main.erl

-module(main).
-export([palindrome/1]).
-import(lists, [filter/2, map/2, reverse/1]).

is_between(X, A, B) ->
  X >= A andalso B >= X.

is_lowercase_char(Ch) ->
  is_between(Ch, $a, $z).

is_uppercase_char(Ch) ->
  is_between(Ch, $A, $Z).


is_letter(Ch) ->
  is_lowercase_char(Ch) orelse
    is_uppercase_char(Ch).

lowercase(Ch) ->
  case is_uppercase_char(Ch) of
    true -> $a + Ch - $A;
    false -> Ch
  end.

palindrome(Str) ->
  Filtered = filter(fun is_letter/1, Str),
  Lowercase = map(fun lowercase/1, Filtered),
  Lowercase == reverse(Lowercase).