I'm Learning Erlang

Here is my first Erlang Code:

-module(factorial).
-export([fact/1,sum/1,flatten/1,flatten1/1,flatten2/1]).

fact (0) -> 1;
fact (N) -> N* fact(N-1).

sum([]) -> 0;
sum([H|T])  -> H + sum(T).

append ([],L) -> L;
append ([H|T],L) -> [H|append(T,L)].

flatten([]) -> [];
flatten([X|L]) when is_list(X)  -> append(flatten(X),flatten(L));
flatten([X|L])           -> [X|flatten(L)].   

%comment

%alternatively with if
flatten1([]) -> [];
flatten1([X|L]) -> if is_list(X)  -> append(flatten1(X),flatten(L));
              true   ->[X|flatten1(L)]
           end.  


%or with ++


flatten2([]) -> [];
flatten2([X|L]) when is_list(X)  -> flatten2(X) ++ flatten2(L);
flatten2([X|L])           -> [X|flatten2(L)].   




To run it

(1) first install Erlang with apt-get install erlang
(2) Then type erl at the command line
(3) Then compile it with c(factorial).
(4) Then try out some expressions e.g.
factorial:flatten2([[1,2],3,[4,5,6]]).
It will say:
[1,2,3,4,5,6]