X-Git-Url: https://git.xandkar.net/?a=blobdiff_plain;f=src%2Fhope_list.erl;h=aba9944f9d5b7ee53c2a3a0a73ee52f3a3e76e34;hb=2a81fbac3178e649d03519f7b1b25e43d7850713;hp=9ce451e927d454940d3dca236266492dd6b61051;hpb=17b5d686d1b6c608c8fab801f44701bb0c6cccf3;p=hope.git

diff --git a/src/hope_list.erl b/src/hope_list.erl
index 9ce451e..aba9944 100644
--- a/src/hope_list.erl
+++ b/src/hope_list.erl
@@ -6,6 +6,8 @@
 
 -export(
     [ unique_preserve_order/1
+    , map_rev/2
+    , map_slow/2
     ]).
 
 
@@ -13,6 +15,30 @@
     [A].
 
 
+%% @doc lists:reverse(map_rev(L, F))
+%% @end
+-spec map_slow([A], fun((A) -> (B))) ->
+    [B].
+map_slow(Xs, F) ->
+    lists:reverse(map_rev(Xs, F)).
+
+
+%% @doc O(N), tail-recursive equivalent to lists:rev(lists:map(F, L))
+%% @end
+-spec map_rev([A], fun((A) -> (B))) ->
+    [B].
+map_rev(Xs, F) ->
+    map_rev_acc(Xs, F, []).
+
+-spec map_rev_acc([A], fun((A) -> (B)), [B]) ->
+    [B].
+map_rev_acc([], _, Ys) ->
+    Ys;
+map_rev_acc([X|Xs], F, Ys) ->
+    Y = F(X),
+    map_rev_acc(Xs, F, [Y|Ys]).
+
+
 -spec unique_preserve_order(t(A)) ->
     t(A).
 unique_preserve_order(L) ->