8.5.1.2 Identifiers

An identifier is a letter or underscore (a-z, A-Z, _), followed by any sequence of digits (0-9), letters, and underscores. Examples of valid identifiers are: i, visibility, NumOps, set2, Metric1_a, _principal.

By convention, attribute names start with a lowercase letter, metric and set names with an uppercase letter, and variables with an underscore. This avoids name conflicts between metrics or sets, attributes, and variables.

Variables

The variable _self yields the model element for which the expression is evaluated. The variable _principal yields the model element for which the current metric, set, or rule is evaluated. Variables _principal and _self need not be the same element. In fact, the variable _principal should only be used if it is not the same element as _self.

Consider the following example. The metric counts, for a class A the supplier classes on which A depends that have more operations than A:

<metric name="SuppliersWithMoreOperations" domain="class"> 
  <description>The number of classes this class depends on that have
     more operations.</description>
  <projection relation="depclient" target="dependency" element="depsupplier" 
    eltype="class" condition="_self.NumOps > _principal.NumOps"/>
</metric>

Note: the condition expression in the above example could have also been written as "NumOps > _principal.NumOps", because the identifier NumOps will be interpreted as a metric of the model element for which the expression is calculated. In fact, variable _self is rarely needed in practice. Useful applications include

• for clarity/readability, as in the above example to make it clear which model elements are being compared.
• to pass the element for which an expression is evaluated into a function, for example in qualifiedname(_self).