fold.erl

-module(folder).

assign(Dict, [LastPart], Value) ->
    Dict:store(LastPart, Value);

assign(Dict, [Part|Tail], Value) ->
    NewDict = case Dict:exists(Part) of
                ok    -> assign(Dict:fetch(Part), Tail, Value);
                error -> assign(dict:new(), Tail, Value)
              end,
    Dict:store(Part, NewDict).

sample() ->
    Dict = dict:new(),
    assign(Dict, ["a", "b", "c"], 1).