Editing Apache Groovy (section)

===Abstract syntax tree transformation===
According to Groovy's own documentation, "When the Groovy compiler compiles Groovy scripts and classes, at some point in the process, the source code will end up being represented in memory in the form of a Concrete Syntax Tree, then transformed into an Abstract Syntax Tree. The purpose of AST Transformations is to let developers hook into the compilation process to be able to modify the AST before it is turned into bytecode that will be run by the JVM. AST Transformations provides Groovy with improved compile-time metaprogramming capabilities allowing powerful flexibility at the language level, without a runtime performance penalty."<ref>{{cite web |url=http://groovy.codehaus.org/Compile-time+Metaprogramming+-+AST+Transformations |title=Compile-time Metaprogramming - AST Transformations |access-date=2012-10-07 |archive-url=https://web.archive.org/web/20121014094900/http://groovy.codehaus.org/Compile-time+Metaprogramming+-+AST+Transformations |archive-date=2012-10-14 |url-status=dead }}</ref>

Examples of ASTs in Groovy are:

* Category and [[Mixin]] transformation
* Immutable AST Macro
* Newify transformation
* Singleton transformation

among others.

The testing framework [[Spock (testing framework)|Spock]] uses AST transformations to allow the programmer to write tests in a syntax not supported by Groovy, but the relevant code is then manipulated in the AST to valid code.<ref>{{cite journal |last1=King |first1=Paul |title=A History of the Groovy Programming Language |journal=Proc. ACM Program. Lang. |date=2020 |volume=4 |pages=53 |doi=10.1145/3386326|doi-access=free }}</ref> An example of such a test is: 
<syntaxhighlight lang="groovy">
def "maximum of #a and #b is #c" () {
  expect:
  Math.max (a, b) == c

  where:
  a | b || c
  3 | 5 || 5
  7 | 0 || 7
  0 | 0 || 0
}
</syntaxhighlight>