Transformation of Applicative Specifications into Imperative ...

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9.6 The Visitor Modules
9.6. THE VISITOR MODULES
) ∗
RPAREN
{p = new RecordPattern (
vovn . value_or_variable_name ( ) ,
i n n e r P a t t e r n L i s t ) ; }
UNDERSCORE { p = new WildcardPattern ( ) ; }
The visitors are built using the visitor design pattern. The purpose of the
visitor design pattern is to separate a hierarchical structure of classes from
tasks which must be performed on the structure itself. When following the
visitor design pattern, tasks that would normally be placed in the functionality
of the individual class are put inside one single class, called a visitor.
When using the visitor design pattern an abstract visitor class is created.
This abstract class contains one method, visitClassName, for each class,
ClassName, in the hierarchical structure. To each class in the structure a
method often called accept or visit is added. This method takes a visitor as
argument and calls the visitClassName method.
In order to fulfill a task a concrete visitor, which can perform the actual
task, is implemented as an implementation of the abstract visitor. Each of
the visit methods in the concrete visitor are implemented such that each of
them implements a part of the whole task.
The advantage of the visitor design pattern is that when a new task is
needed, the only thing to do is to make a new concrete visitor. No changes
are needed in the classes of the structure. If the functionality of a visitor
must be changed, the only class that needs to be changed is the visitor class
itself.
In the transformer program the RSLAstVisitor.rsl is an abstract visitor
and the StringRSLAstVisitor.rsl is a concrete visitor, which can perform
the task of turning an RSL AST into an RSL ASCII specification. Part of a
class diagram showing the use of the visitor design pattern in the transformer
program can be found in Figure 9.6 on the following page. Code snippets
of StringRSLAstVisitor.java can be found in Example 9.2. The entire
source code of the visitor modules can be found in Appendix F.
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