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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
* vim: set ts=8 sts=2 et sw=2 tw=80:
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
/* A type suitable for returning either a value or an error from a function. */
#ifndef mozilla_Result_h
#define mozilla_Result_h
#include <algorithm>
#include <cstdint>
#include <cstring>
#include <type_traits>
#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"
#include "mozilla/CompactPair.h"
#include "mozilla/MaybeStorageBase.h"
namespace mozilla {
/**
* Empty struct, indicating success for operations that have no return value.
* For example, if you declare another empty struct `struct OutOfMemory {};`,
* then `Result<Ok, OutOfMemory>` represents either success or OOM.
*/
struct Ok {};
/**
* A tag used to differentiate between GenericErrorResult created by the Err
* function (completely new error) and GenericErrorResult created by the
* Result::propagateErr function (propagated error). This can be used to track
* error propagation and eventually produce error stacks for logging/debugging
* purposes.
*/
struct ErrorPropagationTag {};
template <typename E>
class GenericErrorResult;
template <typename V, typename E>
class Result;
namespace detail {
enum class PackingStrategy {
Variant,
NullIsOk,
LowBitTagIsError,
PackedVariant,
ZeroIsEmptyError,
};
template <typename T>
struct UnusedZero;
template <typename V, typename E, PackingStrategy Strategy>
class ResultImplementation;
template <typename V>
struct EmptyWrapper : V {
constexpr EmptyWrapper() = default;
explicit constexpr EmptyWrapper(const V&) {}
explicit constexpr EmptyWrapper(std::in_place_t) {}
constexpr V* addr() { return this; }
constexpr const V* addr() const { return this; }
};
// The purpose of AlignedStorageOrEmpty is to make an empty class look like
// std::aligned_storage_t for the purposes of the PackingStrategy::NullIsOk
// specializations of ResultImplementation below. We can't use
// std::aligned_storage_t itself with an empty class, since it would no longer
// be empty.
template <typename V>
using AlignedStorageOrEmpty =
std::conditional_t<std::is_empty_v<V>, EmptyWrapper<V>,
MaybeStorageBase<V>>;
template <typename V, typename E>
class ResultImplementationNullIsOkBase {
protected:
using ErrorStorageType = typename UnusedZero<E>::StorageType;
static constexpr auto kNullValue = UnusedZero<E>::nullValue;
static_assert(std::is_trivially_copyable_v<ErrorStorageType>);
// XXX This can't be statically asserted in general, if ErrorStorageType is
// not a basic type. With C++20 bit_cast, we could probably re-add such as
// assertion. static_assert(kNullValue == decltype(kNullValue)(0));
CompactPair<AlignedStorageOrEmpty<V>, ErrorStorageType> mValue;
public:
explicit constexpr ResultImplementationNullIsOkBase(const V& aSuccessValue)
: mValue(aSuccessValue, kNullValue) {}
explicit constexpr ResultImplementationNullIsOkBase(V&& aSuccessValue)
: mValue(std::move(aSuccessValue), kNullValue) {}
template <typename... Args>
explicit constexpr ResultImplementationNullIsOkBase(std::in_place_t,
Args&&... aArgs)
: mValue(std::piecewise_construct,
std::tuple(std::in_place, std::forward<Args>(aArgs)...),
std::tuple(kNullValue)) {}
explicit constexpr ResultImplementationNullIsOkBase(E aErrorValue)
: mValue(std::piecewise_construct, std::tuple<>(),
std::tuple(UnusedZero<E>::Store(std::move(aErrorValue)))) {
MOZ_ASSERT(mValue.second() != kNullValue);
}
constexpr ResultImplementationNullIsOkBase(
ResultImplementationNullIsOkBase&& aOther)
: mValue(std::piecewise_construct, std::tuple<>(),
std::tuple(aOther.mValue.second())) {
if constexpr (!std::is_empty_v<V>) {
if (isOk()) {
new (mValue.first().addr()) V(std::move(*aOther.mValue.first().addr()));
}
}
}
ResultImplementationNullIsOkBase& operator=(
