Imitating Elixir's Stream.unfold/2 in Ruby

unfold.rb

class Enumerator
  def self.unfold tuple
    new do |yielder|
      loop do
        current, tuple = yield tuple
        yielder << current
      end
    end
  end
end

FiveAndDown = Enumerator.unfold 5 do |n|
  [n, n - 1]
end
FiveAndDown.first 10
#=> [5, 4, 3, 2, 1, 0, -1, -2, -3, -4]

Fibonacci = Enumerator.unfold [0, 1] do |a, b|
  [a, [b, a + b]]
end
Fibonacci.first 10
#=> [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]

# Elixir
defmodule Fib do
  def seq({a, b} \\ {0, 1}) do
    Stream.unfold {a, b}, fn
      {a, b} -> {a, {b, a + b}}
    end
  end 
end

Fib.seq |> Enum.take 5
#=> [0, 1, 1, 2, 3]
Fib.seq({0, -1}) |> Enum.take 5
#=> [0, -1, -1, -2, -3]

# Ruby
module Fib
  def self.seq tuple = [0, 1]
    Enumerator.unfold tuple do |a, b|
      [a, [b, a + b]]
    end
  end
end

Fib.seq.take 5
#=> [0, 1, 1, 2, 3]
Fib.seq([0, -1]).take 5
#=> [0, -1, -1, -2, -3]