"Just to be sure: You suggest to write interfaces ('extract' in my original post as that seems to be the refactoring name you'd use for this kind of stuff in my

"Just to be sure: You suggest to write interfaces ('extract' in my original post as that seems to be the refactoring name you'd use for this kind of stuff in my world) that define a contract for pre-existing characteristics of 3rd party types?" No, in Go you can write new methods for pre-existing 3rd party types that you do not have the source for. It has the feel of monkey patching, but there is no classes in Go. A method is just a function that takes objects of certain type as a parameter. Knowing an object's type does not tell you what actions can be performed on it. To know that, you want to know what interfaces the object implements. This is a one way to solve the so called expression problem, which cannot be solved by simple use of classes and interfaces in a language like Java.