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 Tail-recursive alternative to lists:map/2, which accumulates and
  27 %% returns list in reverse order.
  28 %% @end
  29 -spec map_rev([A], fun((A) -> (B))) ->
  30     [B].
  31 map_rev(Xs, F) ->
  32     map_rev_acc(Xs, F, []).
  33
  34 -spec map_rev_acc([A], fun((A) -> (B)), [B]) ->
  35     [B].
  36 map_rev_acc([], _, Ys) ->
  37     Ys;
  38 map_rev_acc([X|Xs], F, Ys) ->
  39     Y = F(X),
  40     map_rev_acc(Xs, F, [Y|Ys]).
  41
  42
  43 -spec unique_preserve_order(t(A)) ->
  44     t(A).
  45 unique_preserve_order(L) ->
  46     PrependIfNew =
  47         fun (X, Xs) ->
  48             case lists:member(X, Xs)
  49             of  true  ->      Xs
  50             ;   false -> [X | Xs]
  51             end
  52         end,
  53     lists:reverse(lists:foldl(PrependIfNew, [], L)).