Github user paulk-asert commented on a diff in the pull request: https://github.com/apache/groovy/pull/439#discussion_r82512196 --- 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` + +Sometimes we could be only interested in creating a given expression, not the whole statement, in order to do that we +should use any of the `macro` invocations with a boolean parameter: + +[source,groovy] +---- +include::{projectdir}/src/spec/test/metaprogramming/MacroExpressionTest.groovy[tags=addgettwotransformation,indent=0] +---- + +<1> We're telling macro not to wrap the expression in any statement, we're only interested in the expression +<2> Assigning the expression +<3> Creating a `ReturnStatement` using a method from `GeneralUtils` and the expression returned +<4> Adding the code to the new method +<5> Adding the method to the class + +===== Variable substitution + +Macros are great but we can't create anything useful or reusable if our macros couldn't receive parameters or resolve +surrounding variables. + +In the following example we're creating an AST transformation `@MD5` that when applied to a given String field will +add a method returning the MD5 the value of that field. + +[source,groovy] +---- +include::{projectdir}/src/spec/test/metaprogramming/MacroVariableSubstitutionTest.groovy[tags=md5annotation,indent=0] +---- + +And the transformation: + +[source,groovy] +---- +include::{projectdir}/src/spec/test/metaprogramming/MacroVariableSubstitutionTest.groovy[tags=md5transformation,indent=0] +---- + +<1> We need a reference to a variable expression +<2> If using a class outside the standard packages we should whether add needed imports or use the qualified name. When +using the qualified named of a given static method you need to make sure it's resolved in the proper compile phase. In +this particular case we're instructing the macro to resolve it at SEMANTIC_ANALYSIS, which is the first compile phase +with type information. +<3> In order to substitute any `expression` inside the macro we need to use the `$v` method. `$v` receives a closure as an +argument, and the closure is only allowed to substitute expressions, meaning classes inheriting +`org.codehaus.groovy.ast.expr.Expression`. + +===== MacroClass + +As we mentioned earlier the `macro` method is only capable of producing `statements` and `expressions`. But what if we +want to produce other types of nodes, such a method, a field... ? + +`org.codehaus.groovy.macro.transform.MacroClass` can be used to create **classes** (ClassNode instances) in our +transformations the same way we created statements and expressions with the `macro` method before. + +The next example is a local transformation `@Statistics`. When applied to a given class it will add two methods --- End diff -- class it => class, it
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