Merge pull request #1 from trafi/feature/partial-specialization-for-ok-void

Implement partial specialization for result<void, E>.

src/maybe/result.fwd.hpp

@@ -11,5 +11,9 @@
 #pragma once
 
 namespace maybe {
-    template <typename T, typename E> class result;
+    template <typename T, typename E>
+    class result;
+
+    template <typename E>
+    class result<void, E>;
 }

src/maybe/result.hpp

@@ -274,8 +274,15 @@ namespace maybe {
          * @param f F(T) -> U
          * @return maybe::result<U, E>
          */
-        template <typename F>
-        inline auto map(F f) noexcept -> maybe::result<typename std::result_of<F(T)>::type, E>;
+        template <typename F, typename R = typename std::result_of<F(T)>::type>
+        inline auto map(F f) noexcept -> maybe::result<R, E>;
+
+        /**
+         * Maps a result<T, E> to result<void, E>, leaving an err value untouched.
+         *
+         * @return maybe::result<void, E>
+         */
+        inline auto map_void() noexcept -> maybe::result<void, E>;
 
         /**
          * Maps a result<T, E> to result<U, E> by always returning provided U value on success,
@@ -350,6 +357,269 @@ namespace maybe {
     {
         return !(x == y);
     }
+
+    template <typename E>
+    class result<void, E> final {
+    private:
+        std::experimental::optional<E> var_err;
+
+    public:
+        typedef void ok_type;
+        typedef E err_type;
+
+        result()
+        {
+        }
+
+        result(internal::placeholder)
+        {
+        }
+
+        result(internal::placeholder, E&& value) : var_err(std::forward<E>(value))
+        {
+        }
+
+        result(internal::placeholder, const E& value) : var_err(value)
+        {
+        }
+
+        /**
+         * Create a new ok value.
+         *
+         * @param void value
+         * @return result<void, E>
+         */
+        constexpr static result<void, E> ok() noexcept
+        {
+            return std::forward<result<void, E>>(result<void, E>(internal::placeholder{}));
+        }
+
+        /**
+         * Create a new err value.
+         *
+         * @param E value
+         * @return result<void, E>
+         */
+        constexpr static result<void, E> err(E&& value) noexcept
+        {
+            return std::forward<result<void, E>>(
+                result<void, E>(internal::placeholder{}, std::forward<E>(value)));
+        }
+
+        constexpr static result<void, E> err(const E& value) noexcept
+        {
+            return std::forward<result<void, E>>(result<void, E>(internal::placeholder{}, value));
+        }
+
+        /**
+         * Create a new ok value using `void()` constructor.
+         *
+         * @return result<void, E>
+         */
+        constexpr static result<void, E> default_ok() noexcept
+        {
+            return std::experimental::constexpr_move(
+                result<void, E>(void(), internal::placeholder{}));
+        }
+
+        /**
+         * Create a new err value using `E()` constructor.
+         *
+         * @return result<void, E>
+         */
+        constexpr static result<void, E> default_err() noexcept
+        {
+            return std::experimental::constexpr_move(result<void, E>(internal::placeholder{}, E()));
+        }
+
+        // Inspection.
+
+        /**
+         * Check if result contains ok value.
+         *
+         * @return bool
+         */
+        inline bool is_ok() const noexcept
+        {
+            return !var_err;
+        }
+
+        /**
+         * Check if result contains err value.
+         *
+         * @return bool
+         */
+        inline bool is_err() const noexcept
+        {
+            return !!var_err;
+        }
+
+        explicit inline operator bool() const noexcept
+        {
+            return is_ok();
+        }
+
+        void ok_value()
+        {
+            if (is_err()) {
+                throw std::experimental::bad_optional_access("bad optional access");
+            }
+        }
+
+#if OPTIONAL_HAS_MOVE_ACCESSORS == 1
+
+        constexpr E const& err_value() const&
+        {
+            return var_err.value();
+        }
+
+        OPTIONAL_MUTABLE_CONSTEXPR E& err_value() &
+        {
+            return var_err.value();
+        }
+
+        OPTIONAL_MUTABLE_CONSTEXPR E&& err_value() &&
+        {
+            return std::move(var_err.value());
+        }
+
+#else
+
+        /**
+         * Retrieve err value or throw `bad_optional_access` exception.
+         *
+         * @return E
+         */
+        E const& err_value() const
+        {
+            return var_err.value();
+        }
+
+        E& err_value()
+        {
+            return var_err.value();
+        }
+
+#endif
+
+#if OPTIONAL_HAS_THIS_RVALUE_REFS == 1
+
