Ruby DSL Blocks

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.

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

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