Building AST in Groovy 1.6 and Prior

In Groovy 1.6 (and prior) there is one way to build Abstract Syntax Trees (AST) in code: using the constructors on the ASTNode subclasses.

Here is an example of building a block of code that returns the String 'Hello'. A use case for this would be to create a method body implementation that simply returns 'Hello':

Pros

• Documentation is available in Javadoc/Groovydoc
• Supports being invoked from Java
• Supported in all Groovy versions
• Some IDEs support code completion and parameter lookup

Cons

• It can be difficult to determine what AST you need to write
• Verbose - does not communicate the source being created
• Fragile - AST may need to change between major releases
• Author must know what AST looks like in a particular CompilePhase

Building AST in Groovy 1.7

Groovy 1.7 introduces three new ways to build AST:
 * From Strings
 * From Code
 * From a DSL-like Specification
 

AstBuilder.buildFromString

The AstBuilder object provides an API to build AST from Strings of Groovy source code. The original example using buildFromString is:

Pros

• Does not require author to understand ASTNode subtypes
• Allows author to target a CompilePhase
• Communicates source code being generated
• Robust - Should need no changes even if AST is updated in a release

Cons

• IDE cannot check syntax or grammar
• IDE cannot refactor across String
• Some entities cannot be created, like the AST for a field declaration

AstBuilder.buildFromCode

The AstBuilder object also provides an API to create AST from source code. The original example using buildFromCode is:

Pros

• Clearly communicates source being generated
• Does not require author to understand ASTNode subtypes
• Allows author to target a CompilePhase
• Robust - Should need no changes even if AST is updated in a release
• IDE supports syntax checking and refactoring in Closure

Cons

• Some entities cannot be created, like the AST for a field declaration
• buildFromCode requires that the left hand side of the invocation be of type AstBuilder. The best way to ensure this is to invoke it with: new AstBuilder().buildFromCode
  Unknown macro: { ... }
  rather than having a local variable or field of type AstBuilder.

AstBuilder.buildFromSpec

The AstBuilder object also provides a DSL like API for building AST. The original example using buildFromSpec is:  

Pros

• Allows conditionals (or any Groovy code) to be executed during the AST building process.
• Allows any ASTNode subtype to be created
• Fully documented with lengthy examples in TestCase

Cons

Cons

 * It can be difficult to determine what AST you need to write
 * Verbose - does not always communicate the source being created
 * Fragile - AST may need to change between major releases
 * Author must know what AST looks like in a particular CompilePhase
 * IDE does not <i>yet</i> provide code tips
 
Mixing Methods
Sometimes the best solution is to mix several types of the AST Builders. For instance, consider the followihg method:
*/
public String myMethod(String parameter) {
  println 'Hello from a synthesized method!'
  println "Parameter value: $parameter"
}

/*
It might be best to use buildFromSpec to build the method declaration and buildFromCode to create the method body:
*/
List<ASTNode> result = new AstBuilder().buildFromSpec {
       method('myMethod', Opcodes.ACC_PUBLIC, String) {
               parameters

               exceptions {}
               block {
                       owner.expression.addAll new AstBuilder().buildFromCode

               }
               annotations {}
       }
}
*/
Further Resources
The test cases shipping with Groovy are an excellent resource.
More examples can be found in GEP-2, the original proposal. http://docs.codehaus.org/display/GroovyJSR/GEP+2+-+AST+Builder+Support
Examples and questions can be found on the groovy-user and groovy-dev mailing lists.