ResultImplementationNullIsOkBase&& aOther) {
if constexpr (!std::is_empty_v<V>) {
if (isOk()) {
mValue.first().addr()->~V();
}
}
mValue.second() = std::move(aOther.mValue.second());
if constexpr (!std::is_empty_v<V>) {
if (isOk()) {
new (mValue.first().addr()) V(std::move(*aOther.mValue.first().addr()));
}
}
return *this;
}
constexpr bool isOk() const { return mValue.second() == kNullValue; }
constexpr const V& inspect() const { return *mValue.first().addr(); }
constexpr V unwrap() { return std::move(*mValue.first().addr()); }
constexpr void updateAfterTracing(V&& aValue) {
MOZ_ASSERT(isOk());
if (!std::is_empty_v<V>) {
mValue.first().addr()->~V();
new (mValue.first().addr()) V(std::move(aValue));
}
}
constexpr decltype(auto) inspectErr() const {
return UnusedZero<E>::Inspect(mValue.second());
}
constexpr E unwrapErr() { return UnusedZero<E>::Unwrap(mValue.second()); }
constexpr void updateErrorAfterTracing(E&& aErrorValue) {
mValue.second() = UnusedZero<E>::Store(std::move(aErrorValue));
}
};
template <typename V, typename E,
bool IsVTriviallyDestructible = std::is_trivially_destructible_v<V>>
class ResultImplementationNullIsOk;
template <typename V, typename E>
class ResultImplementationNullIsOk<V, E, true>
: public ResultImplementationNullIsOkBase<V, E> {
public:
using ResultImplementationNullIsOkBase<V,
E>::ResultImplementationNullIsOkBase;
};
template <typename V, typename E>
class ResultImplementationNullIsOk<V, E, false>
: public ResultImplementationNullIsOkBase<V, E> {
public:
using ResultImplementationNullIsOkBase<V,
E>::ResultImplementationNullIsOkBase;
ResultImplementationNullIsOk(ResultImplementationNullIsOk&&) = default;
ResultImplementationNullIsOk& operator=(ResultImplementationNullIsOk&&) =
default;
~ResultImplementationNullIsOk() {
if (this->isOk()) {
this->mValue.first().addr()->~V();
}
}
};
/**
* Specialization for when the success type is one of integral, pointer, or
* enum, where 0 is unused, and the error type is an empty struct.
*/
template <typename V, typename E>
class ResultImplementation<V, E, PackingStrategy::ZeroIsEmptyError> {
static_assert(std::is_integral_v<V> || std::is_pointer_v<V> ||
std::is_enum_v<V>);
static_assert(std::is_empty_v<E>);
V mValue;
public:
static constexpr PackingStrategy Strategy = PackingStrategy::ZeroIsEmptyError;
explicit constexpr ResultImplementation(V aValue) : mValue(aValue) {}
explicit constexpr ResultImplementation(E aErrorValue) : mValue(V(0)) {}
constexpr bool isOk() const { return mValue != V(0); }
constexpr V inspect() const { return mValue; }
constexpr V unwrap() { return inspect(); }
constexpr E inspectErr() const { return E(); }
constexpr E unwrapErr() { return inspectErr(); }
constexpr void updateAfterTracing(V&& aValue) {
this->~ResultImplementation();
new (this) ResultImplementation(std::move(aValue));
}
constexpr void updateErrorAfterTracing(E&& aErrorValue) {
this->~ResultImplementation();
new (this) ResultImplementation(std::move(aErrorValue));
}
};
/**
* Specialization for when the success type is default-constructible and the
* error type is a value type which can never have the value 0 (as determined by
* UnusedZero<>).
*/
template <typename V, typename E>
class ResultImplementation<V, E, PackingStrategy::NullIsOk>
: public ResultImplementationNullIsOk<V, E> {
public:
static constexpr PackingStrategy Strategy = PackingStrategy::NullIsOk;
using ResultImplementationNullIsOk<V, E>::ResultImplementationNullIsOk;
};
template <size_t S>
using UnsignedIntType = std::conditional_t<
S == 1, std::uint8_t,
std::conditional_t<
S == 2, std::uint16_t,
std::conditional_t<S == 3 || S == 4, std::uint32_t,
std::conditional_t<S <= 8, std::uint64_t, void>>>>;
/**
* Specialization for when alignment permits using the least significant bit
* as a tag bit.
