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.
