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This chapter is from the book

This chapter is from the book

Defining Functions

XPath has a fairly comprehensive set of functions for doing everything from performing date calculations to evaluating regular expressions. However, sometimes it's useful to be able to build more sophisticated functions out of this core set of basic functions for doing business logic-types of evaluations.

XQuery consequently also supports the capability to create user-definable functions. These functions are XQuery/XPath in origin, and are called in the same context as XPath expressions. For instance, suppose you want to take a date in the standard XSD notation (YYYY-MM-DD) and convert it into the American standard notation MM/DD/YYYY, and you want to do it for several different instances of data that have the following structure (directory.xml):

<directory>
   <file name="chapter3.xml" dateCreated="2002-11-25" dateModified=
"2002-11-28"/>
   <file name="chapter3app1.xml" dateCreated="2002-11-25" dateModified=
"2002-11-29"/>
   <file name="chapter3.toc" dateCreated="2002-11-25" dateModified=
"2002-11-30"/>
   <file name="chapter2.xml" dateCreated="2002-11-18" dateModified=
"2002-11-21"/>
</directory>

You can create an XQuery to provide a report:

<html>
   <body>
   <h1>File Report</h1>
   <table>
      <tr>
         <th>File Name</th>
         <th>Date Created</th>
         <th>Date Last Modified</th>
      </tr>
{for $file in document('directory.xml')//file
return
   <tr>
      <td>{$file/@name}</td>
      <td>{let $refDateStr := string($file/@dateCreated)
   let $year := substring($refDateStr,1,4)
   let $month := substring($refDateStr,6,2)
   let $day := substring($refDateStr,9,2)
   return concat($month,'/',$day,'/',$year)}</td>
   <td>{let $refDateStr := string($file/@dateModified)
   let $year := substring($refDateStr,1,4)
   let $month := substring($refDateStr,6,2)
   let $day := substring($refDateStr,9,2)
   return concat($month,'/',$day,'/',$year)}</td></tr>
}
      </table>
   </body>
</html>

However, there are two difficulties. First, you have a certain degree of code duplication, with similar routines given for formatting the date in the new order. A second related problem is that it is difficult to ascertain exactly what the script is designed to do.

This is a case where working with functions can ameliorate your problems somewhat. You can define a new function called format-date() that takes an XSD type date string as a parameter and formats the date into an American Standard notation:

define function format-date($dt as xs:dateTime) as xs:String
{
let $refDateStr := string($dt)
   let $year := substring($refDateStr,1,4)
   let $month := substring($refDateStr,6,2)
   let $day := substring($refDateStr,9,2)
   return concat($month,'/',$day,'/',$year)
}

First the code creates a new function called format-date in the immediate environment:

define function format-date

The name of the function can include alphanumeric characters along with the underscore (_) and dash (-) characters only.

You can set up zero or more parameters, separated by commas:

define function format-date($dt as xs:dateTime)

The parameters (such as $dt) must be preceded by a dollar sign. A parameter doesn't have to include a data type, as the data type can be inferred dynamically at runtime, but it's generally a good idea to include one if needed. Note, however, that this also precludes overloading (more than one function with the same name and different parametric signatures). The parameter-passing model is thus more akin to languages like JavaScript (or XSLT) than it is Java.

The xs: namespace prefix, discussed earlier, is necessary if you include schema types. Note that some XQuery parsers might support other schema languages, and as such will probably have different data type prefixes:

define function format-date($dt as xs:dateTime) as xs:string

The result type, similarly, need not be specified (as with parameters, it defaults to string and/or numeric general types depending upon the processor) but can be useful to ensure that the results are type-safe. The implication here, of course, is that the resulting output will be a string, rather than a specific nodal type.

The body of the function is itself an XQuery expression—here, a series of lets that breaks the initial text string into chronological pieces:

define function format-date($dt as xs:dateTime) as xs:string
{
   let $refDateStr := string($dt)
   let $year := substring($refDateStr,1,4)
   let $month:= substring($refDateStr,6,2)
   let $day := substring($refDateStr,9,2)
   return concat($month,'/',$day,'/',$year)
}

Note that there are also date-specific functions that do the same thing; however, they are still being finalized in the XQuery working draft.

The outer return clause for the function returns the specific contents of the function to the outside world, and of course should have the data type specified by the return type in the function declaration (here, xs:string):

define function format-date($dt as xs:dateTime) as xs:string
{
   let $refDateStr := string($dt)
   let $year := substring($refDateStr,1,4)
   let $month:= substring($refDateStr,6,2)
   let $day := substring($refDateStr,9,2)
   return concat($month,'/',$day,'/',$year)
}

You can have subordinate return values that create intermediate results, but you must have at least one final outer return value.

There is one implication of XQuery being a "side effect-free" language: You cannot, within XQuery, have a situation where the function changes some external, global variable. Anything passed into an XQuery function is passed by value, not reference. An immediate consequence is that a function must always return something of value—you cannot have a void function type (although you can have one with an empty string or sequence).

