list_functions2.erl

-module(list_functions2).
-export([double/1,evens/1]).
-export([double_test/0,evens_test/0]).

%% double %%%

double([]) -> [];
double([_|_] = L) -> double(L, []).

double([], Ac) -> lists:reverse(Ac);
double([H|T], Ac) when is_integer(H) -> double(T, [2*H|Ac]).

double_test() ->
  [] = double([]),
  [2] = double([1]),
  [6,8] = double([3,4]),
  [-2,0,2,4,6] = double([-1,0,1,2,3]),
  passed.

%% evens %%%

evens([]) -> [];
evens([_|_] = L) -> evens(L, []).

evens([], Ac) -> lists:reverse(Ac);
evens([H|T], Ac) when H rem 2 == 0 ->
  evens(T, [H|Ac]);
evens([_|T], Ac) -> evens(T, Ac).

evens_test() ->
  [] = evens([]),
  [] = evens([1,3,5,7]),
  [2] = evens([2]),
  [2] = evens([1,2,3,5,7,9]),
  [2,4,6,8] = evens([1,2,3,4,5,6,7,8,9]),
  [-2,2] = evens([-2,-1,1,2]),
  passed.

Good job!
Maybe… for completeness sake, you could've added some floating-point values to your tests…