From c66ddf8079f01284853e24ca129062c3d11229b0 Mon Sep 17 00:00:00 2001
From: Siraaj Khandkar <siraaj@khandkar.net>
Date: Sun, 21 Dec 2014 03:10:06 -0500
Subject: [PATCH] Implement hope_list:map_rev/2

O(N), tail-recursive equivalent to lists:rev(lists:map(F, L))
---
 src/hope.app.src         |  2 +-
 src/hope_list.erl        | 17 +++++++++++++++++
 test/hope_list_SUITE.erl |  7 +++++++
 3 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/src/hope.app.src b/src/hope.app.src
index 6ea0b7c..eb6f442 100644
--- a/src/hope.app.src
+++ b/src/hope.app.src
@@ -1,7 +1,7 @@
 {application, hope,
  [
   {description, "Higher Order Programming in Erlang"},
-  {vsn, "1.4.0"},
+  {vsn, "1.5.0"},
   {registered, []},
   {applications, [
                   kernel,
diff --git a/src/hope_list.erl b/src/hope_list.erl
index 9ce451e..b93ad2d 100644
--- a/src/hope_list.erl
+++ b/src/hope_list.erl
@@ -6,6 +6,7 @@
 
 -export(
     [ unique_preserve_order/1
+    , map_rev/2
     ]).
 
 
@@ -13,6 +14,22 @@
     [A].
 
 
+%% @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) ->
diff --git a/test/hope_list_SUITE.erl b/test/hope_list_SUITE.erl
index fd2393c..f591e83 100644
--- a/test/hope_list_SUITE.erl
+++ b/test/hope_list_SUITE.erl
@@ -12,6 +12,7 @@
 -export(
     [ t_unique_preserve_order/1
     , t_hope_list_specs/1
+    , t_map_rev/1
     ]).
 
 
@@ -31,6 +32,7 @@ groups() ->
     Tests =
         [ t_unique_preserve_order
         , t_hope_list_specs
+        , t_map_rev
         ],
     Properties = [parallel],
     [{?GROUP, Properties, Tests}].
@@ -40,6 +42,11 @@ groups() ->
 %%  Test cases
 %% =============================================================================
 
+t_map_rev(_Cfg) ->
+    F = fun (N) -> N + 1 end,
+    [4, 3, 2] = hope_list:map_rev([1, 2, 3], F),
+    []        = hope_list:map_rev([], F).
+
 t_unique_preserve_order(_Cfg) ->
     ?PROPTEST(prop_unique_preserve_order).
 
-- 
2.20.1