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

describe "Bowling Game" do
		it "should score 0 on a gutter game" do
				game = Game.new
				20.times { game.roll(0) }
				game.score.should eql(0)
		end
end

Statemachine

sm = Statemachine.build do
		trans :locked, :coin, :unlocked
		trans :locked, :pass, :locked
		trans :unlocked, :pass, :locked
		trans :unlocked, :coin, :unlocked
end

Parser

parser = Args.expect do
		boolean "l"
		number "p"
		string "d"
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:

Starbucks.order do
		grande.coffee
		short.americano
		venti.breve.half_caff
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:

Starbucks.order do |order|
		order.grande.coffee
		order.short.americano
		order.venti.breve.half_caff
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:

module Starbucks

		def self.order(&block)
				order = Order.new
				order.instance_eval(&block)
				return order.drinks
		end

		class Order

				attr_reader :drinks

				def initialize
						@drinks = []
				end

				def short
						@size = "small"
						return self
				end

				def grande
						@size = "large"
						return self
				end

				def venti
						@size = "extra large"
						return self
				end

				def coffee
						@drink = "coffee"
						build_drink
				end

				def half_caff
						@drink = "regular and decaffeinated coffee mixed together"
						build_drink
				end

				def americano
						@drink = "espresso"
						build_drink
				end

				def breve
						@adjective = "with half and half"
						return self
				end

				private

				def build_drink
						drink = "#{@size} cup of #{@drink}"
						drink << " #{@adjective}" if @adjective
						@drinks << drink

						@size = @drink = @adjective = nil
				end
		end

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 instance_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.

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