+        template <class V>
+        constexpr E err_value_or(V&& v) const&
+        {
+            return var_err.value_or(std::forward<V>(v));
+        }
+
+        template <class V>
+        OPTIONAL_MUTABLE_CONSTEXPR E err_value_or(V&& v) &&
+        {
+            return var_err.value_or(std::forward<V>(v));
+        }
+
+#else
+
+        /**
+         * Retrieve err value or the provided default `V` which can be casted to `E`.
+         *
+         * @return E
+         */
+        template <class V>
+        E err_value_or(V&& v) const
+        {
+            return var_err.value_or(std::forward<V>(v));
+        }
+
+#endif
+
+        // Functional helpers.
+
+        /**
+         * Maps a result<void, E> to result<U, E> (where U is return value of F(void)) by applying a
+         * function F to a
+         * contained ok value, leaving an err value untouched.
+         *
+         * This function can be used to compose the results of two functions.
+         *
+         * @param f F() -> U
+         * @return maybe::result<U, E>
+         */
+        template <typename F>
+        inline auto map(F f) noexcept -> maybe::result<typename std::result_of<F()>::type, E>;
+
+        /**
+         * Maps a result<void, E> to result<U, E> by always returning provided U value on success,
+         * leaving an err value untouched.
+         *
+         * This function can be used to compose the results of two functions.
+         *
+         * @param value U
+         * @return maybe::result<U, E>
+         */
+        template <typename U>
+        inline auto map_value(U value) noexcept -> maybe::result<U, E>;
+
+        /**
+         * Maps a result<void, E> to result<void, U> (where U is return value of F(E)) by applying a
+         * function to a
+         * function to a
+         * contained err value, leaving an ok value untouched.
+         *
+         * This function can be used to pass through a successful result while changing an error.
+         *
+         * @param f F(E) -> U
+         * @return maybe::result<void, U>
+         */
+        template <typename F>
+        inline auto map_err(F f) noexcept
+            -> maybe::result<void, typename std::result_of<F(E)>::type>;
+
+        /**
+         * Maps a result<void, E> to result<void, U> by always returning provided U value on error,
+         * leaving an ok value untouched.
+         *
+         * This function can be used to compose the results of two functions.
+         *
+         * @param value U
+         * @return maybe::result<void, U>
+         */
+        template <typename U>
+        inline auto map_err_value(U value) noexcept -> maybe::result<void, U>;
+
+        /**
+         * Calls op if the result is ok, otherwise returns the err value of self.
+         *
+         * This function can be used for control flow based on result values.
+         *
+         * @param f F() -> maybe::result<U, E>
+         * @return maybe::result<U, E>
+         */
+        template <typename F>
+        inline auto and_then(F op) noexcept -> typename std::result_of<F()>::type;
+
+        /**
+         * Converts into another result with different ok type `R` and forwards the same error.
+         *
+         * The ok type `R` must have `R()` constructor in case the result does not contain an err.
+         *
+         * @return maybe::result<R, E>
+         */
+        template <typename R>
+        inline auto into_err() noexcept -> R;
+    };
+
+    template <typename E>
+    constexpr bool operator==(const result<void, E>& x, const result<void, E>& y)
+    {
+        return x.is_ok() && y.is_ok()
+            ? true
+            : (x.is_err() && y.is_err() ? x.err_value() == y.err_value() : false);
+    }
+
+    template <typename E>
+    constexpr bool operator!=(const result<void, E>& x, const result<void, E>& y)
+    {
+        return !(x == y);
+    }
 }
 
 #include "result.inline.hpp"

src/maybe/result.inline.hpp

@@ -13,15 +13,24 @@
 #include "result.hpp"
 
 template <typename T, typename E>
-template <typename F>
-inline auto maybe::result<T, E>::map(F f) noexcept -> maybe::result<typename std::result_of<F(T)>::type, E>
+template <typename F, typename R>
+inline auto maybe::result<T, E>::map(F f) noexcept -> maybe::result<R, E>
 {
-    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 maybe::result<R, E>::err(std::forward<E>(err_value()));
     }
-    return return_result_t::ok(f(ok_value()));
+
+    return maybe::result<R, E>::ok(f(ok_value()));
+};
+
+template <typename T, typename E>
+inline auto maybe::result<T, E>::map_void() noexcept -> maybe::result<void, E>
+{
+    if (is_err()) {
+        return maybe::result<void, E>::err(std::forward<E>(err_value()));
+    }
+
+    return maybe::result<void, E>::ok();
 };
 
