Writing a DSL like Thor gem in Ruby?

Question

I'm trying to figure out how the Thor gem creates a DSL like this (first example from their README)

class App < Thor                                                 # [1]
  map "-L" => :list                                              # [2]

  desc "install APP_NAME", "install one of the available apps"   # [3]
  method_options :force => :boolean, :alias => :string           # [4]
  def install(name)
    user_alias = options[:alias]
    if options.force?
      # do something
    end
    # other code
  end

  desc "list [SEARCH]", "list all of the available apps, limited by SEARCH"
  def list(search="")
    # list everything
  end
end

Specifically, how does it know which method to map the desc and method_options call to?

Answer 1

desc is pretty easy to implement, the trick is to use Module.method_added :

class DescMethods
  def self.desc(m)
    @last_message = m
  end

  def self.method_added(m)
    puts "#{m} described as #{@last_message}"
  end
end

any class that inherits from DescMethods will have a desc method like Thor . For each method a message will be printed with the method name and description. For example:

class Test < DescMethods
  desc 'Hello world'
  def test
  end
end

when this class is defined the string "test described as Hello world" will be printed.