This post originated from an RSS feed registered with Ruby Buzz by Jeremy Voorhis. Original Post: Specifying Duck Types in Ruby Feed Title: JVoorhis Feed URL: http://feeds.feedburner.com/jvoorhis Feed Description: JVoorhis is a Rubyist in northeast Ohio. He rambles about Ruby on Rails, development practices, other frameworks such as Django, and on other days he is just full of snark. Latest Ruby Buzz Posts Latest Ruby Buzz Posts by Jeremy Voorhis Latest Posts From JVoorhis

While prototyping a RESTful web service, I wrote the following code to allow Rails to parse a request’s parameters from a POST request containing JSON.

ActionController::Base.param_parsers[Mime::JSON] = JSON.method(:parse)

Unfortunately, it did not work, while the following code did:

ActionController::Base.param_parsers[Mime::JSON] = lambda { |data| JSON.parse(data) }

Theoretically, both of these should accomplish the same thing: assign an object that responds to the message call with an arity of 1 to Mime::JSON entry of the param_parsers hash. They lead to different results, however, because a case statement within the Rails HTTP request parsing code watches for an instance of Proc, instead of an object which responds to call.

As a proof of concept, I whipped up this DuckType class:

class DuckType
  def initialize(*args)
    @args = args
  end

  def ===(other)
    @args.all? { |arg| other.respond_to?(arg) }
  end
end

case strategy = ActionController::Base.param_parsers[mime_type]
  when Proc
    strategy.call(raw_post_data)
  # snip...
end

Callable = DuckType.new :call
case strategy = ActionController::Base.param_parsers[mime_type]
  when Callable
    strategy.call(raw_post_data)
  # snip...
end

As you can see, an instance of DuckType has a case comparison operator which asserts that all specified messages are supported, allowing you to focus on an object’s capabilities rather than its class.

My DuckType implementation in its current form is rather crude, but if enough people found this technique useful, I would consider properly packaging and extending it for general use.