Github user mariogarcia commented on a diff in the pull request:
https://github.com/apache/groovy/pull/439#discussion_r82523854
--- 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
+straight forward:
+
+[source,groovy]
+----
+include::{projectdir}/src/spec/test/metaprogramming/MacroStatementTest.groovy[tags=addmethodannotation,indent=0]
+----
+
+How would look like the AST transformation without the use of a macro:
+
+[source,groovy]
+----
+include::{projectdir}/src/spec/test/metaprogramming/MacroStatementTest.groovy[tags=addmethodtransformationwithoutmacro,indent=0]
+----
+
+<1> Create a return statement
+<2> Create a constant expression "42"
+<3> Adding the code to the new method
+<4> Adding the new method to the annotated class
+
+If you're not used to the AST API, that definitely doesn't look like the
code you had in mind. Now look how the
+previous code looks like with the use of macros.
+
+[source,groovy]
+----
+include::{projectdir}/src/spec/test/metaprogramming/MacroStatementTest.groovy[tags=basicWithMacro,indent=0]
+----
+
+<1> Much simpler. You wanted to add a return statement that returned "42"
and that's exactly what you can read inside
+the `macro` utility method. Your plain code will be translated for you to
a `org.codehaus.groovy.ast.stmt.ReturnStatement`
+<2> Adding the return statement to the new method
+<3> Adding the new code to the annotated class
+
+Although `macro` method is used in this example to create an **statement**
the `macro` method could also be used to create
+**expressions** as well, it depends on which `macro` signature you use:
+
+- `macro(Closure)`: Create a given statement with the code inside the
closure.
+- `macro(Boolean,Closure)`: if **true** wrap expressions inside the
closure inside an statement, if **false** then return
+an expression
+- `macro(CompilePhase, Closure)`: Create a given statement with the code
inside the closure in a specific compile phase
+- `macro(CompilePhase, Boolean, Closure)`: Create an statement or an
expression (true == statement, false == expression)
+in a specific compilation phase.
+
+NOTE: All this signatures can be found at
`org.codehaus.groovy.macro.runtime.MacroGroovyMethods`
--- End diff --
Changed!
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