At a high level you’re already on the right track: Pass things as data instead of trying to “pass logic around”. How you encode your invariants into data however depends quite a bit on what really is different between your cases. For simple cases just a struct/map with some keys might be enough. For more complex cases you can create a behaviour and require a module to be supplied, which implements the behaviour (like how genservers work).

For the behaviour/implementation part: Any email library, especially when allowing you to select the sending service via the parameters supplied. Also as already mentioned - genservers - otp provides the code, which runs it, while the module you pass in is in charge of implementing how it behaves.

thank you for the replays - I think abacus is exactly what I’m looking for:
iex> Abacus.eval("a * b", %{"a" => 10, "b" => 10})
{:ok, 100}