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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 <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_