price</td>\n";

 

 } 

The first parameter passed to the function is a handle to the parser itself, followed by the name of the tag (element), optionally followed by the tag attributes as attribute_name, attribute_value pairs. In the above case we print a different label based on the type attribute of the price tag.

The full program is available as a separate listing. Running the program on our XML file produces much more attractive output. A sample looks like:

 <table width=100%>

  <tr>

   <td>

    <img src=images/IBM.gif><br><br>  12/16/1999 4:40PM

   </td>

   <td>

    <table width=100%>

     <tr>

      <td align=left><b>Ask Price</b></td>

      <td align=right>109.1875</td>

      <td align=left><b>Opening Price</b></td>

      <td align=right>108</td>

     </tr>

     <tr>

      <td align=left><b>Today's High</b></td>

      <td align=right>109.6875</td>

     </tr>

    </table>

   </td>

  </tr>

 </table>

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