Mastering Ruby Closures

Implementing Lazy take

Now that you’ve implemented map, you can have a go at implementing the take method. As its name suggests, Enumerable#take(n) returns the first n elements from the Enumerable. As with the lazy version of map, the lazy version of take also returns a Lax instance, this time wrapping the Enumerable#take method. Here’s how it looks:

lazy_enumerable/take/lax.rb

def take(n)
  taken = 0
  Lax.new(self) do |yielder, val|
    if taken < n
      yielder << val
      taken += 1
    else
      raise StopIteration
    end
  end
end

The logic for take should be easy enough for you to follow. The interesting thing here is how take signals that the iteration has ended. When taken reaches the limit, a StopIteration exception is raised ...