calling IRB from ruby

irb_from_ruby.rb

require 'irb'

IRB.init_config(nil)
IRB.conf[:PROMPT_MODE] = :DEFAULT
IRB.conf[:VERBOSE] = true

class IrbStringInputMethod < IRB::InputMethod
  def initialize(line, output)
    @line = line
    @read = false
    @output = output
  end

  def gets
    return nil if @read
    @output << prompt
    @read = true
    @line
  end

  def encoding
    Encoding.default_external
  end
end

script = "1 + 1\n"

old_stdout , string_out = $stdout, StringIO.new

io = IrbStringInputMethod.new(script, string_out)
irb = IRB::Irb.new(nil, io)

$stdout = string_out
irb.eval_input
$stdout = old_stdout

puts string_out.string