week227.erl

-module(week227).
-compile([export_all]).
-include_lib("eunit/include/eunit.hrl").


join(Xs, Ys) -> 
    accumulate(shunt(Xs, []), Ys).

shunt([], Ys) ->
    Ys;
shunt([X|Xs], Ys) ->
    shunt(Xs, [X|Ys]).

accumulate([] ,Result) ->
    Result;
accumulate([X|Xs], Result) ->
    accumulate(Xs, [X|Result]).

concat([]) ->
    [];
concat([X|Xs]) ->
    join(X,concat(Xs)).

member(_N, []) ->
    false;
member(N, [X|Xs]) ->
    case X =:= N of
        true ->
            true;
        false ->
            member(N, Xs)
    end.

perms([]) ->
    [[]];
perms(L) ->
    [[X|Y] || X <- L, Y <- perms(L--[X])].


tests() ->
    ?assertEqual(join([],[]),[]),
    ?assertEqual(join([1,2,3],[4,5,6]),[1,2,3,4,5,6]),
    ?assertEqual(concat(["hello", "world"]),"helloworld"),
    % ?assertEqual(concat([]), []),
    ?assertEqual(perms([]), [[]]),
    ?assertEqual(perms([1,2,3]), [[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]]).

A few things to notice…

1. for ?assertEqual/2… the expected result is the first argument. You will notice that in the output when the assertion fails.
2. You can use pattern-matching more aggressively on member/2…

member(_N, []) -> false;
member(N, [N|_]) -> true;
member(N, [_|Xs]) -> member(N, Xs).

2.5. Or you can use short-circuiting…

member(_, []) -> false;
member(N, [X|Xs]) -> N =:= X orelse member(N, Xs).

3. Do you understand how perms/1 works? I'll leave this here, just in case

Thanks will go thorough