A simple REPL in Ruby

repl.rb

# REPL: Read Evaluate Print Loop
# See http://en.wikipedia.org/wiki/Read%E2%80%93eval%E2%80%93print_loop

# The prompt for the REPL
PROMPT = 'repl> '

# Keep reading, evaluating, and printing forever (or until the user exits)
loop do
  # Display the prompt
  print PROMPT

  # Grab the current line of input, setting the first word to the command
  # variable and the rest of the arguments to the args variable
  input = gets.chomp
  command, *args = input.split(/\s/)

  # Depending on what the command is, do different things and print the results
  case command
  when 'echo'  then puts args.join ' '
  when 'exit'  then exit
  when 'greet' then puts 'Hello, world!'
  when 'help'  then puts 'Commands: echo, exit, greet, help, time'
  when 'time'  then puts "It is now #{Time.now}."
  else puts 'Invalid command' unless command.nil?
  end
end