Github user paulk-asert commented on a diff in the pull request: https://github.com/apache/groovy/pull/439#discussion_r82512035 --- 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 -- this signatures => these signatures
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