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Two days ago, I discovered Module#method_added lurking about in the DRP library. Today, let’s take a look at what it does, along with an example implementation of multiple dispatch for Ruby objects.

Module#method_added accepts one parameter – a symbol allowing us to reference the method that was just defined in a class or module definition. We may do a number of things with only that method name – inspect the method, rename it, change its scope, or – in our case – register it with our multiple dispatch system.

The following code is both crude and simple: it allows us to define multimethods based on their arity, and does not support optional arguments or variable-length argument lists. It also only takes hold within the multi do ... end block. With a little extra work, the multi block could be eliminated, and missing features could be added. What is noteworthy is how reflective callbacks such as Module#method_added and Class#inherited make it possible to use the Ruby language to extend the Ruby language.

module Multi
  module Arity
    def method_added(name)
      if @__multi_def__
        @__multi_def__ = false # Disable method_added behavior while aliasing
        arity = instance_method(name).arity
        @__multi_methods__ |= [name]
        class_eval "private :#{name}; alias __multi__#{name}__#{arity} #{name}" 
        @__multi_def__ = true
      end
    end

    def multi
      @__multi_methods__ = []
      @__multi_def__ = true
      yield
      @__multi_def__ = false

      @__multi_methods__.each do |name|
        define_method(name) { |*args| send("__multi__#{name}__#{args.size}", *args) }
      end
    end
  end
end

if __FILE__ == $0
  require 'test/unit'

  class Example
    extend Multi::Arity
    multi do
      def hello
        "Hello, somebody" 
      end

      def hello(name)
        "Hello, #{name}" 
      end
    end
  end

  class MultiTest < Test::Unit::TestCase
    def test_dispatch
      ex = Example.new
      assert_equal "Hello, somebody", ex.hello()
      assert_equal "Hello, multimethods", ex.hello("multimethods")
    end
  end
end