If you are working with a crappy language then that's probably true. But it doesn't have to be a lot of boilerplate and it can be local in scope. In Clojure it'

If you are working with a crappy language then that's probably true. But it doesn't have to be a lot of boilerplate and it can be local in scope. In Clojure it's a one-liner. (extend-protocol libA/stringable myLib/InputThing as-string [x] (myLib/to-string x) ) (libA/some-func (myLib/make-a-thing)) And sure it can just be doing something as simple as returning an internal value directly or calling some common stringifying method. You do have a good point that you may have redundant stringifying protocols across libraries - which sucks. > This will explode very quickly into lots of boilerplate code and is a good reason to use 'primitive types' in interfaces. I feel you were so close and missed :) The natural conclusion would be to have primitive interfaces - not primitive types. This way libraries only need to agree to adhere to an interface (and not an implementation)