Github user mariogarcia commented on a diff in the pull request:
https://github.com/apache/groovy/pull/439#discussion_r82523189
--- Diff: src/spec/doc/core-metaprogramming.adoc ---
@@ -2868,6 +3015,90 @@ The difference is that when you use `assertScript`,
the code in the `assertScrip
unit test is executed*. That is to say that this time, the `Subject` class
will be compiled with debugging active, and
the breakpoint is going to be hit.
+===== ASTMatcher
+
+Sometimes you may want to make assertions over AST nodes whether to filter
those nodes, or to make sure a given
+transformation has built the expected AST node.
+
+**Filtering nodes**
+
+For instance if you would like to apply a given transformation only to a
specific set of AST nodes, you could
+use **ASTMatcher** to filter these nodes. The following example shows how
to transform a given expression to
+another. Using **ASTMatcher** it looks for a specific expression `1 + 1`
and it transforms it to `3`. That's why
+I called it the `@Joking` example.
+
+First we create the `@Joking` annotation that only can be applied to
methods:
+
+[source,groovy]
+----
+include::{projectdir}/src/spec/test/metaprogramming/ASTMatcherFilteringTest.groovy[tags=jokingannotation,indent=0]
+----
+
+Then the transformation, that only applies an instance of
`org.codehaus.groovy.ast.ClassCodeExpressionTransformer`
+to all the expressions within the method code block.
+
+[source,groovy]
+----
+include::{projectdir}/src/spec/test/metaprogramming/ASTMatcherFilteringTest.groovy[tags=jokingtransformation,indent=0]
+----
+<1> Get the method's code statement and apply the expression transformer
+
+And then is when the **ASTMatcher** is used to apply the transformation
only to those expressions matching
--- End diff --
:+1:
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