OOP in Erlang. Part 2. Polymorphism

Encapsulation was covered in previous article of my OOP in Erlang series.

Polymorphism

Polymorphism is ability to present the same interface for different instances (types).

Processes

First of all, process is a unit of encapsulation in Erlang, polymorphism could be implemented on that level.
Client can send the same message to different processes, which will handle the message differently.
For example, we can implement gen_server behaviour in two modules.
serv1.erl

handle_call(message, _From, State) ->
  {reply, 1, State}.

serv2.erl

handle_call(message, _From, State) ->
  {reply, 2, State}.

Then client chooses one of gen_servers and starts it, saving Pid of the process. And then gen_server:call(Pid, message) can be called and caller experiences different behaviour based on module chosen in the beginning.

Dynamic types

Erlang is dynamically typed language, as a result module and even function name could be taken from variable during function call. For example, interfaces of dict and orddict from OTP are unified, and following code shows polymorphism implementation.

Module =
       case Ordering of
           unordered ->
               dict;
           ordered ->
               ordict
       end,
Container = Module:from_list(Values),
Module:find(Key, Container).

Pattern matching

Pattern matching is a powerful feature of the language, which can be used for polymorphism implementation. The idea is that function changes it's behaviour depending on arguments it gets. The most obvious data type to match on is record.

move(#duck{}) -> move_duck();
move(#fish{}) -> move_fish().

Extensive usage of pattern matching for that purpose leads to decoupling of "object" from it's behaviour (business logic), as a result for adding of a new "type" changes in multiple modules are required. This is very similar to Anemic domain model, which has some advantages though.

Conclusion

Polymorphism in Erlang could me implemented in different ways. Even though processes provide the most clear interface for that, developer should not create processes only for modeling business logic, because:

1. Logic changes quite often and changing all boilerplate code of processes is an overhead.
2. System becomes more complicated with each new type of process spawned.