 template <typename T, typename E>
@@ -80,3 +89,73 @@ inline auto maybe::result<T, E>::into_err() noexcept -> R
     }
     return R::default_ok();
 };
+
+template <typename E>
+template <typename F>
+inline auto maybe::result<void, E>::map(F f) noexcept
+    -> maybe::result<typename std::result_of<F()>::type, E>
+{
+    typedef maybe::result<typename std::result_of<F()>::type, E> return_result_t;
+
+    if (is_err()) {
+        return return_result_t::err(std::forward<E>(err_value()));
+    }
+    return return_result_t::ok(f());
+};
+
+template <typename E>
+template <typename U>
+inline auto maybe::result<void, E>::map_value(U value) noexcept -> maybe::result<U, E>
+{
+    if (is_err()) {
+        return maybe::result<U, E>::err(std::forward<E>(err_value()));
+    }
+
+    return maybe::result<U, E>::ok(std::forward<U>(value));
+};
+
+template <typename E>
+template <typename F>
+inline auto maybe::result<void, E>::map_err(F f) noexcept
+    -> maybe::result<void, typename std::result_of<F(E)>::type>
+{
+    typedef maybe::result<void, typename std::result_of<F(E)>::type> return_result_t;
+
+    if (is_ok()) {
+        return return_result_t::ok();
+    }
+    return return_result_t::err(f(err_value()));
+};
+
+template <typename E>
+template <typename U>
+inline auto maybe::result<void, E>::map_err_value(U value) noexcept -> maybe::result<void, U>
+{
+    if (is_ok()) {
+        return maybe::result<void, U>::ok();
+    }
+
+    return maybe::result<void, U>::err(std::forward<U>(value));
+};
+
+template <typename E>
+template <typename F>
+inline auto maybe::result<void, E>::and_then(F f) noexcept -> typename std::result_of<F()>::type
+{
+    typedef typename std::result_of<F()>::type result_t;
+
+    if (is_err()) {
+        return maybe::result<typename result_t::ok_type, E>::err(std::forward<E>(err_value()));
+    }
+    return f();
+};
+
+template <typename E>
+template <typename R>
+inline auto maybe::result<void, E>::into_err() noexcept -> R
+{
+    if (is_err()) {
+        return R::err(std::forward<E>(err_value()));
+    }
+    return R::default_ok();
+};

tests/result_and_then_tests.cpp

@@ -30,6 +30,21 @@ TEST_CASE("result_and_then")
         REQUIRE(b.ok_value().value == "hello world");
     }
 
+    SECTION("chains another function returning different void result if previous one was successful")
+    {
+        auto a = result<A, int>::ok(A("hello"));
+        auto b = a.and_then([](A v) { return result<void, int>::ok(); });
+        REQUIRE(b);
+    }
+
+    SECTION("chains another function returning different result if previous one was successful void")
+    {
+        auto a = result<void, int>::ok();
+        auto b = a.and_then([]() { return result<B, int>::ok(B("Void world")); });
+        REQUIRE(b);
+        REQUIRE(b.ok_value().value == "Void world");
+    }
+
     SECTION(
         "should not run another function returning different result if previous one returned error")
     {
@@ -38,4 +53,22 @@ TEST_CASE("result_and_then")
         REQUIRE(!b);
         REQUIRE(43 == b.err_value());
     }
+
+    SECTION(
+            "should not run another function returning different void result if previous one returned error")
+    {
+        auto a = result<A, int>::err(43);
+        auto b = a.and_then([](A v) { return result<void, int>::ok(); });
+        REQUIRE(!b);
+        REQUIRE(43 == b.err_value());
+    }
+
+    SECTION(
+            "should not run another function returning different result if previous void one returned error")
+    {
+        auto a = result<void, int>::err(43);
+        auto b = a.and_then([]() { return result<B, int>::ok(B("Void world")); });
+        REQUIRE(!b);
+        REQUIRE(43 == b.err_value());
+    }
 }

tests/result_map_err_tests.cpp

@@ -30,6 +30,14 @@ TEST_CASE("result_map_err")
         REQUIRE(42 == b.err_value());
     }
 
+    SECTION("converts err bool to err int for void function")
+    {
+        auto a = result<void, bool>::err(true);
+        auto b = a.map_err([](bool v) { return v ? 42 : 43; });
+        REQUIRE(!b);
+        REQUIRE(42 == b.err_value());
+    }
+
     SECTION("does not convert anything and returns ok if not err")
     {
         auto a = result<A, bool>::ok(A("hi"));
@@ -38,6 +46,13 @@ TEST_CASE("result_map_err")
         REQUIRE(b.ok_value().value == "hi");
     }
 
