Memoization using a `defmem` macro and a gen_server.

memoize.ex

defmodule Memoize do
  alias Memoize.ResultTable

  defmacro defmem(header, do: body) do
    { name, _meta, vars } = header

    quote do
      def unquote(header) do
        case ResultTable.get(unquote(name), unquote(vars)) do
          { :hit, result } ->
            result
          :miss ->
            result = unquote(body)
            ResultTable.put(unquote(name), unquote(vars), result)
            result
        end
      end
    end
  end

  # gen_server keeping results for function calls
  defmodule ResultTable do
    use GenServer

    def start_link do
      GenServer.start_link(__MODULE__, %{}, name: :result_table)
    end

    def init do
      { :ok, HashDict.new }
    end
   
    def handle_call({ :get, fun, args }, _sender, dict) do
      if Dict.has_key?(dict, { fun, args }) do
        { :reply, { :hit, dict[{ fun, args }] }, dict }
      else
        { :reply, :miss, dict }
      end
    end

    def handle_cast({ :put, fun, args, result }, dict) do
      { :noreply, Dict.put(dict, { fun, args }, result) }
    end
   
    def get(fun, args),         do: GenServer.call(:result_table, { :get, fun, args })
    def put(fun, args, result), do: GenServer.cast(:result_table, { :put, fun, args, result })
  end 
end

defmodule Fib do
  require Memoize

  Memoize.defmem fibs(0), do: 0
  Memoize.defmem fibs(1), do: 1
  Memoize.defmem fibs(n), do: fibs(n - 1) + fibs(n - 2)
end

defmodule Time do
  # measure running time of f
  def running_time(f) do
    t_before = :erlang.timestamp()
    return   = f.()
    t_after  = :erlang.timestamp()
    { return, :timer.now_diff(t_after, t_before) }
  end
end

# only oe instance is needed
Memoize.ResultTable.start_link

IO.puts "function -> {result, running time}"
IO.puts "=================================="
IO.puts "fibs(20) -> #{inspect Time.running_time fn -> Fib.fibs(20) end}"
IO.puts "fibs(20) -> #{inspect Time.running_time fn -> Fib.fibs(20) end}"
IO.puts "fibs(30) -> #{inspect Time.running_time fn -> Fib.fibs(30) end}"