There’s a common pattern I’ve seen for developing DSL in Ruby. It’s used in RSpec, the Statemachine Gem, and Unclebob’s Clean Code talk at RailsConf 2007. I haven’t seen a name for this pattern so I’ll call it the DSL Block Pattern.

RSpec

1 describe "Bowling Game" do
2     it "should score 0 on a gutter game" do
3         game = Game.new
4         20.times { game.roll(0) }
5         game.score.should eql(0)
6     end
7 end

Statemachine

1 sm = Statemachine.build do
2     trans :locked, :coin, :unlocked
3     trans :locked, :pass, :locked
4     trans :unlocked, :pass, :locked
5     trans :unlocked, :coin, :unlocked
6 end

Parser

1 parser = Args.expect do
2     boolean "l"
3     number "p"
4     string "d"
5 end

Here’s the problem. You’ve got to write code for specific domain such as writing specifications (RSpec), defining a Statemachine, or defining command line arguments (Unclebob’s Clean Code talk).

These domains have a contained and well defined terminology set. Often the cleanest, most elegant way to express this code is to create a DSL.

Before diving into the example, let me say that I like coffee as much as the next guy. But I feel lost when ever I go to a Starbucks. As you know, Starbucks has a it’s own language, DSL if you will, for ordering coffee. What follows is a DSL Block for ordering Starbucks coffee.

The general grammar for ordering coffee is: Size, Adjective (optional), Type of Coffee. This is by no means comprehensive but it’s sufficient for the example. So if you wanted to order a large coffee, for example, you would say, Grande Coffee.

A small espresso: Short Americano. An extra large mixture of regular and decaffeinated coffee with some half and half: Venti Breve Half Caff.

Given the task to code these coffee orders, I’d like to code it like this:

1 Starbucks.order do
2     grande.coffee
3     short.americano
4     venti.breve.half_caff
5 end

Ok that looks good, but as you look closely, you’ll start to wonder about those methods, grande, short, and venti “Do they have to be defined on the Kernel?” you may ask.

Defining them on the Kernel is a scary prospect. And that may convince you to clutter the syntax by passing an object into the block like this:

1  
2 Starbucks.order do |order|
3     order.grande.coffee
4     order.short.americano
5     order.venti.breve.half_caff
6 end 

This would allow you to define the grande, short, and venti methods on the object passed into the block. Although you do need an object where grande, short, and venti will be defined, you don’t need to add an argument to the block.

You’ll find code out there, such as migrations, that uses this less optimal route. It’s not necessary. The trick to get rid of the argument is below:

 1  
 2 module Starbucks
 3 
 4   def self.order(&block)
 5     order = Order.new
 6     order.instance_eval(&block)
 7     return order.drinks
 8   end
 9 
10   class Order
11 
12     attr_reader :drinks
13 
14     def initialize
15       @drinks = []
16     end
17 
18     def short
19       @size = "small"
20       return self
21     end
22 
23     def grande
24       @size = "large"
25       return self
26     end
27 
28     def venti
29       @size = "extra large"
30       return self
31     end
32 
33     def coffee
34       @drink = "coffee"
35       build_drink
36     end
37 
38     def half_caff
39       @drink = "regular and decaffeinated coffee mixed together"
40       build_drink
41     end
42 
43     def americano
44       @drink = "espresso"
45       build_drink
46     end
47 
48     def breve
49       @adjective = "with half and half"
50       return self
51     end
52 
53     private
54 
55     def build_drink
56       drink = "#{@size} cup of #{@drink}"
57       drink << " #{@adjective}" if @adjective
58       @drinks << drink
59 
60       @size = @drink = @adjective = nil
61     end
62   end
63 
64 end 

You can see that the Order object is doing all the work. It’s got the responsibility of interpreting the DSL, so let’s call it the Interpreter Object. The Module::order method simply creates an instance of Order and calls istance_eval on it.

This causes the block to execute using the binding of the Order instance. All of the methods on Order will be accessible to the block.

The Interpreter Object can do any number of things as it interprets the DSL. In this case it simply generates a translation for Starbucks newbies. But, the sky’s the limit really.

Show all the source code.

Micah Martin is co-founder of 8th Light and is known for his open source work such as FitNesse, Limelight, Joodo, and Speclj.

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