Reverse polish calculator in Erlang

calc.erl

-module(calc).
-export([main_loop/1]).

read_tokens() ->
    case io:get_line("calc> ") of
        [] -> [];
        L -> string:tokens(lists:droplast(L), " ")
    end.

exec_token(S, T) ->
    try
        F = list_to_float(T),
        [F|S]
    catch error:badarg ->
        {Fa, Fb, Tl} = {hd(S), hd(tl(S)), tl(tl(S))},
        case T of
            "+" -> [Fa + Fb|Tl];
            "-" -> [Fa - Fb|Tl];
            "*" -> [Fa * Fb|Tl];
            "/" -> [Fa / Fb|Tl];
            "clear" -> []
        end
    end.

exec_tokens(S, []) -> S;
exec_tokens(S, [T|R]) ->
    exec_tokens(exec_token(S, T), R).

main_loop(S) when S =:= [] ->
    main_loop(exec_tokens(S, read_tokens()));
main_loop(S) ->
    io:format("~f~n", [hd(S)]),
    main_loop(exec_tokens(S, read_tokens())).