From 52c783731e02b79dce1b6b9f71fd1ceb27141708 Mon Sep 17 00:00:00 2001
From: Nerijus Arlauskas <nercury@gmail.com>
Date: Thu, 7 Jul 2016 01:10:45 +0300
Subject: [PATCH] Implement and_then and use simpler magic.

---
 src/maybe/result.hpp            |  7 +++++-
 src/maybe/result.inline.hpp     | 16 +++++++++++--
 tests/CMakeLists.txt            |  4 +++-
 tests/result_and_then_tests.cpp | 41 +++++++++++++++++++++++++++++++++
 tests/result_chaining_tests.cpp | 28 ----------------------
 tests/result_map_tests.cpp      | 40 ++++++++++++++++++++++++++++++++
 6 files changed, 104 insertions(+), 32 deletions(-)
 create mode 100644 tests/result_and_then_tests.cpp
 delete mode 100644 tests/result_chaining_tests.cpp
 create mode 100644 tests/result_map_tests.cpp

diff --git a/src/maybe/result.hpp b/src/maybe/result.hpp
index bfa8d57..b2e5e56 100644
--- a/src/maybe/result.hpp
+++ b/src/maybe/result.hpp
@@ -50,6 +50,8 @@ namespace maybe {
     template <typename T, typename E>
     class result final {
     public:
+        typedef T ok_type;
+
         result() : tag(internal::Value::NONE)
         {
         }
@@ -194,7 +196,10 @@ namespace maybe {
         }
 
         template <typename F>
-        constexpr auto map(F f) noexcept -> maybe::result<decltype(f(std::declval<T>())), E>;
+        constexpr auto map(F f) noexcept -> maybe::result<typename std::result_of<F(T)>::type, E>;
+
+        template <typename F>
+        constexpr auto and_then(F f) noexcept -> typename std::result_of<F(T)>::type;
 
     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 b00cf5c..c7885f6 100644
--- a/src/maybe/result.inline.hpp
+++ b/src/maybe/result.inline.hpp
@@ -99,12 +99,24 @@ constexpr void maybe::result<T, E>::clear() noexcept
 template <typename T, typename E>
 template <typename F>
 constexpr auto maybe::result<T, E>::map(F f) noexcept
-    -> maybe::result<decltype(f(std::declval<T>())), E>
+    -> maybe::result<typename std::result_of<F(T)>::type, E>
 {
-    typedef maybe::result<decltype(f(std::declval<T>())), E> return_result_t;
+    typedef maybe::result<typename std::result_of<F(T)>::type, E> return_result_t;
 
     if (is_err()) {
         return return_result_t::err(std::forward<E>(err_value()));
     }
     return return_result_t::ok(f(std::forward<T>(ok_value())));
+};
+
+template <typename T, typename E>
+template <typename F>
+constexpr auto maybe::result<T, E>::and_then(F f) noexcept -> typename std::result_of<F(T)>::type
+{
+    typedef typename std::result_of<F(T)>::type result_t;
+
+    if (is_err()) {
+        return maybe::result<typename result_t::ok_type, E>::err(std::forward<E>(err_value()));
+    }
+    return f(std::forward<T>(ok_value()));
 };
\ No newline at end of file
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 1da5cfe..df38a68 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -5,7 +5,9 @@ set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++14")
 add_executable(${TARGET}
         main.cpp
         result_tests.cpp
-        result_chaining_tests.cpp)
+        result_map_tests.cpp
+        result_and_then_tests.cpp
+        )
 
 target_include_directories(${TARGET}
         PUBLIC $<TARGET_PROPERTY:maybe_result,INTERFACE_INCLUDE_DIRECTORIES>
diff --git a/tests/result_and_then_tests.cpp b/tests/result_and_then_tests.cpp
new file mode 100644
index 0000000..f0ec392
--- /dev/null
+++ b/tests/result_and_then_tests.cpp
@@ -0,0 +1,41 @@
+#include "catch.hpp"
+
+#include <maybe/result.hpp>
+
+class A final {
+public:
+    A(std::string value) : value(value)
+    {
+    }
+    std::string value;
+};
+
+class B final {
+public:
+    B(std::string value) : value(value)
+    {
+    }
+    std::string value;
+};
+
+using maybe::result;
+
+TEST_CASE("result_and_then")
+{
+    SECTION("chains another function returning different result if previous one was successful")
+    {
+        auto a = result<A, int>::ok(A("hello"));
+        auto b = a.and_then([](A v) { return result<B, int>::ok(B(v.value + " world")); });
+        REQUIRE(b);
+        REQUIRE(b.ok_value().value == "hello world");
+    }
+
+    SECTION(
+        "should not run another function returning different result if previous one returned error")
+    {
+        auto a = result<A, int>::err(43);
+        auto b = a.and_then([](A v) { return result<B, int>::ok(B(v.value + " world")); });
+        REQUIRE(!b);
+        REQUIRE(b.err_value() == 43);
+    }
+}
\ No newline at end of file
diff --git a/tests/result_chaining_tests.cpp b/tests/result_chaining_tests.cpp
deleted file mode 100644
index 399361f..0000000
--- a/tests/result_chaining_tests.cpp
+++ /dev/null
@@ -1,28 +0,0 @@
-#include "catch.hpp"
-
-#include <maybe/result.hpp>
-
-class A final {
-public:
-    A(std::string value) : value(value) {}
-    std::string value;
-};
-
-class B final {
-public:
-    B(std::string value) : value(value) {}
-    std::string value;
-};
-
-using maybe::result;
-
-TEST_CASE("result_chaining")
-{
-    SECTION("converts result A to result B")
-    {
-        auto a = result<A, int>::ok(A("hello"));
-        auto b = a.map([](A v) { return B(v.value); });
-        REQUIRE(b);
-        REQUIRE(b.ok_value().value == "hello");
-    }
-}
\ No newline at end of file
diff --git a/tests/result_map_tests.cpp b/tests/result_map_tests.cpp
new file mode 100644
index 0000000..1d97f62
--- /dev/null
+++ b/tests/result_map_tests.cpp
@@ -0,0 +1,40 @@
+#include "catch.hpp"
+
+#include <maybe/result.hpp>
+
+class A final {
+public:
+    A(std::string value) : value(value)
+    {
+    }
+    std::string value;
+};
+
+class B final {
+public:
+    B(std::string value) : value(value)
+    {
+    }
+    std::string value;
+};
+
+using maybe::result;
+
+TEST_CASE("result_map")
+{
+    SECTION("converts result A to result B")
+    {
+        auto a = result<A, int>::ok(A("hello"));
+        auto b = a.map([](A v) { return B(v.value + " world"); });
+        REQUIRE(b);
+        REQUIRE(b.ok_value().value == "hello world");
+    }
+
+    SECTION("does not convert result A to result B if value was error")
+    {
+        auto a = result<A, int>::err(43);
+        auto b = a.map([](A v) { return B(v.value = " world"); });
+        REQUIRE(!b);
+        REQUIRE(b.err_value() == 43);
+    }
+}
\ No newline at end of file