The functions are defined ahead of time within the XQuery command, and then are invoked as you would expect for functions. Taking the date-modifying report code mentioned earlier, the functional notation simplifies it considerably (see Figure 3.5):

define function format-date($dt as xs:dateTime) as xs:string
{
   let $refDateStr := string($dt)
   let $year := substring($refDateStr,1,4)
   let $month:= substring($refDateStr,6,2)
   let $day := substring($refDateStr,9,2)
   return concat($month,'/',$day,'/',$year)
}

<html>
   <body>
   <h1>File Report</h1>
   <table>
      <tr>
         <th>File Name</th>
         <th>Date Created</th>
         <th>Date Last Modified</th>
      </tr>
{for $file in document('directory.xml')//file
return
   <tr>
      <td>{string($file/@name)}</td>
      <td>{format-date($file/@dateCreated)}</td>
      <td>{format-date($file/@dateModified)}</td>
   </tr>
}
      </table>
   </body>
</html>

The body of the code now contains much less code, and the intent of the programming becomes considerably clearer in this example. The expressions

<td>{format-date($file/@dateCreated)}</td>
<td>{format-date($file/@dateModified)}</td>

indicate that the dateCreated attribute and then the dateModified attribute of the file element (here contained in the $file variable) be changed into the MM/DD/YYYY notation, to produce the final output in HTML:

Figure 3.5Figure 3.5 Consolidating reused code into functions can significantly simplify the source query, and encourages the development of code libraries.

<html>
   <body>
      <h1>File Report</h1>
      <table>
         <tr>
            <th>File Name</th>
            <th>Date Created</th>
            <th>Date Last Modified</th>
         </tr>
         <tr>
            <td>chapter3.xml</td>
            <td>11/25/2002</td>
            <td>11/28/2002</td>
         </tr>
         <tr>
            <td>chapter3app1.xml</td>
            <td>11/25/2002</td>
            <td>11/29/2002</td>
         </tr>
         <tr>
            <td>chapter3.toc</td>
            <td>11/25/2002</td>
            <td>11/30/2002</td>
         </tr>
         <tr>
            <td>chapter2.xml</td>
            <td>11/18/2002</td>
            <td>11/21/2002</td>
         </tr>
      </table>
   </body>
</html>

There are two issues to be aware of when dealing with functions: namespaces and libraries. The following sections describe these issues.

Namespaces

The XQuery language has its own specific (default) namespace that defines the functions that are commonly available. Although you can create functions into that namespace (which you are doing implicitly when you create a function without a namespace prefix), you stand the possibility of overwriting a system function with one of your own. In the current context, where all functions are local, this is not necessarily a bad thing, but in situations where more than one person is relying on these functions, that situation could prove disastrous.

Namespaces Can Be Useful, Not Just a Nuisance

People who work with XML only periodically sometimes see namespaces as something of a nuisance. Namespace identifiers are often long and unwieldy, and namespace prefixes can make a seemingly straightforward block of XML seem much more complex. However, namespaces can come in handy.

For instance, I've dealt with XML schemas describing such things as framework components (such as describing a form in Visual Basic via XML). A number of us were working on this application, and it was very important to ensure that the XML code one person wrote with samples wouldn't end up contaminating the base code until it had properly been approved.

To get around this, each developer was assigned their own namespace. The interpreter of the XML was programmed so that only certain namespaces would be enabled in each respective build (that is, you could run your own test-code, but other people running this code without the namespace enabled wouldn't have your functionality). After the XML code was deemed to be working correctly, a particular code's prefixes were swapped over to the formal implementation.

This system worked surprisingly well, especially as we moved more of our operant code into XML form. Without the namespaces, it would have been impossible to keep the coding straight; with them, not only could we tell at a glance whose code we were dealing with, but the software applications could use the same namespaces to assign functionality.

The declare namespace command lets you define other namespaces for use within function declarations. For instance, you might decide to create a package of date functions, and associate them with a given namespace URI (http://www.kurtcagle.net/schemas/xquery/date, for instance). This namespace would then be used to refer to all functions within that package as follows:

declare namespace dates = "http://www.kurtcagle.net/schemas/xquery/date"

define function dates:format-date($dt as xs:dateTime) as xs:string
{
   let $refDateStr := string($dt)
   let $year := substring($refDateStr,1,4)
   let $month:= substring($refDateStr,6,2)
   let $day := substring($refDateStr,9,2)
   return concat($month,'/',$day,'/',$year)
}

<html>
   <body>
   <h1>File Report</h1>
   <table>
      <tr>
         <th>File Name</th>
         <th>Date Created</th>
         <th>Date Last Modified</th>
      </tr>
{for $file in document('directory.xml')//file
return
   <tr>
      <td>{string($file/@name)}</td>
      <td>{dates:format-date($file/@dateCreated)}</td>
      <td>{dates:format-date($file/@dateModified)}</td>
   </tr>
}
      </table>
   </body>
</html>

By doing this, you avoid the problem of namespace collision, and not coincidentally, make it easier to organize your code.

Code Libraries

The second issue is a little more irksome, and has to do with the creation of code libraries. One of the principal reasons for working with functions is the capability to build function libraries that you (and others working in the same space) can use in your own code.

Currently (as of November 15, 2002) there is no provision within the XQuery specification for indicating code libraries, although there is an open-issue item concerning it. The primary difficulty in working with such external libraries revolves around the fact that such function libraries should realistically be in their own namespaces.

One speculative form for adding such functional libraries might look something like the following for the date library:

import "dateFunctions.xquery" in namespace dates = "http://www.kurtcagle.net/
schemas/xquery/dates"

In this case, dateFunctions.xquery is a URL that contains all the custom functions associated with dates. Once declared in this manner, the functions require the namespace prefix to be invoked (for example, dates:format-date($myDate)). Note that by associating the namespace (and its prefix) with the function set, you can use a different prefix than any defined within the imported XQuery call.

Ultimately, functions in XQuery serve much the same purpose as stored procedures (SPROCS) within SQL: They simplify the coding involved within queries, and also form a mechanism for encapsulating business logic within queries (a topic to be covered in Chapter 4 and elsewhere). This becomes critically important in dealing with pipelined architectures, in which the XQuery acts as a filter on a dataset to be passed to another component (such as a Web service, or an XSLT transform). Expect to see more on function libraries in the final specification.

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