Any colour you like. On one condition

We've now covered all of the fundamentals and a good deal of the system knowledge needed to write complex applications and, as Sir Earnest Shackleton instructed his men, we can now move on to the cool stuff.

We're now sufficiently advanced we can dispense with the Nannying applications that produce XHTML output. It's hardcore XML from now on. After all the NetKernel isn't just an XHTML-server it's an XML processing system and can be used for any XML application.

If /i/were/a='rich man'

For a processing language to do useful things it requires conditionals. The mother of all conditionals is the if statement. Here's an application that shows the if in action.

View the application here
Try this application here

This application first creates a stupid literal and puts the value in var:fiddler. Based upon an xpath evaluation of the contents of var:fiddler one of two messages is returned. The condition is identified by an if block.

The <if> block contains a <cond> block, a <then> block and an optional <else> block. Each of these elements is just a block of instructions that can contain more instructions or blocks of instructions.

The <cond> block is the condition block and has a special document this:cond associated with it. this:cond is evaluated prior to leaving the <cond>, therefore at some point within a cond block an instruction must write a boolean to this:cond or the if sequence will fail. We now need a short diversion to discuss canonical types.

Spring in the air Archbishop. Spring in the air yourself, Canon

In the last section we said that an instruction should write a boolean to this:cond. But what is a boolean? In other XML languages, such as XQuery and XML Schema , types are generally special names usually set as attribute values, for example xsd:boolean.

DPML defines a small number of basic types, and since everthing in DPML is either a document or a URI, the types are special documents called the canonical documents - we will see later that having types that are processable documents turns out to be very convenient.

The basic types are:

Since DPML is declarative it doesn't much care what you do to a canonical or how it is generated, it is after all just another document. The only requirement is that when a document is evaluated in a context that requires a canonical form it must either have been transformed to a canonical or be in a form that is equivalent to the canonical form.

Here's an example:

is equivalent to

Both of these are true when used as the value of this:cond. As we continue we will use canonical documents frequently.

Detour Over

Having established what canonical documents are we can show how they're used in the if example above and introduce a new accessor. The conditional block in our example has one instruction

The xpatheval accessor evaluates an xpath expression on the operand document. xpatheval requires a canonical xpath document as the operator and returns a canonical boolean document based upon whether the xpath is true for the given operand document. Truth as applied to an Xpath expression can include evaluations of xpath functions, values or even if the path is valid. See Xpath standard for details. Xpatheval is very useful and is frequently used for conditional evaluations.

Choose your weapons

<choose> is the DPML equivalent of a switch/case statement. Like if it takes <cond> and <then> blocks. However it will process each <cond> block in turn until one evaluates true at which point the associated <then> block is processed after which the <choose> exits.

As with any <cond> block an instruction must write a canonical boolean to this:cond or the choose will fail.

Examine an example choose application here
Try this application here

In a While Crocadile

<while> is used to perform iterative loops. It has a similar structure to if and choose. If the <cond> block evaluates to true the <seq> is executed. The while repeats until the <cond> block evaluates to false at which point it exits. Take care to avoid infinite loops.

Examine an example while application here
Try this application here

Summary

In this section we've covered conditional blocks. In addition we discussed the canonical documents and showed how the canonical boolean, in the form of this:cond is used to evaluate the result of <cond> block.

In the next section we'll introduce transactions...