Haskell's IO implemented in Ruby

io.rb

class Io
  attr_reader :action
  def initialize(&action)
    @action = action
  end

  def bind(&f)
    Io.new do
      result = @action.call
      f.call(result)
    end
  end

  # This is what the compiler calls.
  def run
    result = @action.call
    if result.is_a?(Io)
      result.run
    else
      result
    end
  end
end

def get_line(io = $stdin)
  Io.new { io.gets }
end

def put_line(s, io = $stdout)
  Io.new { io.puts s }
end

# You type this:
main = get_line.bind { |s| put_line s }

# And then the compiler does this:
main.run