[doyougnu]

Clojure doesn't solve the expression problem and the expression problem tends to be trivial in dynamically typed languages.

Strictly speaking the expression problem is only defined for _static_ type systems because its concerned with _static_ type safety and extensions without recompilation.

[tmountain]

It does solve it and doing so was a key design goal of protocols. It's right there in the docs:

There are several motivations for protocols:

Avoid the 'expression problem' by allowing independent extension of the set of types, protocols, and implementations of protocols on types, by different parties

I agree that it's a concern in static type systems, but the same issue rears its head when defining methods which operate on specific types of strongly typed data, so no, it's not always trivial, nor is it a problem exclusive to statically typed languages, and if it was, there wouldn't be a need to introduce a new abstraction for which addressing the problem is a key goal.