Groovy DSL - A Simple Example

Domain Specific Languages (DSLs) have become a valuable part of the Groovy idiom. DSLs are used in native Groovy builders, Grails and GORM, and testing frameworks. To a developer, DSLs are consumable and understandable, which makes implementation more fluid as compared to traditional programming. But how is a DSL implemented? How does it work behind the scenes? This article will demonstrate a simple DSL that can get a developer kick started on the basic concepts.

What is a DSL

DSL's are meant to target a particular type of problem. They are short expressive means of programming that fit well in a narrow context. For example with GORM, you can express hibernate mapping with a DSL rather than XML.

  static mapping = {
    table 'person'
    columns {
      name column:'name'
    }
  }  

Much of the theory for DSLs and the benefits they deliver are well documented. Refer to these sources as a starting point:

• Martin Fowler - DSL
• Writing Domain Specific Languages - Groovy
• Programming Groovy - Chapter 18

A Simple DSL Example in Groovy

The following example offers a simplified view of implementing an internal DSL. Frameworks have much more advanced methods of creating a DSL. However this example does highlight closure delegation and meta object protocol concepts that are essential to understanding the inner workings of a DSL.

Pragmatic Thinking: Novice vs Expert

Recently I started reading Andy Hunt's fine book "Pragmatic Thinking And Learning." Hunt is notorious for writing books which offer practical, insightful advice in which developers can apply to their work on a daily basis.  His most famous book is "The Pragmatic Programmer", widely considered one of the top agile programmer books of all time. Even after reading the book 7 years ago, I still refer to it a few times a month. My colleagues and I bring up the "broken window" theory, or often throw out the phrase "Don't Assume, Prove it," sometimes to the chagrin of the unfamiliar. :)

4 Groovy Tips You May Not Know

I have been programming in Groovy full time for almost two years now, and I feel quite comfortable with it. In fact, the language's idioms, shortcuts, and syntax has become part of my daily programming thinking. This is fantastic as it has opened my mind to unique programming elegance and artistry, but unfortunately on the flip-side it has cause me to cringe when looking at traditional Java code. This speaks to the efficiency and conciseness of Groovy.

Recently I decided that even though I am adequetely familiar with the language, I wanted to read Programming In Groovy by Venkat Subramanium. I was pleasantly surprised that I learned some useful tips. Below are some tips that I learned:

1. Variant Looping Mechanisms

In addition to the traditional looping constructs, such as "for (i in iterm)" or range (0..5), Groovy provides alternate ways to loop.

Use the upto function to operate on a range of values:

def sum = 0
10.upto(15) {
  sum += it
}
println sum

result:
75

Use the step function to skip values:

3.step(20, 3) {
  print "$it "
}

result:
3 6 9 12 15 18