% Imperative Solution
Histogram = histogram(List),

-module(histogram).
-export([start/0]).

start() ->

List = [a, b, a, c, b, b],

% Imperative Solution
Histogram = histogram(List),

io:format("~w~n", [Histogram]).

histogram(List) ->
AccTbl = ets:new(the_map_name, [ordered_set, private, {keypos, 1}]),
ets:insert(AccTbl, lists:zip(List, lists:duplicate(length(List), 0))),
lists:foreach(fun (X) -> ets:update_counter(AccTbl, X, 1) end, List),
Histogram = ets:tab2list(AccTbl),
ets:delete(AccTbl),
Histogram.

% Functional (1) Solution
Histogram = histogram(List),

-module(histogram).
-export([start/0]).

start() ->

List = [a, b, a, c, b, b],

% Functional (1) Solution
Histogram = histogram(List),

io:format("~w~n", [Histogram]).

histogram(List) -> histogram(List, dict:new()).

histogram([], Dict) -> dict:to_list(Dict);

histogram([H|T], Dict) ->
case dict:find(H, Dict) of error -> Value = 0 ; {ok, Value} -> Value end,
histogram(T, dict:store(H, Value + 1, Dict)).