Github user paulk-asert commented on a diff in the pull request:
https://github.com/apache/groovy/pull/439#discussion_r82511988
--- Diff: src/spec/doc/core-metaprogramming.adoc ---
@@ -2829,6 +2829,153 @@ to use the Groovy Console, in particular the AST
browser tool, to gain knowledge
resource for learning is the
https://github.com/apache/groovy/tree/master/src/test/org/codehaus/groovy/ast/builder[AST
Builder]
test suite.
+==== Macros
+
+===== Introduction
+
+Until version 2.5.0, when developing AST transformations, developers
should have a deep knowledge about how the AST
+(Abstract source tree) was built by the compiler in order to know how to
add new expressions or statements during
+compile time.
+
+Although the use of `org.codehaus.groovy.ast.tool.GeneralUtils` static
methods could mitigate the burden of creating
+expressions and statements, it's still a low-level way of writing those
AST nodes directly.
+We needed something to abstract us from writing the AST directly and
that's exactly what Groovy macros were made for.
+They allow you to add code during compile time directly, without having
translate the code you had in mind to the
+`org.codehaus.groovy.ast.*` node related classes.
+
+===== Statements and expressions
+
+Lets see an example, lets create a local AST transformation.
`@AddMessageMethod`. When applied to a given class it
+will add a new method called `getMessage` to that class. The method will
return "42". The annotation it's pretty
--- End diff --
it's => is
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