Revision control

Line Code
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 * vim: set ts=8 sts=4 et sw=4 tw=99:
 * 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
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

/* A type suitable for returning either a value or an error from a function. */

#ifndef mozilla_Result_h
#define mozilla_Result_h

#include "mozilla/Alignment.h"
#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"
#include "mozilla/Types.h"
#include "mozilla/TypeTraits.h"
#include "mozilla/Variant.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 {};

template <typename E> class GenericErrorResult;
template <typename V, typename E> class Result;

namespace detail {

enum class PackingStrategy {
  Variant,
  NullIsOk,
  LowBitTagIsError,
  PackedVariant,
};

template <typename V, typename E, PackingStrategy Strategy>
class ResultImplementation;

template <typename V, typename E>
class ResultImplementation<V, E, PackingStrategy::Variant>
{
  mozilla::Variant<V, E> mStorage;

public:
  explicit ResultImplementation(V aValue) : mStorage(aValue) {}
  explicit ResultImplementation(E aErrorValue) : mStorage(aErrorValue) {}

  bool isOk() const { return mStorage.template is<V>(); }

  // The callers of these functions will assert isOk() has the proper value, so
  // these functions (in all ResultImplementation specializations) don't need
  // to do so.
  V unwrap() const { return mStorage.template as<V>(); }
  E unwrapErr() const { return mStorage.template as<E>(); }
};

/**
 * mozilla::Variant doesn't like storing a reference. This is a specialization
 * to store E as pointer if it's a reference.
 */
template <typename V, typename E>
class ResultImplementation<V, E&, PackingStrategy::Variant>
{
  mozilla::Variant<V, E*> mStorage;

public:
  explicit ResultImplementation(V aValue) : mStorage(aValue) {}
  explicit ResultImplementation(E& aErrorValue) : mStorage(&aErrorValue) {}

  bool isOk() const { return mStorage.template is<V>(); }
  V unwrap() const { return mStorage.template as<V>(); }
  E& unwrapErr() const { return *mStorage.template as<E*>(); }
};

/**
 * Specialization for when the success type is Ok (or another empty class) and
 * the error type is a reference.
 */
template <typename V, typename E>
class ResultImplementation<V, E&, PackingStrategy::NullIsOk>
{
  E* mErrorValue;

public:
  explicit ResultImplementation(V) : mErrorValue(nullptr) {}
  explicit ResultImplementation(E& aErrorValue) : mErrorValue(&aErrorValue) {}

  bool isOk() const { return mErrorValue == nullptr; }

  V unwrap() const { return V(); }
  E& unwrapErr() const { return *mErrorValue; }
};

/**
 * Specialization for when the success type is Ok (or another empty class) 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>
{
  static constexpr E NullValue = E(0);

  E mErrorValue;

public:
  explicit ResultImplementation(V) : mErrorValue(NullValue) {}
  explicit ResultImplementation(E aErrorValue) : mErrorValue(aErrorValue)
  {
    MOZ_ASSERT(aErrorValue != NullValue);
  }

  bool isOk() const { return mErrorValue == NullValue; }

  V unwrap() const { return V(); }
  E unwrapErr() const { return mErrorValue; }
};


/**
 * 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>
{
  uintptr_t mBits;

public:
  explicit ResultImplementation(V* aValue)
    : mBits(reinterpret_cast<uintptr_t>(aValue))
  {
    MOZ_ASSERT((uintptr_t(aValue) % MOZ_ALIGNOF(V)) == 0,
               "Result value pointers must not be misaligned");
  }
  explicit ResultImplementation(E& aErrorValue)
    : mBits(reinterpret_cast<uintptr_t>(&aErrorValue) | 1)
  {
    MOZ_ASSERT((uintptr_t(&aErrorValue) % MOZ_ALIGNOF(E)) == 0,
               "Result errors must not be misaligned");
  }

  bool isOk() const { return (mBits & 1) == 0; }

  V* unwrap() const { return reinterpret_cast<V*>(mBits); }
  E& unwrapErr() const { return *reinterpret_cast<E*>(mBits ^ 1); }
};

