Protocols are sort of an extension to pattern-matching:
- Normal pattern-matching only allows the person writing ModuleX to add function clauses to a certain function.
- Protocols allow both the person writing, as well as any people using ModuleX as dependency to add function clauses to one (or multiple) of ModuleX’s functions.
Which protocol-function-clause is used depends on the type of the first argument passed to the function, so in that way this extra freedom is somewhat bounded (but I have yet to find a case where this does not provide enough flexibility).
A third thing you might want to compare with pattern-matching/Protocols are Behaviours: Rather than swapping what what kind of data-type is passed to a function, you are swapping what module a certain function is called on. Behaviours are often used on modules that define a number of complex functions (often on a module that is run as a separate process/GenServer). These basically list a number of functions (including their typespecs) that a module that implements a certain behaviour has to define. When not following a Behaviour, a compile-time warning is shown.
