Coolline is a readline-like library written in pure Ruby.

It offers all of the core readline features, but with a cleaner, simpler implementation, and the ability to easily customize its behaviour.

Customizations include: modifying all key bindings, binding keys custom functions, full control over history and tab completion, and control over what's displayed to the user (transforms).

If you don't need anything fancy, it can work like Ruby's built-in Readline.readline:

result = Coolline.readline

But, of course you want something fancy, otherwise you'd be using readline! Here's how to create a simple REPL with live syntax highlighting and tab completion:

require 'coolline'
require 'coderay'
require 'pp'

cool = Coolline.new do |c|

  # Before the line is displayed, it gets passed through this proc,
  # which performs syntax highlighting.
  c.transform_proc = proc do
    CodeRay.scan(c.line, :ruby).term
  end

  # Add tab completion for constants (and classes)
  c.completion_proc = proc do
    word = c.completed_word
    Object.constants.map(&:to_s).select { |w| w.start_with? word }
  end

  # Alt-R should reverse the line, because we like to look at our code in the mirror
  c.bind "\er" do |cool|
    cool.line.reverse!
  end

end

loop do
  # READ
  line = cool.readline

  # EVAL
  obj = eval(line)

  # PRINT
  print "=> "
  pp obj

  # LOOP
end

Coolline automatically loads a config file before starting, which allows adding new key bindings to it. The file is just a chunk of arbitrary ruby code located at $XDG_CONFIG_HOME/coolline/coolline.rb.

Coolline.bind "\C-z" do |cool|
  puts "Testing key binding with #{cool}!"
end

gem install coolline

Note: If your Ruby version is less than 1.9.3, you also need to install the io-console gem.