// Return true if any of the struct can fit in a word.
template<typename V, typename E>
struct IsPackableVariant
{
  struct VEbool {
      V v;
      E e;
      bool ok;
  };
  struct EVbool {
      E e;
      V v;
      bool ok;
  };

  using Impl = typename Conditional<sizeof(VEbool) <= sizeof(EVbool),
                                    VEbool, EVbool>::Type;

  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:
  explicit ResultImplementation(V aValue)
  {
    data.v = aValue;
    data.ok = true;
  }
  explicit ResultImplementation(E aErrorValue)
  {
    data.e = aErrorValue;
    data.ok = false;
  }

  bool isOk() const { return data.ok; }

  V unwrap() const { return data.v; }
  E unwrapErr() const { return data.e; }
};

// 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;
};

// References can't be null.
template<typename T>
struct UnusedZero<T&>
{
  static const bool value = true;
};

// 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.
template <typename T> struct HasFreeLSB { 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 = (MOZ_ALIGNOF(T) & 1) == 0;
};

// We store references as pointers, so they have a free bit if a pointer would
// have one.
template <typename T> struct HasFreeLSB<T&> {
  static const bool value = HasFreeLSB<T*>::value;
};

// 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 =
      (IsEmpty<V>::value && UnusedZero<E>::value)
    ? PackingStrategy::NullIsOk
    : (detail::HasFreeLSB<V>::value && detail::HasFreeLSB<E>::value)
    ? PackingStrategy::LowBitTagIsError
    : (IsDefaultConstructible<V>::value && IsDefaultConstructible<E>::value &&
       IsPackableVariant<V, E>::value)
    ? PackingStrategy::PackedVariant
    : PackingStrategy::Variant;

  using Type = detail::ResultImplementation<V, E, value>;
};

template <typename T>
struct IsResult : FalseType { };

template <typename V, typename E>
struct IsResult<Result<V, E>> : TrueType { };

} // namespace detail

template <typename V, typename E>
auto
ToResult(Result<V, E>&& aValue)
  -> decltype(Forward<Result<V, E>>(aValue))
{
  return 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 the success type is Ok (or another empty class) and the error type is a
 *   reference, Result<V, E&> is guaranteed to be pointer-sized and all zero
 *   bits on success. Do not change this representation! There is JIT code that
 *   depends on it.
 *
 * - If the success type is a pointer type and the error type is a reference
 *   type, and the least significant bit is unused for both types when stored
 *   as a pointer (due to alignment rules), Result<V*, E&> is guaranteed to be
 *   pointer-sized. In this case, we use the lowest bit as tag bit: 0 to
 *   indicate the Result's bits are a V, 1 to indicate the Result's bits (with
 *   the 1 masked out) encode an E*.
 *
 * The purpose of Result is to reduce the screwups caused by using `false` or
 * `nullptr` to indicate errors.
 * What screwups? See <https://bugzilla.mozilla.org/show_bug.cgi?id=912928> for
 * a partial list.
 */
template <typename V, typename E>
class MOZ_MUST_USE_TYPE Result final
{
  using Impl = typename detail::SelectResultImpl<V, E>::Type;

  Impl mImpl;

public:
  /**
   * Create a success result.
   */
  MOZ_IMPLICIT Result(V aValue) : mImpl(aValue) { MOZ_ASSERT(isOk()); }

  /**
   * Create an error result.
   */
  explicit Result(E aErrorValue) : mImpl(aErrorValue) { MOZ_ASSERT(isErr()); }

