If, like me, you were wondering how contracts.ruby works, the comments in here are quite instructive: https://github.com/egonSchiele/contracts.ruby/blob/master/li...

very cool.

Ruby is duck-typed, so it is advantageous to write code and tests that are not tied to classes at all.

http://www.poodr.com/ goes in-depth about this.

Well, the same kind of contracts approach could be used to enforce a duck-typed set of behaviors:

  [:quacks], [:barks] => Maybe[:flies]

Some Rubyists get pedantic about duck typing. It's just a tool to design good systems, not an article of faith.

Yeah the contract here is a form of nominative type checking, it could just as well be structural.