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document the visitor pattern in Java

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Krasimir Angelov
2017-08-29 18:03:26 +02:00
parent e9e5952eac
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@@ -561,7 +561,7 @@ in the abstract syntax of the grammar. If the functions is called
will be called each time when the corresponding function is encountered, will be called each time when the corresponding function is encountered,
and its arguments will be the arguments from the original tree. and its arguments will be the arguments from the original tree.
If there is no matching method name then the runtime will If there is no matching method name then the runtime will
to call the method <tt>default</tt>. The following is an example: call the method <tt>default</tt>. The following is an example:
<pre class="python"> <pre class="python">
>>> class ExampleVisitor: >>> class ExampleVisitor:
def on_DetCN(self,quant,cn): def on_DetCN(self,quant,cn):
@@ -587,6 +587,45 @@ correspondingly. In this example we just print a message and
we call <tt>visit</tt> recursively to go deeper into the tree. we call <tt>visit</tt> recursively to go deeper into the tree.
</p> </p>
</span> </span>
<span class="java">
<p>
For more complex analyses you can use the visitor pattern.
In object oriented languages this is just a clumpsy way to do
what is called pattern matching in most functional languages.
You need to define a class which has one method for each function
in the abstract syntax of the grammar. If the functions is called
<tt>f</tt> then you need a method called <tt>on_f</tt>. The method
will be called each time when the corresponding function is encountered,
and its arguments will be the arguments from the original tree.
If there is no matching method name then the runtime will
call the method <tt>defaultCase</tt>. The following is an example:
<pre class="java">
e2.visit(new Object() {
public void on_DetCN(Expr quant, Expr cn) {
System.out.println("found DetCN");
cn.visit(this);
}
public void on_AdjCN(Expr adj, Expr cn) {
System.out.println("found AdjCN");
cn.visit(this);
}
public void defaultCase(Expr e) {
System.out.println("found "+e);
}
});
Found DetCN
Found AdjCN
</pre>
Here we call the method <tt>visit</tt> from the tree e2 and we give
it, as parameter, an instance of a class with two methods <tt>on_DetCN</tt>
and <tt>on_AdjCN</tt> which are called when the top function of
the current tree is <tt>DetCN</tt> or <tt>AdjCN</tt>
correspondingly. In this example we just print a message and
we call <tt>visit</tt> recursively to go deeper into the tree.
</p>
</span>
Constructing new trees is also easy. You can either use Constructing new trees is also easy. You can either use
<tt>readExpr</tt> to read trees from strings, or you can <tt>readExpr</tt> to read trees from strings, or you can