  /**
   * Implementation detail of MOZ_TRY().
   * Create an error result from another error result.
   */
  template <typename E2>
  MOZ_IMPLICIT Result(const GenericErrorResult<E2>& aErrorResult)
    : mImpl(aErrorResult.mErrorValue)
  {
    static_assert(mozilla::IsConvertible<E2, E>::value,
                  "E2 must be convertible to E");
    MOZ_ASSERT(isErr());
  }

  Result(const Result&) = default;
  Result& operator=(const Result&) = default;

  /** True if this Result is a success result. */
  bool isOk() const { return mImpl.isOk(); }

  /** True if this Result is an error result. */
  bool isErr() const { return !mImpl.isOk(); }

  /** Get the success value from this Result, which must be a success result. */
  V unwrap() const {
    MOZ_ASSERT(isOk());
    return mImpl.unwrap();
  }

  /** Get the error value from this Result, which must be an error result. */
  E unwrapErr() const {
    MOZ_ASSERT(isErr());
    return mImpl.unwrapErr();
  }

  /**
   * Map a function V -> W over this result's success variant. If this result is
   * an error, do not invoke the function and return a copy of 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(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(res2.unwrap() == 11);
   *
   * Mapping over an error does not invoke the function and copies the error:
   *
   *     Result<V, int> res(5);
   *     MOZ_ASSERT(res.isErr());
   *     Result<W, int> res2 = res.map([](V v) { ... });
   *     MOZ_ASSERT(res2.isErr());
   *     MOZ_ASSERT(res2.unwrapErr() == 5);
   */
  template<typename F>
  auto map(F f) const -> Result<decltype(f(*((V*) nullptr))), E> {
      using RetResult = Result<decltype(f(*((V*) nullptr))), E>;
      return isOk() ? RetResult(f(unwrap())) : RetResult(unwrapErr());
  }

  /**
   * Given a function V -> Result<W, E>, apply it to this result's success value
   * and return its result. If this result is an error value, then return a
   * copy.
   *
   * 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
   * produces a new copy of 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 = typename EnableIf<
          detail::IsResult<decltype((*((F*) nullptr))(*((V*) nullptr)))>::value
      >::Type
  >
  auto andThen(F f) const -> decltype(f(*((V*) nullptr))) {
      return isOk() ? f(unwrap()) : GenericErrorResult<E>(unwrapErr());
  }
};

/**
 * 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 MOZ_MUST_USE_TYPE GenericErrorResult
{
  E mErrorValue;

  template<typename V, typename E2> friend class Result;

public:
  explicit GenericErrorResult(E aErrorValue) : mErrorValue(aErrorValue) {}
};

template <typename E>
inline GenericErrorResult<E>
Err(E&& aErrorValue)
{
  return GenericErrorResult<E>(aErrorValue);
}

} // namespace mozilla

/**
 * MOZ_TRY(expr) is the C++ equivalent of Rust's `try!(expr);`. First, it
 * evaluates expr, which must produce a Result value. On success, it
 * discards the result altogether. On error, it immediately returns an error
 * Result from the enclosing function.
 */
#define MOZ_TRY(expr) \
  do { \
    auto mozTryTempResult_ = ::mozilla::ToResult(expr); \
    if (mozTryTempResult_.isErr()) { \
      return ::mozilla::Err(mozTryTempResult_.unwrapErr()); \
    } \
  } while (0)

/**
 * MOZ_TRY_VAR(target, expr) is the C++ equivalent of Rust's `target = try!(expr);`.
 * First, it evaluates expr, which must produce a Result value.
 * On success, the result's success value is assigned to target.
 * On error, immediately returns the error result.
 * |target| must evaluate to a reference without any side effects.
 */
#define MOZ_TRY_VAR(target, expr) \
  do { \
    auto mozTryVarTempResult_ = (expr); \
    if (mozTryVarTempResult_.isErr()) { \
      return ::mozilla::Err( \
          mozTryVarTempResult_.unwrapErr()); \
    } \
    (target) = mozTryVarTempResult_.unwrap(); \
  } while (0)

#endif // mozilla_Result_h