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     , map_slow/2
  11     ]).
  12
  13
  14 -type t(A) ::
  15     [A].
  16
  17
  18 %% @doc lists:reverse(map_rev(L, F))
  19 %% @end
  20 -spec map_slow([A], fun((A) -> (B))) ->
  21     [B].
  22 map_slow(Xs, F) ->
  23     lists:reverse(map_rev(Xs, F)).
  24
  25
  26 %% @doc O(N), tail-recursive equivalent to lists:rev(lists:map(F, L))
  27 %% @end
  28 -spec map_rev([A], fun((A) -> (B))) ->
  29     [B].
  30 map_rev(Xs, F) ->
  31     map_rev_acc(Xs, F, []).
  32
  33 -spec map_rev_acc([A], fun((A) -> (B)), [B]) ->
  34     [B].
  35 map_rev_acc([], _, Ys) ->
  36     Ys;
  37 map_rev_acc([X|Xs], F, Ys) ->
  38     Y = F(X),
  39     map_rev_acc(Xs, F, [Y|Ys]).
  40
  41
  42 -spec unique_preserve_order(t(A)) ->
  43     t(A).
  44 unique_preserve_order(L) ->
  45     PrependIfNew =
  46         fun (X, Xs) ->
  47             case lists:member(X, Xs)
  48             of  true  ->      Xs
  49             ;   false -> [X | Xs]
  50             end
  51         end,
  52     lists:reverse(lists:foldl(PrependIfNew, [], L)).