A proof of concept which wrap IRB in a different feed-driven API

rep.rb

require 'irb'

module IRB
  WAITING_FOR_INPUT_IDENTIFIER = "~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~"

  class MyInputMethod < StdioInputMethod
    def gets
      puts WAITING_FOR_INPUT_IDENTIFIER
      print @prompt + "\n"
      line = @stdin.gets
      @line[@line_no += 1] = line
    end
  end
  
  def self.start_session(binding)
    IRB.setup(nil)
    workspace = WorkSpace.new(binding)
    irb = Irb.new(workspace, MyInputMethod.new)
    @CONF[:MAIN_CONTEXT] = irb.context
    
    catch(:IRB_EXIT) do
      irb.eval_input
    end
  end
end

class IrbWrapper
  def initialize(binding)
    @rd, @wr = IO.pipe # for sending input to IRB process
    @rd2, @wr2 = IO.pipe # for receiving result from IRB process
    
    @irb_pid = Process.fork do
      STDIN.reopen(@rd)
      STDOUT.reopen(@wr2)
      STDERR.reopen(@wr2)
      # Make IRB believe it's talking to terminal, so the prompt string will be returned back
      STDIN.instance_eval("def tty?; true; end") 
      IRB.start_session(binding)
    end
    
    @rd2.gets # Read the identifier when first prompted
  end
    
  # Should be used carefully between REP round, since read is blocking
  def get_prompt()
    @rd2.gets.chomp()
  end
  
  # The given block will recieve a line when IRB output one until done
  def send_input(input, &block)
    @wr.write(input)
    loop do
      str = @rd2.gets
      break if str.chomp == IRB::WAITING_FOR_INPUT_IDENTIFIER
      if str.include?(IRB::WAITING_FOR_INPUT_IDENTIFIER)
        yield str.chomp[0..(-IRB::WAITING_FOR_INPUT_IDENTIFIER.length-1)] + "\n"
        break
      end 
      yield str
    end
  end
  
  def exit
    @wr.write("exit\n")
    Process.waitpid(@irb_pid)
  end
end

rep = IrbWrapper.new(binding)

puts "Welcome :-)"

loop do # Loop
  print rep.get_prompt
  str = gets # Read

  if str.chomp == "exit"
    rep.exit
    puts "bye"
    exit 0
  end

  rep.send_input(str) do |line| # Eval and Print
    print line
  end
end

Simply download the file, run ruby rep.rb and you got a interactive ruby shell built upon IRB.

Make improvement today to provide a more consistent communication between IRB and wrapper class by using our own input method to inject a custom identifier IRB::WAITING_FOR_INPUT_IDENTIFIER before IRB call gets.

[Update] The halting status triggered by gets could be solved by using IO.select to detect if input or output available.