When discussing AST transformations in Groovy, we refer to local and global AST transformations. A local transformation is one that is targeted at a specific "local" piece of code. Currently, the only way to tell the Groovy compiler to implement a local transformation is to use an annotation. The annotation gives the compiler the clue as to what part of the compiled code should be transformed. Annotations can be placed next to any part of the Groovy source code, but are typically bound to a class, field, or method.

Let's start out by implementing a very simple annotation based on an AST transformation. Ultimately, we will evolve this example into an AST that implements the pretty printing functionality we built back in Chapter 7, Power Groovy DSL Features, as both an Expando closure method and via a trait. However, let's start simple and just add a method to a class that prints a string.

The functionality we want to implement can be captured in the following Spock...