*/
template <typename V, typename E>
class ResultImplementation<V, E, PackingStrategy::LowBitTagIsError> {
static_assert(std::is_trivially_copyable_v<V> &&
std::is_trivially_destructible_v<V>);
static_assert(std::is_trivially_copyable_v<E> &&
std::is_trivially_destructible_v<E>);
static constexpr size_t kRequiredSize = std::max(sizeof(V), sizeof(E));
using StorageType = UnsignedIntType<kRequiredSize>;
#if defined(__clang__)
alignas(std::max(alignof(V), alignof(E))) StorageType mBits;
#else
// Some gcc versions choke on using std::max with alignas, see
// regressed in some gcc 9.x version before being fixed again) Keeping the
// code above since we would eventually drop this when we no longer support
// gcc versions with the bug.
alignas(alignof(V) > alignof(E) ? alignof(V) : alignof(E)) StorageType mBits;
#endif
public:
static constexpr PackingStrategy Strategy = PackingStrategy::LowBitTagIsError;
explicit constexpr ResultImplementation(V aValue) : mBits(0) {
if constexpr (!std::is_empty_v<V>) {
std::memcpy(&mBits, &aValue, sizeof(V));
MOZ_ASSERT((mBits & 1) == 0);
} else {
(void)aValue;
}
}
explicit constexpr ResultImplementation(E aErrorValue) : mBits(1) {
if constexpr (!std::is_empty_v<E>) {
std::memcpy(&mBits, &aErrorValue, sizeof(E));
MOZ_ASSERT((mBits & 1) == 0);
mBits |= 1;
} else {
(void)aErrorValue;
}
}
constexpr bool isOk() const { return (mBits & 1) == 0; }
constexpr V inspect() const {
V res;
std::memcpy(&res, &mBits, sizeof(V));
return res;
}
constexpr V unwrap() { return inspect(); }
constexpr E inspectErr() const {
const auto bits = mBits ^ 1;
E res;
std::memcpy(&res, &bits, sizeof(E));
return res;
}
constexpr E unwrapErr() { return inspectErr(); }
constexpr void updateAfterTracing(V&& aValue) {
this->~ResultImplementation();
new (this) ResultImplementation(std::move(aValue));
}
constexpr void updateErrorAfterTracing(E&& aErrorValue) {
this->~ResultImplementation();
new (this) ResultImplementation(std::move(aErrorValue));
}
};
// Return true if any of the struct can fit in a word.
template <typename V, typename E>
struct IsPackableVariant {
struct VEbool {
explicit constexpr VEbool(V&& aValue) : v(std::move(aValue)), ok(true) {}
explicit constexpr VEbool(E&& aErrorValue)
: e(std::move(aErrorValue)), ok(false) {}
V v;
E e;
bool ok;
};
struct EVbool {
explicit constexpr EVbool(V&& aValue) : v(std::move(aValue)), ok(true) {}
explicit constexpr EVbool(E&& aErrorValue)
: e(std::move(aErrorValue)), ok(false) {}
E e;
V v;
bool ok;
};
using Impl =
std::conditional_t<sizeof(VEbool) <= sizeof(EVbool), VEbool, EVbool>;
static const bool value = sizeof(Impl) <= sizeof(uintptr_t);
};
/**
* Specialization for when both type are not using all the bytes, in order to
* use one byte as a tag.
*/
template <typename V, typename E>
class ResultImplementation<V, E, PackingStrategy::PackedVariant> {
using Impl = typename IsPackableVariant<V, E>::Impl;
Impl data;
public:
static constexpr PackingStrategy Strategy = PackingStrategy::PackedVariant;
explicit constexpr ResultImplementation(V aValue) : data(std::move(aValue)) {}
explicit constexpr ResultImplementation(E aErrorValue)
: data(std::move(aErrorValue)) {}
constexpr bool isOk() const { return data.ok; }
constexpr const V& inspect() const { return data.v; }
constexpr V unwrap() { return std::move(data.v); }
constexpr const E& inspectErr() const { return data.e; }
constexpr E unwrapErr() { return std::move(data.e); }
constexpr void updateAfterTracing(V&& aValue) {
MOZ_ASSERT(data.ok);
this->~ResultImplementation();
new (this) ResultImplementation(std::move(aValue));
}
constexpr void updateErrorAfterTracing(E&& aErrorValue) {
MOZ_ASSERT(!data.ok);
this->~ResultImplementation();
new (this) ResultImplementation(std::move(aErrorValue));
}
};
// To use nullptr as a special value, we need the counter part to exclude zero
// from its range of valid representations.
//
// By default assume that zero can be represented.
template <typename T>
struct UnusedZero {
static const bool value = false;
};
// This template can be used as a helper for specializing UnusedZero for scoped
// enum types which never use 0 as an error value, e.g.
