Writing the function "once" in Elixir

Question

I'm coming to Elixir from primarily a Javascript background. in JS, it's possible to write a higher order function "once" which returns a function that will invoke the passed in function only once, and returns the previous result on subsequent calls- the trick is manipulating variables that were captured via closure:

var once = (func) => {
    var wasCalled = false, prevResult;
    return (...args) => {
        if (wasCalled) return prevResult;
        wasCalled = true;
        return prevResult = func(...args);
    }
}

It seems to me that it's not possible to create this function in Elixir, due to its different variable rebinding behavior. Is there some other clever way to do it via pattern matching or recursion, or is it just not possible? Without macros that is, I'd imagine those might enable it. Thanks

Answer 1

Using the current process dictionary:

defmodule A do
  def once(f) do
    key = make_ref()
    fn ->
      case Process.get(key) do
        {^key, val} -> val
        nil -> 
          val = f.()
          Process.put(key, {key, val})
          val
      end
    end
  end
end

Or if the function will be passed across processes, an ets table can be used:

# ... during application initialization
:ets.new(:cache, [:set, :public, :named_table])


defmodule A do
  def once(f) do
    key = make_ref()
    fn ->
      case :ets.lookup(:cache, key) do
        [{^key, val}] -> val
        [] -> 
          val = f.()
          :ets.insert(:cache, {key, val})
          val
      end
    end
  end
end

Application.put_env / Application.get_env can also be used to hold global state, though usually is used for configuration settings.