FutureLearn - Functional Programming in Erlang 2.12 - The 'nub' function

nub.erl

-module(nub).
-export([all_tests/0,nub/1,nub2/1]).

%% Helper to add element to list with no duplicate elements.
add(X, []) -> [X];
add(X, [Y|Uniqs]) when X == Y -> [Y|Uniqs];
add(X, [Y|Uniqs]) -> [Y|add(X, Uniqs)].

%% Helper to create a list of unique elements.
uniqs([],Uniqs) -> Uniqs;
uniqs([X|Xs],Uniqs) ->
    uniqs(Xs,add(X,Uniqs)).

%% Reverse a list.
-spec reverse([T]) -> [T].
reverse(Xs) ->  reverse(Xs, []).

-spec reverse([T], [T]) -> [T].
reverse([], Ys) -> Ys;
reverse([X|Xs], Ys) -> reverse(Xs, [X|Ys]).

%% Removes all duplications from a list (keeping the first occurence)
-spec nub([T]) -> [T].
nub([]) -> [];
nub(Xs) -> uniqs(Xs,[]).

%% Removes all duplications from a list (keeping the last occurence)
-spec nub2([T]) -> [T].
nub2([]) -> [];
nub2(Xs) -> reverse(uniqs(reverse(Xs),[])).

all_tests() ->
    [2,4,1,3] = nub([2,4,1,3,3,1]),
    [2,4,3,1] = nub2([2,4,1,3,3,1]),
    success.