//
// namespace mozilla::detail {
//
// template <>
// struct UnusedZero<MyEnumType> : UnusedZeroEnum<MyEnumType> {};
//
// } // namespace mozilla::detail
//
template <typename T>
struct UnusedZeroEnum {
using StorageType = std::underlying_type_t<T>;
static constexpr bool value = true;
static constexpr StorageType nullValue = 0;
static constexpr T Inspect(const StorageType& aValue) {
return static_cast<T>(aValue);
}
static constexpr T Unwrap(StorageType aValue) {
return static_cast<T>(aValue);
}
static constexpr StorageType Store(T aValue) {
return static_cast<StorageType>(aValue);
}
};
// A bit of help figuring out which of the above specializations to use.
//
// We begin by safely assuming types don't have a spare bit, unless they are
// empty.
template <typename T>
struct HasFreeLSB {
static const bool value = std::is_empty_v<T>;
};
// As an incomplete type, void* does not have a spare bit.
template <>
struct HasFreeLSB<void*> {
static const bool value = false;
};
// The lowest bit of a properly-aligned pointer is always zero if the pointee
// type is greater than byte-aligned. That bit is free to use if it's masked
// out of such pointers before they're dereferenced.
template <typename T>
struct HasFreeLSB<T*> {
static const bool value = (alignof(T) & 1) == 0;
};
// Select one of the previous result implementation based on the properties of
// the V and E types.
template <typename V, typename E>
struct SelectResultImpl {
static const PackingStrategy value =
(UnusedZero<V>::value && std::is_empty_v<E>)
? PackingStrategy::ZeroIsEmptyError
: (HasFreeLSB<V>::value && HasFreeLSB<E>::value)
? PackingStrategy::LowBitTagIsError
: (UnusedZero<E>::value && sizeof(E) <= sizeof(uintptr_t))
? PackingStrategy::NullIsOk
: (std::is_default_constructible_v<V> &&
std::is_default_constructible_v<E> && IsPackableVariant<V, E>::value)
? PackingStrategy::PackedVariant
: PackingStrategy::Variant;
using Type = ResultImplementation<V, E, value>;
};
template <typename T>
struct IsResult : std::false_type {};
template <typename V, typename E>
struct IsResult<Result<V, E>> : std::true_type {};
} // namespace detail
template <typename V, typename E>
constexpr auto ToResult(Result<V, E>&& aValue)
-> decltype(std::forward<Result<V, E>>(aValue)) {
return std::forward<Result<V, E>>(aValue);
}
/**
* Result<V, E> represents the outcome of an operation that can either succeed
* or fail. It contains either a success value of type V or an error value of
* type E.
*
* All Result methods are const, so results are basically immutable.
* This is just like Variant<V, E> but with a slightly different API, and the
* following cases are optimized so Result can be stored more efficiently:
*
* - If both the success and error types do not use their least significant bit,
* are trivially copyable and destructible, Result<V, E> is guaranteed to be as
* large as the larger type. This is determined via the HasFreeLSB trait. By
* default, empty classes (in particular Ok) and aligned pointer types are
* assumed to have a free LSB, but you can specialize this trait for other
* types. If the success type is empty, the representation is guaranteed to be
* all zero bits on success. Do not change this representation! There is JIT
* code that depends on it. (Implementation note: The lowest bit is used as a
* tag bit: 0 to indicate the Result's bits are a success value, 1 to indicate
* the Result's bits (with the 1 masked out) encode an error value)
*
* - Else, if the error type can't have a all-zero bits representation and is
* not larger than a pointer, a CompactPair is used to represent this rather
* than a Variant. This has shown to be better optimizable, and the template
* code is much simpler than that of Variant, so it should also compile faster.
* Whether an error type can't be all-zero bits, is determined via the
* UnusedZero trait. MFBT doesn't declare any public type UnusedZero, but
* nsresult is declared UnusedZero in XPCOM.
*
* The purpose of Result is to reduce the screwups caused by using `false` or
* `nullptr` to indicate errors.
* a partial list.
*
* Result<const V, E> or Result<V, const E> are not meaningful. The success or
* error values in a Result instance are non-modifiable in-place anyway. This
* guarantee must also be maintained when evolving Result. They can be
* unwrap()ped, but this loses const qualification. However, Result<const V, E>
* or Result<V, const E> may be misleading and prevent movability. Just use
* Result<V, E>. (Result<const V*, E> may make sense though, just Result<const
* V* const, E> is not possible.)
