behaviour.ex

defmodule Datastore do
    use Behaviour
    
    @type key :: any
    @type value :: any
    
    defcallback start_link() :: Agent.t
    defcallback get(Agent.t, key) :: value
    defcallback put(Agent.t, key, value) :: :ok   
    
    defmacro __using__(_) do
        quote do
            @behaviour Datastore
            
            def start_link() do
                Agent.start_link(fn -> %{ } end, name: __MODULE__)
            end
            
            def get(datastore, key) do 
                Agent.get(datastore, &Map.get(&1, key))
            end
    
            def put(datastore, key, value) do
                Agent.update(datastore, &Map.put(&1, key, value))
            end
            
            defoverridable start_link: 0, get: 2, put: 3
        end
    end
    
    def put(datastore, key, val) do
      __MODULE__.put(datastore, key, val) # How do I get the name/module ?
    end
end

defmodule Driver do    
    use Datastore

    def start_link() do
        Agent.start_link(fn -> HashDict.new end) # Could use __MODULE__ as the name?
    end

    def get(datastore, key) do
        Agent.get(datastore, &HashDict.get(&1, key))
    end

    def put(datastore, key, value) do
        IO.puts "Rumpen stumpen"
        Agent.update(datastore, fn dict -> HashDict.put(dict, key, value) end)
    end
end

{:ok, mem} = Driver.start_link()

Datastore.put(mem, "hello", "world") # fail