src/hope_list.erl

   1 -module(hope_list).
   2
   3 -export_type(
   4     [ t/1
   5     ]).
   6
   7 -export(
   8     [ unique_preserve_order/1
   9     , map_rev/2
  10     ]).
  11
  12
  13 -type t(A) ::
  14     [A].
  15
  16
  17 %% @doc O(N), tail-recursive equivalent to lists:rev(lists:map(F, L))
  18 %% @end
  19 -spec map_rev([A], fun((A) -> (B))) ->
  20     [B].
  21 map_rev(Xs, F) ->
  22     map_rev_acc(Xs, F, []).
  23
  24 -spec map_rev_acc([A], fun((A) -> (B)), [B]) ->
  25     [B].
  26 map_rev_acc([], _, Ys) ->
  27     Ys;
  28 map_rev_acc([X|Xs], F, Ys) ->
  29     Y = F(X),
  30     map_rev_acc(Xs, F, [Y|Ys]).
  31
  32
  33 -spec unique_preserve_order(t(A)) ->
  34     t(A).
  35 unique_preserve_order(L) ->
  36     PrependIfNew =
  37         fun (X, Xs) ->
  38             case lists:member(X, Xs)
  39             of  true  ->      Xs
  40             ;   false -> [X | Xs]
  41             end
  42         end,
  43     lists:reverse(lists:foldl(PrependIfNew, [], L)).