Override IRB Prompt

irb.rb

require 'irb'                                                                                                                                                                                                                                                                 

module IRB 
  class << self 
    alias :orig_init_config :init_config

    def init_config(ap_path)
      begin
        puts "loading init config: #{Rails.env}"
        # Set up the prompt to be slightly more informative
        rails_env = Rails.env
        current_app = Rails.application.class.parent_name

        # a string we can sub the 2nd to last character depending on the context
        prompt_string_root = "#{current_app}(#{rails_env})%%3n%c> " 
        # calculate manually since we don't count the trailing '> ' and %100n will be 3 char
        normal_prompt_string_length = current_app.length + 1 + rails_env.length + 1 + 3 
        empty_string_root = "#{' ' * normal_prompt_string_length}%c> "

        # http://tagaholic.me/2009/05/29/exploring-how-to-configure-irb.html#prompt
        prompt_config = { 
          # Normal prompt (%zn – Line number with optional number z for printf width)
          :PROMPT_I => sprintf(prompt_string_root, '>'),
          # indent prompt
          :PROMPT_N => sprintf(empty_string_root, ' '),
          # string continue prompt
          :PROMPT_S => sprintf(empty_string_root, '"'),
          # statement continue prompt
          :PROMPT_C => sprintf(empty_string_root, '?'),
          # prefix output  (leaves a space between the side effect ouput and return val)
          :RETURN => "\n=> %s\n"
        }   

        #actaully do the init
        orig_init_config(ap_path)
        # and override with our preferred prompt
        @CONF[:PROMPT].reverse_merge!(:RAILS_ENV => prompt_config)
        @CONF[:PROMPT_MODE] = :RAILS_ENV
      rescue Exception => e
        puts "Error loading IRB PROMPT", e
      end 
    end 
  end 
end