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Creating Domain Specific Languages with Groovy

by Craig Wickesser on Feb 21, 2008 |

Today marked the first day of the Groovy/Grails Experience, also known as 2GX, in Reston, Virginia. The conference spans three days and includes forty 90-minute sessions, panel discussions and code workshops. One of the first sessions of the day was Venkat Subramaniam's "DSL In Groovy." Domain specific languages have appeared on InfoQ in the past including an Introduction to Domain Specific Languages by Martin Fowler and Ian Roughley's article on Building Domain-Specific Languages in JRuby.

Venkat's session provided information about what DSLs are, there characteristics, the types of DSLs (internal vs. external), as well as, the Groovy features for creating and using them. Venkat primarily focused on creating internal DSLs using some of Groovy's built-in features:

Method Missing - a special method that is called when a method can not be found allowing a developer to intercept the call
Categories - allows new methods to be added to any class at runtime
ExpandoMetaClass - a dynamically expandable bean

Besides these three features Venkat discussed the lack of constraints that Groovy places on developers and additional features that promote DSLs including:

Closures
Relaxed rules on the use of parentheses
Built-in list and map syntax
Property Missing
Ability to dynamically add methods to an interface and have concrete classes also have the methods

In deciding which method to use when creating DSLs Venkat did explain the primary differences between using Categories versus the ExpandoMetaClass,

Categories allows you to tactically enhance a class

ExpandMetaClass is far reaching, global in nature

You may not want to affect a class globally

Categories provide controlled flexibility

Domain specific languages are continuing to stay in the spotlight and can be created and used with Groovy with its built-in features.