Go is not very impressive when it comes to type system power though. My bet is that if you wrote your statically typed code in Crystal instead, then you wouldn'

Go is not very impressive when it comes to type system power though. My bet is that if you wrote your statically typed code in Crystal instead, then you wouldn't have any problems creating your mocks. There is nothing in the concept of static typing that forbids duck typing - it is just that most type systems are weak enough that it doesn't work.