That’s not possible with pattern matching and, as far as I know, with Dialyzer.
The thing is, you won’t have static typing guarantees in a dynamic language like Elixir (save some things that can be detected by Dialyzer), and at runtime, a Protocol.UndefinedError saying that “protocol X not implemented for y of type Y” will be more descriptive than a failed pattern match anyway.
If your concern is documentation, you can still do something akin to what the Enum module does: it defines its functions as taking an Enumerable.t() type, which is actually defined as term, but it makes it clear in the docs that anything implementing Enumerable is a valid argument.
One of the differences between protocols and duck-typing in OOP is that protocols allow you to define implementations in an open way without having to “own” the protocol or the data type. A protocol can be implemented by anyone, not only by the project defining the data type or by the project defining the protocol. It’s an elegant solution to the open-closed principle and the expression problem.
