From 9b2c4842ae9cf83cf76272a441e46d74f726b4de Mon Sep 17 00:00:00 2001
From: Nerijus Arlauskas <nercury@gmail.com>
Date: Wed, 6 Jul 2016 20:47:34 +0300
Subject: [PATCH] Use crazy magic and infer returned result type from return
 type of lambda.

---
 src/maybe/result.hpp            |  4 ++--
 src/maybe/result.inline.hpp     | 11 +++++++----
 tests/result_chaining_tests.cpp |  2 +-
 3 files changed, 10 insertions(+), 7 deletions(-)

diff --git a/src/maybe/result.hpp b/src/maybe/result.hpp
index ce77c43..bfa8d57 100644
--- a/src/maybe/result.hpp
+++ b/src/maybe/result.hpp
@@ -193,8 +193,8 @@ namespace maybe {
             return std::addressof(err_val);
         }
 
-        template <typename R>
-        constexpr result<R, E> map(std::function<R(T v)> closure) noexcept;
+        template <typename F>
+        constexpr auto map(F f) noexcept -> maybe::result<decltype(f(std::declval<T>())), E>;
 
     private:
         constexpr void copy_from(const result<T, E>& other) noexcept;
diff --git a/src/maybe/result.inline.hpp b/src/maybe/result.inline.hpp
index 51c6cbe..b00cf5c 100644
--- a/src/maybe/result.inline.hpp
+++ b/src/maybe/result.inline.hpp
@@ -97,11 +97,14 @@ constexpr void maybe::result<T, E>::clear() noexcept
 }
 
 template <typename T, typename E>
-template <typename R>
-constexpr maybe::result<R, E> maybe::result<T, E>::map(std::function<R(T v)> closure) noexcept
+template <typename F>
+constexpr auto maybe::result<T, E>::map(F f) noexcept
+    -> maybe::result<decltype(f(std::declval<T>())), E>
 {
+    typedef maybe::result<decltype(f(std::declval<T>())), E> return_result_t;
+
     if (is_err()) {
-        return maybe::result<R, E>::err(err_value());
+        return return_result_t::err(std::forward<E>(err_value()));
     }
-    return maybe::result<R, E>::ok(closure(ok_value()));
+    return return_result_t::ok(f(std::forward<T>(ok_value())));
 };
\ No newline at end of file
diff --git a/tests/result_chaining_tests.cpp b/tests/result_chaining_tests.cpp
index 56052c0..399361f 100644
--- a/tests/result_chaining_tests.cpp
+++ b/tests/result_chaining_tests.cpp
@@ -21,7 +21,7 @@ TEST_CASE("result_chaining")
     SECTION("converts result A to result B")
     {
         auto a = result<A, int>::ok(A("hello"));
-        auto b = a.map<B>([](const A& v) { return B(v.value); });
+        auto b = a.map([](A v) { return B(v.value); });
         REQUIRE(b);
         REQUIRE(b.ok_value().value == "hello");
     }