Reverse polish calculator in Elixir

calc.exs

defmodule Calc do
    defp exec_op(_, "clear"), do: []
    defp exec_op([fa | [fb | r]], token) do
        case token do
            "+" -> [fa + fb | r]
            "-" -> [fa - fb | r]
            "*" -> [fa * fb | r]
            "/" -> [fa / fb | r]
            "swap" -> [fb | [fa | r]]
        end
    end

    defp exec_token(stack, token) do
        case Float.parse(token) do
            {f, _} -> [f | stack]
            :error -> exec_op(stack, token)
        end
    end

    defp exec_tokens(stack, []), do: stack
    defp exec_tokens(stack, tokens) do
        exec_token(stack, hd(tokens))
            |> fn s -> exec_tokens(s, tl(tokens)) end.()
    end

    def mainloop(stack) do
        if stack != [] do
            IO.puts("#{hd(stack)}")
        end

        IO.gets("calc> ")
            |> String.split()
            |> fn tokens -> exec_tokens(stack, tokens) end.()
            |> mainloop()
    end
end

Calc.mainloop([])