*/
template <typename V, typename E>
class [[nodiscard]] Result final {
// See class comment on Result<const V, E> and Result<V, const E>.
static_assert(!std::is_const_v<V>);
static_assert(!std::is_const_v<E>);
static_assert(!std::is_reference_v<V>);
static_assert(!std::is_reference_v<E>);
using Impl = typename detail::SelectResultImpl<V, E>::Type;
Impl mImpl;
// Are you getting this error?
// > error: implicit instantiation of undefined template
// > 'mozilla::detail::ResultImplementation<$V,$E,
// > mozilla::detail::PackingStrategy::Variant>'
// You need to include "ResultVariant.h"!
public:
static constexpr detail::PackingStrategy Strategy = Impl::Strategy;
using ok_type = V;
using err_type = E;
/** Create a success result. */
MOZ_IMPLICIT constexpr Result(V&& aValue) : mImpl(std::move(aValue)) {
MOZ_ASSERT(isOk());
}
/** Create a success result. */
MOZ_IMPLICIT constexpr Result(const V& aValue) : mImpl(aValue) {
MOZ_ASSERT(isOk());
}
/** Create a success result in-place. */
template <typename... Args>
explicit constexpr Result(std::in_place_t, Args&&... aArgs)
: mImpl(std::in_place, std::forward<Args>(aArgs)...) {
MOZ_ASSERT(isOk());
}
/** Create an error result. */
explicit constexpr Result(const E& aErrorValue) : mImpl(aErrorValue) {
MOZ_ASSERT(isErr());
}
explicit constexpr Result(E&& aErrorValue) : mImpl(std::move(aErrorValue)) {
MOZ_ASSERT(isErr());
}
/**
* Create a (success/error) result from another (success/error) result with
* different but convertible value and error types.
*/
template <typename V2, typename E2,
typename = std::enable_if_t<std::is_convertible_v<V2, V> &&
std::is_convertible_v<E2, E>>>
MOZ_IMPLICIT constexpr Result(Result<V2, E2>&& aOther)
: mImpl(aOther.isOk() ? Impl{aOther.unwrap()}
: Impl{aOther.unwrapErr()}) {}
/**
* Implementation detail of MOZ_TRY().
* Create an error result from another error result.
*/
template <typename E2>
MOZ_IMPLICIT constexpr Result(GenericErrorResult<E2>&& aErrorResult)
: mImpl(std::move(aErrorResult.mErrorValue)) {
static_assert(std::is_convertible_v<E2, E>, "E2 must be convertible to E");
MOZ_ASSERT(isErr());
}
/**
* Implementation detail of MOZ_TRY().
* Create an error result from another error result.
*/
template <typename E2>
MOZ_IMPLICIT constexpr Result(const GenericErrorResult<E2>& aErrorResult)
: mImpl(aErrorResult.mErrorValue) {
static_assert(std::is_convertible_v<E2, E>, "E2 must be convertible to E");
MOZ_ASSERT(isErr());
}
Result(const Result&) = delete;
Result(Result&&) = default;
Result& operator=(const Result&) = delete;
Result& operator=(Result&&) = default;
/** True if this Result is a success result. */
constexpr bool isOk() const { return mImpl.isOk(); }
/** True if this Result is an error result. */
constexpr bool isErr() const { return !mImpl.isOk(); }
/** Take the success value from this Result, which must be a success result.
*/
constexpr V unwrap() {
MOZ_ASSERT(isOk());
return mImpl.unwrap();
}
/**
* Take the success value from this Result, which must be a success result.
* If it is an error result, then return the aValue.
