Ruby Enumerable Iterators

enumerable_spec.rb

require 'active_support/core_ext/string'

RSpec.describe 'Ruby Enumerator Examples' do

  def ext_each(enum)
    while true
      begin
        values = enum.next_values
      rescue StopIteration => err
        return err.result
      end
      y = yield(*values)
      enum.feed y
    end
  end

  obj = Object.new

  def obj.each &block
    yield 0
    yield 1
    yield 1,2
    3
  end

  describe 'using obj.each as an internal iterator' do
    it 'should work via stdout' do
      stdout_expected = <<-HEREDOC.strip_heredoc
        oh hai! [0]
        ["blah", 0]oh hai! [1]
        ["blah", 1]oh hai! [1, 2]
        ["blah", 1, 2]3
      HEREDOC

      expect {
        puts obj.each {|*x| print(puts "oh hai! #{x}"); print(['blah', *x]) }
      }.to output(stdout_expected).to_stdout
    end

    it 'works but is ugly using rspec block matchers' do
      def wow_probe_it obj
        yield obj.each {|*x| yield "oh hai! #{x}"; yield ['blah', *x] }
      end

      expected = [
        "oh hai! [0]",
        ["blah", 0], "oh hai! [1]",
        ["blah", 1], "oh hai! [1, 2]",
        ["blah", 1, 2],
        3
      ]

      expect {|probe| wow_probe_it(obj, &probe) }.to yield_successive_args(*expected)
    end
  end

  describe 'using obj.each as an external iterator' do
    it 'should work via stdout' do
      stdout_expected = <<-HEREDOC.strip_heredoc
        oh hai! [0]
        ["blah", 0]oh hai! [1]
        ["blah", 1]oh hai! [1, 2]
        ["blah", 1, 2]3
      HEREDOC

      expect {
        # variant that makes it an external iterator: ext_each(obj.to_enum)
        puts ext_each(obj.to_enum) {|*x| print(puts "oh hai! #{x}"); print(['blah', *x]) }
      }.to output(stdout_expected).to_stdout
    end

    it 'works but is ugly using rspec block matchers' do
      def wow_probe_it obj
        # variant that makes it an external iterator: ext_each(obj.to_enum)
        yield ext_each(obj.to_enum) {|*x| yield "oh hai! #{x}"; yield ['blah', *x] }
      end

      expected = [
        "oh hai! [0]",
        ["blah", 0], "oh hai! [1]",
        ["blah", 1], "oh hai! [1, 2]",
        ["blah", 1, 2],
        3
      ]

      expect {|probe| wow_probe_it(obj, &probe) }.to yield_successive_args(*expected)
    end
  end
end