example of a homebrew external command with a start to categorized cask listing built for conversation in https://github.com/phinze/homebrew-cask/issues/198

brew-casklist.rb

BREW_CASK_PREFIX = `brew --prefix brew-cask`.chomp
$LOAD_PATH << File.join(BREW_CASK_PREFIX, 'rubylib')

require 'cask'

module CaskCategories
  CATEGORIES = {
    'adium' => 'chat',
    'rdio' => 'music',
    'firefox' => 'browser',
    'google-chrome' => 'browser'
  }
  def self.category_for(cask)
    CATEGORIES[cask.title] || 'no-category'
  end
end

puts "Here's a categorized list of casks:"

Cask.all.select(&:installed?).each do |cask|
  puts "cask: #{cask.title}, category: #{CaskCategories.category_for(cask)}"
end

example_output.txt

$ brew casklist

Here's a categorized list of casks:
cask: adium, category: chat
cask: bartender, category: no-category
cask: caffeine, category: no-category
cask: f-lux, category: no-category
cask: firefox, category: browser
cask: gfxcardstatus, category: no-category
cask: google-chrome, category: browser
cask: istat-menus, category: no-category
cask: keepass-x, category: no-category
cask: lime-chat, category: no-category
cask: mou, category: no-category
cask: rdio, category: music
cask: skitch, category: no-category
cask: slate, category: no-category
cask: steam, category: no-category
cask: transmission, category: no-category
cask: vine-server, category: no-category
cask: vlc, category: no-category