*/
constexpr V unwrapOr(V aValue) {
return MOZ_LIKELY(isOk()) ? mImpl.unwrap() : std::move(aValue);
}
/** Take the error value from this Result, which must be an error result. */
constexpr E unwrapErr() {
MOZ_ASSERT(isErr());
return mImpl.unwrapErr();
}
/** Used only for GC tracing. If used in Rooted<Result<...>>, V must have a
* GCPolicy for tracing it. */
constexpr void updateAfterTracing(V&& aValue) {
mImpl.updateAfterTracing(std::move(aValue));
}
/** Used only for GC tracing. If used in Rooted<Result<...>>, E must have a
* GCPolicy for tracing it. */
constexpr void updateErrorAfterTracing(E&& aErrorValue) {
mImpl.updateErrorAfterTracing(std::move(aErrorValue));
}
/** See the success value from this Result, which must be a success result. */
constexpr decltype(auto) inspect() const {
static_assert(!std::is_reference_v<
std::invoke_result_t<decltype(&Impl::inspect), Impl>> ||
std::is_const_v<std::remove_reference_t<
std::invoke_result_t<decltype(&Impl::inspect), Impl>>>);
MOZ_ASSERT(isOk());
return mImpl.inspect();
}
/** See the error value from this Result, which must be an error result. */
constexpr decltype(auto) inspectErr() const {
static_assert(
!std::is_reference_v<
std::invoke_result_t<decltype(&Impl::inspectErr), Impl>> ||
std::is_const_v<std::remove_reference_t<
std::invoke_result_t<decltype(&Impl::inspectErr), Impl>>>);
MOZ_ASSERT(isErr());
return mImpl.inspectErr();
}
/** Propagate the error value from this Result, which must be an error result.
*
* This can be used to propagate an error from a function call to the caller
* with a different value type, but the same error type:
*
* Result<T1, E> Func1() {
* Result<T2, E> res = Func2();
* if (res.isErr()) { return res.propagateErr(); }
* }
*/
constexpr GenericErrorResult<E> propagateErr() {
MOZ_ASSERT(isErr());
return GenericErrorResult<E>{mImpl.unwrapErr(), ErrorPropagationTag{}};
}
/**
* Map a function V -> V2 over this result's success variant. If this result
* is an error, do not invoke the function and propagate the error.
*
* Mapping over success values invokes the function to produce a new success
* value:
*
* // Map Result<int, E> to another Result<int, E>
* Result<int, E> res(5);
* Result<int, E> res2 = res.map([](int x) { return x * x; });
* MOZ_ASSERT(res.isOk());
* MOZ_ASSERT(res2.unwrap() == 25);
*
* // Map Result<const char*, E> to Result<size_t, E>
* Result<const char*, E> res("hello, map!");
* Result<size_t, E> res2 = res.map(strlen);
* MOZ_ASSERT(res.isOk());
* MOZ_ASSERT(res2.unwrap() == 11);
*
* Mapping over an error does not invoke the function and propagates the
* error:
*
* Result<V, int> res(5);
* MOZ_ASSERT(res.isErr());
* Result<V2, int> res2 = res.map([](V v) { ... });
* MOZ_ASSERT(res2.isErr());
* MOZ_ASSERT(res2.unwrapErr() == 5);
*/
template <typename F>
constexpr auto map(F f) -> Result<std::invoke_result_t<F, V>, E> {
using RetResult = Result<std::invoke_result_t<F, V>, E>;
return MOZ_LIKELY(isOk()) ? RetResult(f(unwrap())) : RetResult(unwrapErr());
}
/**
* Map a function E -> E2 over this result's error variant. If this result is
* a success, do not invoke the function and move the success over.
*
* Mapping over error values invokes the function to produce a new error
* value:
*
* // Map Result<V, int> to another Result<V, int>
* Result<V, int> res(5);
* Result<V, int> res2 = res.mapErr([](int x) { return x * x; });
* MOZ_ASSERT(res2.isErr());
* MOZ_ASSERT(res2.unwrapErr() == 25);
*
* // Map Result<V, const char*> to Result<V, size_t>
* Result<V, const char*> res("hello, mapErr!");
* Result<V, size_t> res2 = res.mapErr(strlen);
* MOZ_ASSERT(res2.isErr());
* MOZ_ASSERT(res2.unwrapErr() == 14);
*
* Mapping over a success does not invoke the function and moves the success:
*
* Result<int, E> res(5);
* MOZ_ASSERT(res.isOk());
* Result<int, E2> res2 = res.mapErr([](E e) { ... });
* MOZ_ASSERT(res2.isOk());
* MOZ_ASSERT(res2.unwrap() == 5);
*/
template <typename F>
constexpr auto mapErr(F f) {
using RetResult = Result<V, std::invoke_result_t<F, E>>;
return MOZ_UNLIKELY(isErr()) ? RetResult(f(unwrapErr()))
: RetResult(unwrap());
}
/**
* Map a function E -> Result<V, E2> over this result's error variant. If
* this result is a success, do not invoke the function and move the success
* over.
