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From paulk-asert <>
Subject [GitHub] groovy pull request #439: WIP Add groovy-macro docs
Date Sun, 09 Oct 2016 00:32:17 GMT
Github user paulk-asert commented on a diff in the pull request:
    --- 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[AST
     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:
    +How would look like the AST transformation without the use of a macro:
    +<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.
    +<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:
    +<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
    +<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.
    +And the transformation:
    +<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
    +===== 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
    +The next example is a local transformation `@Statistics`. When applied to a given class
it will add two methods
    +**getMethodCount()** and **getFieldCount()** which return how many methods and fields
has the class respectively. Here
    --- End diff --
    has the class => within the class

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