Phone mnemonics in Erlang

mnemonics.erl

-module(mnemonics).
-export([translate/1]).
-define(MNEMONICS, [{2, "ABC"}, {3, "DEF"}, {4, "GHI"}, {5, "JKL"}, {6, "MNO"}, {7, "PQRS"}, {8, "TUV"}, {9, "WXYZ"}]).
-define(WORDS, []).

% Public API

translate(StringNumber) ->
    Result2 = encode(StringNumber, fun(X) -> sets:to_list(sets:from_list(slab(X))) end),
    Result = sets:from_list(Result2),
    Strings = sets:fold(fun(Words, AccIn) -> [string:join(filter_empty(Words), " ")|AccIn] end, [], Result),
    filter_empty(Strings).

% Private API

wordCode(Word, CharCode) ->
    lists:map(fun(Char) -> dict:fetch(Char, CharCode) end, string:to_upper(Word)).

wordsForNum(StringNumber) ->
    CharCodeNested = lists:map(fun({N, Str}) -> lists:map(fun(Char) -> {Char, N} end, Str) end, ?MNEMONICS),
    CharCode = dict:from_list(lists:flatten(CharCodeNested)),
    NumWords = lists:map(fun(Word) -> {Word, wordCode(Word, CharCode)} end, ?WORDS),
    WordsDict = lists:foldl(fun({Word, Code}, AccIn) ->
                                CodeString = string:join(lists:map(fun(Char) -> integer_to_list(Char) end, Code), ""),
                                dict:append(CodeString, Word, AccIn) end, dict:new(), NumWords),
    case dict:is_key(StringNumber, WordsDict) of
        true ->
            dict:fetch(StringNumber, WordsDict);
        false ->
            []
    end.

encode(StringNumber, Fun) ->
    L = length(StringNumber),
    Res = lists:map(fun(Idx) -> Word = wordsForNum(string:substr(StringNumber, 1, Idx)),
                                Rest = encode(string:substr(StringNumber, Idx + 1, L), fun(X) -> X end),
                                Fun([Word|Rest]) end, lists:seq(1, L)),
    Res.

filter_empty(List) ->
    filter_empty(List, []).

filter_empty([], AccIn) -> AccIn;
filter_empty([H|T], AccIn) when is_list(H) andalso length(H) == 0 ->
    filter_empty(T, AccIn);
filter_empty([H|T], AccIn) ->
    filter_empty(T, [H|AccIn]).

slab([]) ->
    [];
slab([F|R]) ->
    case io_lib:char_list(F) of
        true -> [F|slab(R)];
        false -> slab(F) ++ slab(R)
    end.