*
* `orElse`ing over error values invokes the function to produce a new
* result:
*
* // `orElse` Result<V, int> error variant to another Result<V, int>
* // error variant or Result<V, int> success variant
* auto orElse = [](int x) -> Result<V, int> {
* if (x != 6) {
* return Err(x * x);
* }
* return V(...);
* };
*
* Result<V, int> res(5);
* auto res2 = res.orElse(orElse);
* MOZ_ASSERT(res2.isErr());
* MOZ_ASSERT(res2.unwrapErr() == 25);
*
* Result<V, int> res3(6);
* auto res4 = res3.orElse(orElse);
* MOZ_ASSERT(res4.isOk());
* MOZ_ASSERT(res4.unwrap() == ...);
*
* // `orElse` Result<V, const char*> error variant to Result<V, size_t>
* // error variant or Result<V, size_t> success variant
* auto orElse = [](const char* s) -> Result<V, size_t> {
* if (strcmp(s, "foo")) {
* return Err(strlen(s));
* }
* return V(...);
* };
*
* Result<V, const char*> res("hello, orElse!");
* auto res2 = res.orElse(orElse);
* MOZ_ASSERT(res2.isErr());
* MOZ_ASSERT(res2.unwrapErr() == 14);
*
* Result<V, const char*> res3("foo");
* auto res4 = ress.orElse(orElse);
* MOZ_ASSERT(res4.isOk());
* MOZ_ASSERT(res4.unwrap() == ...);
*
* `orElse`ing over a success does not invoke the function and moves the
* success:
*
* Result<int, E> res(5);
* MOZ_ASSERT(res.isOk());
* Result<int, E2> res2 = res.orElse([](E e) { ... });
* MOZ_ASSERT(res2.isOk());
* MOZ_ASSERT(res2.unwrap() == 5);
*/
template <typename F>
auto orElse(F f) -> Result<V, typename std::invoke_result_t<F, E>::err_type> {
return MOZ_UNLIKELY(isErr()) ? f(unwrapErr()) : unwrap();
}
/**
* Given a function V -> Result<V2, E>, apply it to this result's success
* value and return its result. If this result is an error value, it is
* propagated.
*
* This is sometimes called "flatMap" or ">>=" in other contexts.
*
* `andThen`ing over success values invokes the function to produce a new
* result:
*
* Result<const char*, Error> res("hello, andThen!");
* Result<HtmlFreeString, Error> res2 = res.andThen([](const char* s) {
* return containsHtmlTag(s)
* ? Result<HtmlFreeString, Error>(Error("Invalid: contains HTML"))
* : Result<HtmlFreeString, Error>(HtmlFreeString(s));
* }
* });
* MOZ_ASSERT(res2.isOk());
* MOZ_ASSERT(res2.unwrap() == HtmlFreeString("hello, andThen!");
*
* `andThen`ing over error results does not invoke the function, and just
* propagates the error result:
*
* Result<int, const char*> res("some error");
* auto res2 = res.andThen([](int x) { ... });
* MOZ_ASSERT(res2.isErr());
* MOZ_ASSERT(res.unwrapErr() == res2.unwrapErr());
*/
template <typename F, typename = std::enable_if_t<detail::IsResult<
std::invoke_result_t<F, V&&>>::value>>
constexpr auto andThen(F f) -> std::invoke_result_t<F, V&&> {
return MOZ_LIKELY(isOk()) ? f(unwrap()) : propagateErr();
}
};
/**
* A type that auto-converts to an error Result. This is like a Result without
* a success type. It's the best return type for functions that always return
* an error--functions designed to build and populate error objects. It's also
* useful in error-handling macros; see MOZ_TRY for an example.
*/
template <typename E>
class [[nodiscard]] GenericErrorResult {
E mErrorValue;
template <typename V, typename E2>
friend class Result;
public:
explicit constexpr GenericErrorResult(const E& aErrorValue)
: mErrorValue(aErrorValue) {}
explicit constexpr GenericErrorResult(E&& aErrorValue)
: mErrorValue(std::move(aErrorValue)) {}
constexpr GenericErrorResult(const E& aErrorValue, const ErrorPropagationTag&)
: GenericErrorResult(aErrorValue) {}
constexpr GenericErrorResult(E&& aErrorValue, const ErrorPropagationTag&)
: GenericErrorResult(std::move(aErrorValue)) {}
};
template <typename E>
inline constexpr auto Err(E&& aErrorValue) {
return GenericErrorResult<std::decay_t<E>>(std::forward<E>(aErrorValue));
}
} // namespace mozilla
#endif // mozilla_Result_h