FTL: Add Optional<T>::transform
Bug: 185536303
Test: ftl_test
Change-Id: If4cb9894c615499af04bb9793d9f900485e18fe2
diff --git a/include/ftl/optional.h b/include/ftl/optional.h
new file mode 100644
index 0000000..daf4502
--- /dev/null
+++ b/include/ftl/optional.h
@@ -0,0 +1,71 @@
+/*
+ * Copyright 2022 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <functional>
+#include <optional>
+#include <type_traits>
+#include <utility>
+
+namespace android::ftl {
+
+// Superset of std::optional<T> with monadic operations, as proposed in https://wg21.link/P0798R8.
+//
+// TODO: Remove in C++23.
+//
+template <typename T>
+struct Optional final : std::optional<T> {
+ using std::optional<T>::optional;
+
+ using std::optional<T>::has_value;
+ using std::optional<T>::value;
+
+ // Returns Optional<U> where F is a function that maps T to U.
+ template <typename F>
+ constexpr auto transform(F&& f) const& {
+ using U = std::remove_cv_t<std::invoke_result_t<F, decltype(value())>>;
+ if (has_value()) return Optional<U>(std::invoke(std::forward<F>(f), value()));
+ return Optional<U>();
+ }
+
+ template <typename F>
+ constexpr auto transform(F&& f) & {
+ using U = std::remove_cv_t<std::invoke_result_t<F, decltype(value())>>;
+ if (has_value()) return Optional<U>(std::invoke(std::forward<F>(f), value()));
+ return Optional<U>();
+ }
+
+ template <typename F>
+ constexpr auto transform(F&& f) const&& {
+ using U = std::invoke_result_t<F, decltype(std::move(value()))>;
+ if (has_value()) return Optional<U>(std::invoke(std::forward<F>(f), std::move(value())));
+ return Optional<U>();
+ }
+
+ template <typename F>
+ constexpr auto transform(F&& f) && {
+ using U = std::invoke_result_t<F, decltype(std::move(value()))>;
+ if (has_value()) return Optional<U>(std::invoke(std::forward<F>(f), std::move(value())));
+ return Optional<U>();
+ }
+};
+
+// Deduction guide.
+template <typename T>
+Optional(T) -> Optional<T>;
+
+} // namespace android::ftl