FutureLearn - Functional Programming in Erlang 2.15 - Palindrome

pal.erl

-module(pal).
-export([test/0,alpha_nums/1,palindrome/1]).

%% alph_nums filter outs punctuation or non printable characters.
alpha_nums([]) -> [];
alpha_nums([X|Xs]) when X >= $A andalso X =< $z ->
    [X|alpha_nums(Xs)];
alpha_nums([X|Xs]) when X >= $0 andalso X =< $9 ->
    [X|alpha_nums(Xs)];
alpha_nums([_|Xs]) ->
    alpha_nums(Xs).

%% palindrome makes use or erlang modules (lists and string)
-spec palindrome([_]) -> boolean().
palindrome([]) -> true;
palindrome(Xs) ->
    A = string:to_lower(alpha_nums(Xs)),
    A == lists:reverse(A).

test() ->
    true = palindrome("Madam I\'m Adam"),
    true = palindrome("12321"),
    true = palindrome(""),
    true = palindrome([]),
    success.