Functional Programming in Erlang, 2.15

where3.erl

-module(where3).
-export([palindrome/1]).

palindrome(List) -> 
	real_palindrome(lcase(strip(List))).

real_palindrome([]) ->
	true;
real_palindrome([_X|[]]) ->
	true;
real_palindrome([F|Xs]) ->
	{Rest,[L]} = lists:split(length(Xs) - 1, Xs),
	case lowercase(F) =:= lowercase(L) of
		true  ->
			palindrome(Rest);
		false ->
			false
	end.

ctype(C) when C < 65 -> punctuation;
ctype(C) when C > 64 -> letter.

lowercase(C) when C > 65, C < 91 -> C + 32;
lowercase(C) when C > 96, C < 123 -> C;
lowercase(_) -> badarg.

lcase([]) -> [];
lcase([X|Xs]) ->
	[lowercase(X)|lcase(Xs)].

strip([]) -> [];
strip([X|Xs]) -> 
	case ctype(X) of
		punctuation ->
			strip(Xs);
		letter ->
			[X|strip(Xs)]
	end.