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July 11, 2010

Book Review: Groovy for Domain-Specific Languages

Title: Groovy for Domain-Specific Languages
Paperback : 312 pages (235mm x 191mm)
ISBN : 184719690X
ISBN 13 : 978-1-847196-90-3
Author(s) : Fergal Dearle
Publisher: Packt Publishing
Sample chapter: Building a builder (chapter 7)

Groovy for Domain-Specific Languages has the subtitle 'Extend and enhance your Java applications with Domain-Specific Language in Groovy'. The book is aimed at Java developers who want to write a Domain-Specific Language (DSL) and integrate it with their applications.

Because the book targets a Java audience the first chapters are mostly an introduction to Groovy. The first chapter is an introduction to Groovy and Domain-Specific Languages (DSLs). The author explains some common concepts for DSLs and common examples of DSLs. In the second chapter we learn how to install Groovy and how to write and run scripts. Next we get a brief introduction into the Groovy language features, so as a Java developer we get enough knowledge to get started with Groovy. Features like metaprogramming are covered in a later chapter.

In chapter three the author covers closures. Because Groovy supports closures and they are important for writing DSLs a complete chapter is devoted to closures. This is a good thing, because as Java developers we get a good understanding of the power of closures, we don't have in Java. We learn how to write closures, pass parameters, curry parameters and invoke them. The book doesn't cover the latests Groovy 1.7.2 curry methods, but it is enough to get started.

After the first three chapters we learned enough to write our first DSL. The author explains how we can write our own DSL for a Twitter client. First we learn about the Twitter API and how to use it with the Twitter4J library. Next we are going to make the API calls more Groovy by using closures and Groovy's collection support. This results already in more readable code that doesn't read like normal Java code. We even learn how we can write a command-line script to use the Twitter DSL. The author explains that we can extend from the Script class to provide a very clean DSL that just read like English.

So after the fourth chapter we already can write our own DSL in Groovy with jsut the basic Groovy features. In chapter five the author introduces more powerful Groovy features. We learn about named parameters, builders and the builder design pattern, metaprogramming with Groovy's Meta-Object Protocol (MOP) and the ExpandoMetaClass to enhance classes.

In chapter six the author shows some existing DSLs from the Groovy eco-system. This is a bit theoretical but we get to see Grails' GORM, Gant, and test frameworks like Spock and EasyB. For a Java developer this might be new and shows at least the power of Groovy and what type of DSLs can be achieved.

Builders are a powerful way to work with hierarchical data and constructs. So chapter seven is completely dedicated to builders. This chapter is also available as free chapter for download. The author first explains how we can use closures and the invokeMethod and methodMissing methods to construct a builder. Next we learn how to use the BuilderSupport classes to create our own builders.

In chapter eight we learn how to write our own rules DSL and that is where it really gets interesting. We learn how we can use the bindings support in Groovy for a business rules DSL. We create a DSL to define rewards and promotion rules for an imaginery online media provider. Step by step we build the DSL and write supporting classes to load the DSL rules and execute them with concrete domain object classes.

In the last chapter we learn different ways to integrate the rules DSL from the previous chapter in a Java application. First we see how Groovy and Java integrate. And next we learn how we can integrate the DSL script with GroovyClassLoader, GroovyShell and GroovyScriptEngine.

Groovy for Domain-Specific Languages is a good book to learn how to write a DSL in Groovy. As a Java developer we get enough introduction to Groovy to get started, but if we already know Groovy we can skip the chapters about Groovy's features. It is good that in chapter four we already learned enough to write a DSL for the Twitter API. This shows that writing a DSL doesn't have to be difficult. The rest of the book introduces some more powerful features so in the end we can write a builder and a extensive rules DSL that can be integrated in our Java (or Groovy) applications. The book is a good starting point and provides useful code to get stared with writing our own Domain-Specific Language in Groovy. The author doesn't cover AST transformations as a another way to write DSLs, but with the provided information we can already achieve a lot.