+    SECTION("does not convert anything and returns ok if not err for void ok type")
+    {
+        auto a = result<void, bool>::ok();
+        auto b = a.map_err([](bool v) { return v ? 42 : 43; });
+        REQUIRE(b);
+    }
+
     SECTION("replaces err A with err B")
     {
         auto a = result<int, A>::err(A("hello"));
@@ -46,6 +61,14 @@ TEST_CASE("result_map_err")
         REQUIRE(b.err_value().value == "bye");
     }
 
+    SECTION("replaces err A with err B for void ok type")
+    {
+        auto a = result<void, A>::err(A("hello"));
+        auto b = a.map_err_value(B("bye"));
+        REQUIRE(!b);
+        REQUIRE(b.err_value().value == "bye");
+    }
+
     SECTION("does not replace err A with err B if A is ok")
     {
         auto a = result<int, A>::ok(43);
@@ -53,4 +76,11 @@ TEST_CASE("result_map_err")
         REQUIRE(b);
         REQUIRE(43 == b.ok_value());
     }
+
+    SECTION("does not replace err A with err B if A is void ok")
+    {
+        auto a = result<void, A>::ok();
+        auto b = a.map_err_value(B("bye"));
+        REQUIRE(b);
+    }
 }

tests/result_map_tests.cpp

@@ -30,6 +30,14 @@ TEST_CASE("result_map")
         REQUIRE(b.ok_value().value == "hello world");
     }
 
+    SECTION("converts result void to result B")
+    {
+        auto a = result<void, int>::ok();
+        auto b = a.map([]() { return B("Void world"); });
+        REQUIRE(b);
+        REQUIRE(b.ok_value().value == "Void world");
+    }
+
     SECTION("does not convert result A to result B if value was error")
     {
         auto a = result<A, int>::err(43);
@@ -38,6 +46,14 @@ TEST_CASE("result_map")
         REQUIRE(43 == b.err_value());
     }
 
+    SECTION("does not convert result A to result void if value was error")
+    {
+        auto a = result<A, int>::err(43);
+        auto b = a.map_void();
+        REQUIRE(!b);
+        REQUIRE(43 == b.err_value());
+    }
+
     SECTION("replaces result A with result B")
     {
         auto a = result<A, int>::ok(A("hello"));

tests/result_tests.cpp

@@ -71,6 +71,21 @@ TEST_CASE("result")
         REQUIRE(res != other_different_err);
     }
 
+    SECTION("created err result returns err value in result<void, E>")
+    {
+        auto res = result<void, int>::err(12);
+        REQUIRE(res.is_err());
+        REQUIRE(!res.is_ok());
+        REQUIRE(!res);
+        REQUIRE(12 == res.err_value());
+
+        auto other_err = result<void, int>::err(12);
+        REQUIRE(res == other_err);
+
+        auto other_different_err = result<void, int>::err(42);
+        REQUIRE(res != other_different_err);
+    }
+
     SECTION("created ok not equal created result")
     {
         auto ok = result<int, int>::ok(12);
@@ -78,6 +93,13 @@ TEST_CASE("result")
         REQUIRE(ok != err);
     }
 
+    SECTION("created ok not equal created result in result<void, E>")
+    {
+        auto ok = result<void, int>::ok();
+        auto err = result<void, int>::err(12);
+        REQUIRE(ok != err);
+    }
+
     SECTION("throws exception if invalid value accessed")
     {
         auto ok = result<int, int>::ok(12);
@@ -86,6 +108,14 @@ TEST_CASE("result")
         REQUIRE_THROWS(err.ok_value());
     }
 
+    SECTION("throws exception if invalid value accessed in result<void, E>")
+    {
+        auto ok = result<void, int>::ok();
+        auto err = result<void, int>::err(12);
+        REQUIRE_THROWS(ok.err_value());
+        REQUIRE_THROWS(err.ok_value());
+    }
+
     SECTION("returns default values")
     {
         auto ok = result<int, int>::ok(12);
@@ -96,6 +126,14 @@ TEST_CASE("result")
         REQUIRE(err.ok_value_or(42) == 42);
     }
 
+    SECTION("returns default values in result<void, E>")
+    {
+        auto ok = result<void, int>::ok();
+        auto err = result<void, int>::err(12);
+        REQUIRE(err.err_value_or(42) == 12);
+        REQUIRE(ok.err_value_or(42) == 42);
+    }
+
     SECTION("destroys ok value correctly")
     {
         std::ostringstream ss;