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- // Copyright 2021 the V8 project authors. All rights reserved.
- // Use of this source code is governed by a BSD-style license that can be
- // found in the LICENSE file.
- #ifndef INCLUDE_V8_MAYBE_H_
- #define INCLUDE_V8_MAYBE_H_
- #include "v8-internal.h" // NOLINT(build/include_directory)
- #include "v8config.h" // NOLINT(build/include_directory)
- namespace v8 {
- namespace api_internal {
- // Called when ToChecked is called on an empty Maybe.
- V8_EXPORT void FromJustIsNothing();
- } // namespace api_internal
- /**
- * A simple Maybe type, representing an object which may or may not have a
- * value, see https://hackage.haskell.org/package/base/docs/Data-Maybe.html.
- *
- * If an API method returns a Maybe<>, the API method can potentially fail
- * either because an exception is thrown, or because an exception is pending,
- * e.g. because a previous API call threw an exception that hasn't been caught
- * yet, or because a TerminateExecution exception was thrown. In that case, a
- * "Nothing" value is returned.
- */
- template <class T>
- class Maybe {
- public:
- V8_INLINE bool IsNothing() const { return !has_value_; }
- V8_INLINE bool IsJust() const { return has_value_; }
- /**
- * An alias for |FromJust|. Will crash if the Maybe<> is nothing.
- */
- V8_INLINE T ToChecked() const { return FromJust(); }
- /**
- * Short-hand for ToChecked(), which doesn't return a value. To be used, where
- * the actual value of the Maybe is not needed like Object::Set.
- */
- V8_INLINE void Check() const {
- if (V8_UNLIKELY(!IsJust())) api_internal::FromJustIsNothing();
- }
- /**
- * Converts this Maybe<> to a value of type T. If this Maybe<> is
- * nothing (empty), |false| is returned and |out| is left untouched.
- */
- V8_WARN_UNUSED_RESULT V8_INLINE bool To(T* out) const {
- if (V8_LIKELY(IsJust())) *out = value_;
- return IsJust();
- }
- /**
- * Converts this Maybe<> to a value of type T. If this Maybe<> is
- * nothing (empty), V8 will crash the process.
- */
- V8_INLINE T FromJust() const {
- if (V8_UNLIKELY(!IsJust())) api_internal::FromJustIsNothing();
- return value_;
- }
- /**
- * Converts this Maybe<> to a value of type T, using a default value if this
- * Maybe<> is nothing (empty).
- */
- V8_INLINE T FromMaybe(const T& default_value) const {
- return has_value_ ? value_ : default_value;
- }
- V8_INLINE bool operator==(const Maybe& other) const {
- return (IsJust() == other.IsJust()) &&
- (!IsJust() || FromJust() == other.FromJust());
- }
- V8_INLINE bool operator!=(const Maybe& other) const {
- return !operator==(other);
- }
- private:
- Maybe() : has_value_(false) {}
- explicit Maybe(const T& t) : has_value_(true), value_(t) {}
- bool has_value_;
- T value_;
- template <class U>
- friend Maybe<U> Nothing();
- template <class U>
- friend Maybe<U> Just(const U& u);
- };
- template <class T>
- inline Maybe<T> Nothing() {
- return Maybe<T>();
- }
- template <class T>
- inline Maybe<T> Just(const T& t) {
- return Maybe<T>(t);
- }
- // A template specialization of Maybe<T> for the case of T = void.
- template <>
- class Maybe<void> {
- public:
- V8_INLINE bool IsNothing() const { return !is_valid_; }
- V8_INLINE bool IsJust() const { return is_valid_; }
- V8_INLINE bool operator==(const Maybe& other) const {
- return IsJust() == other.IsJust();
- }
- V8_INLINE bool operator!=(const Maybe& other) const {
- return !operator==(other);
- }
- private:
- struct JustTag {};
- Maybe() : is_valid_(false) {}
- explicit Maybe(JustTag) : is_valid_(true) {}
- bool is_valid_;
- template <class U>
- friend Maybe<U> Nothing();
- friend Maybe<void> JustVoid();
- };
- inline Maybe<void> JustVoid() { return Maybe<void>(Maybe<void>::JustTag()); }
- } // namespace v8
- #endif // INCLUDE_V8_MAYBE_H_
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