David Chelimsky

random thoughtlessness

let it be @-less

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If you use RSpec and you’re disciplined about the red/green/refactor of Test Driven Development, you probably find yourself doing this from time to time. We start off with a single example:

describe BowlingGame do
it "scores all gutters with 0" do
  game = BowlingGame.new
  20.times { game.roll(0) }
  game.score.should == 0
end
end

Then add second example:

describe BowlingGame do
it "scores all gutters with 0" do
  game = BowlingGame.new
  20.times { game.roll(0) }
  game.score.should == 0
end

it "scores all 1's with 20" do
  game = BowlingGame.new
  20.times { game.roll(1) }
  game.score.should == 20
end
end

Once we get the second example passing, we remove duplication in the examples, typically like this:

describe BowlingGame do
before(:each) do
  @game = BowlingGame.new
end

it "scores all gutters with 0" do
  20.times { @game.roll(0) }
  @game.score.should == 0
end

it "scores all 1's with 20" do
  20.times { @game.roll(1) }
  @game.score.should == 20
end
end

This last step involves copying the first line of each example to a before(:each) block, and then converting the references to game to an instance variable using an @ symbol. This is tedious and error prone, but we accept that in the interest of keeping things clean.

a convention emerges

One convention that has emerged over time is to introduce a helper method, like so:

describe BowlingGame do
def game
  @game ||= BowlingGame.new
end

it "scores all gutters with 0" do
  20.times { game.roll(0) }
  game.score.should == 0
end

it "scores all 1's with 20" do
  20.times { game.roll(1) }
  game.score.should == 20
end
end

With the first call to the game method in each example, ruby invokes the right side of the expression and stores it in the @game variable on the left, which is returned to the current and each subsquent caller. This approach takes up the same number of lines as the before(:each) block, but note the lack of @ symbols in the examples themselves.

Consider that there will likely be several more examples that have a similar structure, and you can see that the removal of @ symbols reduces a fair amount of noise. Not that there’s anything inherently wrong or noisy about instance variables, but for many this may just feel cleaner.

from convention emerges solution

Thanks to a suggestion from Stuart Halloway, rspec-1.2.9 introduces a new let() method that lets examples move towards @-less-ness by encapsulating the process of caching the instance variable. Here’s the previous example using let():

describe BowlingGame do
let(:game) { BowlingGame.new }

it "scores all gutters with 0" do
  20.times { game.roll(0) }
  game.score.should == 0
end

it "scores all 1's with 20" do
  20.times { game.roll(1) }
  game.score.should == 20
end
end

The call to let() in this example defines a game method. The first time game is called in each example, it invokes the block, caches the result (an instance of BowlingGame), and returns it. Each subsequent call to game returns the same object, just like the game method in the previous example.

This will, admittedly, not change your life. Nor will it appeal to all of you, though I expect it to appeal to the lispier among you. The thing that I’ve noticed is that as more of these appear in a codebase, the more meaning they begin to establish, and it actually helps to compartmentalize this pattern and separate it from other helper methods that might be floating around.

So experiment away and please feel free to provide feedback here or on the rspec-users mailing list.

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