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From: mariogarcia <git@git.apache.org>
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Subject: [GitHub] groovy pull request #439: WIP Add groovy-macro docs
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Message-Id: <20161009140702.D1C7CDFB78@git1-us-west.apache.org>
Date: Sun,  9 Oct 2016 14:07:02 +0000 (UTC)
archived-at: Sun, 09 Oct 2016 14:07:04 -0000

Github user mariogarcia commented on a diff in the pull request:

    https://github.com/apache/groovy/pull/439#discussion_r82523034
  
    --- 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... ?
    --- End diff --
    
    :+1: 


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