Provides an unflatten function. Useful when you flatten a list to do something and then need to unflatten an associated result list.

mylists.erl

-module(mylists).

-export([ unflatten/2 ]).

unflatten([],[]) ->
  [];
unflatten(List,[PSubList|PList]) ->
  unflatten(List,[],[],PSubList,PList).

unflatten([],SubList,Lists,[],[]) ->
  lists:reverse([lists:reverse(SubList)|Lists]);
unflatten(L,SubList,Lists,[],[PSubList|PLists]) ->
  unflatten(L,[],[lists:reverse(SubList)|Lists],PSubList,PLists);
unflatten([H|T],SubList,Lists,[PSubH|PSubT],PLists) ->
  unflatten(T,[H|SubList],Lists,PSubT,PLists).

usage.erl

example() ->
  ML = [[1,2,3],[4,5,6],[],[7,8],[9]],
  MLF = lists:flatten(ML),
  RL = [9,8,7,6,5,4,3,2,1],
  RLUF = mylists:unflatten(RL,ML),
  [[9,8,7],[6,5,4],[],[3,2],[1]] = RLUF.