add map_err_or_else to result

tests/test_result.cpp

@@ -259,6 +259,49 @@ BOOST_AUTO_TEST_CASE(test_map_or_else)
     }
 }
 
+BOOST_AUTO_TEST_CASE(test_map_err_or_else)
+{
+    {
+        const toml::result<int, std::string> result(toml::ok(42));
+        const auto mapped = result.map_err_or_else(
+                [](const std::string i) -> std::string {
+                    return i + i;
+                }, "foobar");
+
+        BOOST_CHECK_EQUAL(mapped, "foobar");
+    }
+    {
+        toml::result<std::unique_ptr<int>, std::string>
+            result(toml::ok(std::unique_ptr<int>(new int(42))));
+        const auto mapped = std::move(result).map_err_or_else(
+                [](const std::string i) -> std::string {
+                    return i + i;
+                }, "foobar");
+
+        BOOST_CHECK_EQUAL(mapped, "foobar");
+    }
+    {
+        const toml::result<int, std::string> result(toml::err<std::string>("hoge"));
+        const auto mapped = result.map_err_or_else(
+                [](const std::string i) -> std::string {
+                    return i + i;
+                }, "foobar");
+
+        BOOST_CHECK_EQUAL(mapped, "hogehoge");
+    }
+    {
+        toml::result<std::unique_ptr<int>, std::string>
+            result(toml::err<std::string>("hoge"));
+        const auto mapped = result.map_err_or_else(
+                [](const std::string i) -> std::string {
+                    return i + i;
+                }, "foobar");
+
+        BOOST_CHECK_EQUAL(mapped, "hogehoge");
+    }
+}
+
+
 BOOST_AUTO_TEST_CASE(test_and_then)
 {
     {

toml/result.hpp

@@ -494,6 +494,31 @@ struct result
         return f(std::move(this->as_ok()));
     }
 
+    // prerequisities
+    // F: E -> U
+    // retval: U
+    template<typename F, typename U>
+    return_type_of_t<F, error_type&>
+    map_err_or_else(F&& f, U&& opt) &
+    {
+        if(this->is_ok()){return std::forward<U>(opt);}
+        return f(this->as_err());
+    }
+    template<typename F, typename U>
+    return_type_of_t<F, error_type const&>
+    map_err_or_else(F&& f, U&& opt) const&
+    {
+        if(this->is_ok()){return std::forward<U>(opt);}
+        return f(this->as_err());
+    }
+    template<typename F, typename U>
+    return_type_of_t<F, error_type&&>
+    map_err_or_else(F&& f, U&& opt) &&
+    {
+        if(this->is_ok()){return std::forward<U>(opt);}
+        return f(std::move(this->as_err()));
+    }
+
     // prerequisities:
     // F: func T -> U
     // toml::err(error_type) should be convertible to U.