Mocking is an infamous technique from object-oriented programming. The goal is to be able to test stateful systems in small pieces by simulating the behaviour of certain objects. The problem with mocking is that it usually requires heavyweight frameworks and clutters test code. There are countless rants on that topic, but this talk isn't one. Instead, we'll explore the functional approach in Haskell: Designing a small language supporting the desired behaviour, and then writing interpreters which can execute its semantics in various ways. Testing I/O code was never easier.