Lazy `#reduce` and `#join` in Ruby

lazy_reduce.rb

require "rspec/autorun"

class Enumerator::Lazy
  def reduce(*)
    Lazy.new([nil]) do |yielder, _|
      yielder << super
    end
  end

  def join(separator="")
    reduce do |str, each|
      "%s%s%s" % [str, separator, each]
    end
  end
end

RSpec.describe Enumerator::Lazy do
  shared_examples_for "a lazy method" do
    it "doesn't call its block" do
      called = false
      method.call do
        called = true
      end
      expect(called).to be_falsy
    end
  end

  subject { [1, 2, 3].lazy }

  describe "#reduce" do
    it_behaves_like "a lazy method" do
      let(:method) { subject.public_method(:reduce) }
    end

    it "accepts the same number of arguments as Enumerable#reduce" do
      enumerable = Object.new.extend(Enumerable).public_method(:reduce)
      lazy = subject.public_method(:reduce)
      expect(lazy.arity).to eq(enumerable.arity)
    end

    it "yields the result of calling reduce when forced" do
      eager_sum = [1, 2, 3].reduce(0, :+)
      lazy_sum = subject.reduce(0, :+)
      expect(lazy_sum.first).to eq(eager_sum)
    end
  end

  describe "#join" do
    it_behaves_like "a lazy method" do
      let(:method) { subject.public_method(:join) }
    end

    it "stringifies and joins values" do
      expect(subject.join(",").first).to eq("1,2,3")
    end
  end
end