// 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 <type_traits> #include <utility> #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. If this Maybe<> is * nothing (empty), V8 will crash the process. */ V8_INLINE T FromJust() && { if (V8_UNLIKELY(!IsJust())) api_internal::FromJustIsNothing(); return std::move(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) {} explicit Maybe(T&& t) : has_value_(true), value_(std::move(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 U, std::enable_if_t<!std::is_lvalue_reference_v<U>>*> friend Maybe<U> Just(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); } // Don't use forwarding references here but instead use two overloads. // Forwarding references only work when type deduction takes place, which is not // the case for callsites such as Just<Type>(t). template <class T, std::enable_if_t<!std::is_lvalue_reference_v<T>>* = nullptr> inline Maybe<T> Just(T&& t) { return Maybe<T>(std::move(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_