So most type checking, especially in how people write pre- and post-checks, is of the form "is this an X", for example, "is this object an instance of the Strin

So most type checking, especially in how people write pre- and post-checks, is of the form "is this an X", for example, "is this object an instance of the String class". In duck-typed languages, that's not good enough, because you're not passing an instance of a subclass of a string, you're passing "something that quacks like a string" (aka implements the necessary interfaces to act string-like). So the type of an object in a duck-typed language is based on the functions and fields it has _now_, not its instance type. Since there is no name for that type, you cannot do "nominal" typing. Instead, you must use "structural" typing, which checks its structure. This is exactly what Typed Clojure does. An example of how that works in practice, is you say that the 2nd parameter must be a map with one of the following configurations: a key named :foo and a key named :bar, both mapping to strings, or a key named :error, mapping to a string.