Reverse Polish notation calculator

rpn.erl

% -*- coding: utf8 -*-
% From LearnYouSomeErlang Functionally Solving Problems
-module(main).
-export([main/0]).

main() ->
    5 = rpn("2 3 +"),
    87 = rpn("90 3 -"),
    -4 = rpn("10 4 3 + 2 * -"),
    -2.0 = rpn("10 4 3 + 2 * - 2 /"),
    ok = try
        rpn("90 34 12 33 55 66 + * - +")
    catch
        error:{badmatch,[_|_]} -> ok
    end,
    4037 = rpn("90 34 12 33 55 66 + * - + -"),
    8.0 = rpn("2 3 ^"),
    true = math:sqrt(2) == rpn("2 0.5 ^"),
    true = math:log(2.7) == rpn("2.7 ln"),
    true = math:log10(2.7) == rpn("2.7 log10").
%% TODO--
%%    50 = rpn("10 10 10 20 sum"),
%%    10.0 = rpn("10 10 10 20 sum 5 /"),
%%    1000.0 = rpn("10 10 20 0.5 prod").

rpn(L) when is_list(L) ->
    [Res] = lists:foldl(fun rpn/2, [], string:tokens(L," ")),
    Res.

rpn("+", [N1, N2|S]) -> [N2+N1|S];
rpn("-", [N1, N2|S]) -> [N2-N1|S];
rpn("*", [N1, N2|S]) -> [N2*N1|S];
rpn("/", [N1, N2|S]) -> [N2/N1|S];
rpn("^", [N1, N2|S]) -> [math:pow(N2,N1)|S];
rpn("ln", [N|S]) -> [math:log(N)|S];
rpn("log10", [N|S]) -> [math:log10(N)|S];
rpn(X,Stack) -> [read(X) | Stack].

read(N) ->
    case string:to_float(N) of
        {error,no_float} -> list_to_integer(N);
        {F,_} -> F
    end.