Difference between protocol & behaviour in elixir

Question

Behaviours define callbacks & protocols define methods without signatures. Modules implementing a protocol should give definition for all those methods. Same for modules using a behaviour. What is the semantic difference?

One difference I can think of is, a protocol can be implemented for a single type only once where as we can implement a behaviour for a module multiple times based on our requirements. I am clear with when to use what. Is there any other difference other than this?

Answer 1

Protocol is type/data based polymorphism. When I call Enum.each(foo, ...), the concrete enumeration is determined from the type of foo.

Behaviour is a typeless plug-in mechanism. When I call GenServer.start(MyModule), I explicitly pass MyModule as a plug-in, and the generic code from GenServer will call into this module when needed.

Answer 2

Answered by José Valim on the same topic ( from google thread, https://groups.google.com/forum/#!msg/elixir-lang-talk/S0NlOoc4ThM/J2aD2hKrtuoJ )

A protocol is indeed a behaviour + dispatching logic.

However I think you are missing the point of behaviours. Behaviours are extremely useful. For example, a GenServer defines a behaviour. A behaviour is a way to say: give me a module as argument and I will invoke the following callbacks on it, which these argument and so on. A more complex example for behaviours besides a GenServer are the Ecto adapters.

However, this does not work if you have a data structure and you want to dispatch based on the data structure. Hence protocols.