erlang hailstone

hailstone.erl

-module(hailstone).
-import(io).
-export([main/1]).

hailstone(1) -> [1];
hailstone(N) when N band 1 == 1 -> [N|hailstone(N * 3 + 1)];
hailstone(N) when N band 1 == 0 -> [N|hailstone(N div 2)].

main(End) ->
    F = fun (N) -> length(hailstone(N)) end,
    Lengths = lists:map(F, lists:seq(1, End)),
    average(Lengths).

average(List) ->
    lists:sum(List) / length(List).