An IRB implementation in Ruby.

irb.rb

require 'readline'

first_run = true

loop do
  if first_run
    puts
    puts "Type a command to get started."
    puts "Type \"exit\" to quit."
    first_run = false
  else
    str = Readline.readline(">> ", true)
    if str == "exit" then exit end
    begin
      puts eval str, TOPLEVEL_BINDING
    rescue Exception => e
      puts e
    end
  end
end