The Gang ofFour'sInterpreter design pattern encourages extending a languageby building a new language from it. Most functional languages letyou extend the languagein a variety of ways, such as operator overloading and pattern matching. Although Java™ doesn't permitanyofthesetechniques, next-generation JVM languages do, with varying implementation details. In this article, Neal Ford investigateshow Groovy, Scala, and Clojure realizethe intent of the Interpreter design pattern byallowing functional extensions in ways thatJava does not.