msort.erl

-module(msort).
-export([sort/1]).

sort([]) ->
    [];
sort([X]) ->
    [X];
sort(List) ->
    {Xs, Ys} = split(List),
    merge(sort(Xs), sort(Ys)).

merge(Xs, []) ->
    Xs;
merge([], Ys) ->
    Ys;
merge([X|Xs], [Y|Ys]) ->
    case X =< Y of
        true ->
            [X|merge(Xs, [Y|Ys])];
        false ->
            [Y|merge([X|Xs], Ys)]
    end.

split(List) ->
    lists:split(length(List) div 2, List).

msort_tests.erl

-module(msort_tests).
-include_lib("eunit/include/eunit.hrl").

msort_test() ->
    ?assertEqual([1, 2, 3, 4], msort:sort([1, 2, 3, 4])),
    ?assertEqual([1, 2, 3, 4], msort:sort([1, 3, 2, 4])),
    ?assertEqual([1, 2, 3, 4], msort:sort([4, 3, 2, 1])),
    ?assertEqual([1, 2, 3], msort:sort([1, 2, 3])),
    ?assertEqual([1, 2, 3], msort:sort([2, 1, 3])),
    ?assertEqual([1, 2, 3], msort:sort([3, 2, 1])).