I recently asked about a very similar situation (calling a function from a NIF) over on the Erlang forums. There is no straightforward way to do this, but there are a number of interesting workarounds.
The core approach is similar to what @garazdawi also mentions: Use a separate Elixir/Erlang process which you exchange messages with. To hide this async communication inside a function that expects to run in a normal synchronious fashion, using any kind of mutex to wait for a reply (/or crash) from the process.
