From a80ca0b203dc3398a0117d2e1fc6ae5d77250783 Mon Sep 17 00:00:00 2001
From: Siraaj Khandkar <siraaj@khandkar.net>
Date: Tue, 24 Mar 2015 07:18:40 -0400
Subject: [PATCH] Implement hope_result:(map_error and tag_error)

---
 src/hope_result.erl        | 18 +++++++++++++++++-
 test/hope_result_SUITE.erl | 20 +++++++++++++++++---
 2 files changed, 34 insertions(+), 4 deletions(-)

diff --git a/src/hope_result.erl b/src/hope_result.erl
index 7cae0a3..dec6e79 100644
--- a/src/hope_result.erl
+++ b/src/hope_result.erl
@@ -9,6 +9,8 @@
 -export(
     [ return/1
     , map/2
+    , map_error/2
+    , tag_error/2
     , pipe/2
     , lift_exn/1
     , lift_exn/2
@@ -33,6 +35,20 @@ map({ok, X}, F) ->
 map({error, _}=Error, _) ->
     Error.
 
+-spec map_error(t(A, B), fun((B) -> (C))) ->
+    t(A, C).
+map_error({ok, _}=Ok, _) ->
+    Ok;
+map_error({error, Reason}, F) ->
+    {error, F(Reason)}.
+
+-spec tag_error(t(A, Reason), Tag) ->
+    t(A, {Tag, Reason}).
+tag_error({ok, _}=Ok, _) ->
+    Ok;
+tag_error({error, Reason}, Tag) ->
+    {error, {Tag, Reason}}.
+
 -spec pipe([F], X) ->
     t(Ok, Error)
     when X     :: any()
@@ -76,6 +92,6 @@ lift_exn(F, Label) when is_function(F, 1) ->
         try
             {ok, F(X)}
         catch Class:Reason ->
-            {error, {Label, {Class, Reason}}}
+            tag_error({error, {Class, Reason}}, Label)
         end
     end.
diff --git a/test/hope_result_SUITE.erl b/test/hope_result_SUITE.erl
index 9e60e85..d323cee 100644
--- a/test/hope_result_SUITE.erl
+++ b/test/hope_result_SUITE.erl
@@ -1,8 +1,5 @@
 -module(hope_result_SUITE).
 
-%% TODO: Import only what is used.
--include_lib("proper/include/proper.hrl").
-
 %% Callbacks
 -export(
     [ all/0
@@ -19,6 +16,8 @@
     , t_lift_exn/1
     , t_return/1
     , t_map/1
+    , t_map_error/1
+    , t_tag_error/1
     ]).
 
 
@@ -53,6 +52,8 @@ groups() ->
     OtherTests =
         [ t_return
         , t_map
+        , t_map_error
+        , t_tag_error
         ],
     Properties = [parallel],
     [ {?GROUP_PIPE, Properties, PipeTests}
@@ -110,3 +111,16 @@ t_map(_Cfg) ->
     F = fun (foo) -> Y end,
     {ok, Y}    = hope_result:map({ok, X}, F),
     {error, X} = hope_result:map({error, X}, F).
+
+t_map_error(_Cfg) ->
+    X = foo,
+    Y = bar,
+    XtoY = fun (foo) -> Y end,
+    {ok   , X} = hope_result:map_error({ok   , X}, XtoY),
+    {error, Y} = hope_result:map_error({error, X}, XtoY).
+
+t_tag_error(_Cfg) ->
+    X = foo,
+    Tag = bar,
+    {ok   ,       X } = hope_result:tag_error({ok   , X}, Tag),
+    {error, {Tag, X}} = hope_result:tag_error({error, X}, Tag).
-- 
2.20.1