1 // Copyright 2023 The Go Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style 3 // license that can be found in the LICENSE file. 4 5 /* 6 Package iter provides basic definitions and operations related to 7 iterators over sequences. 8 9 # Iterators 10 11 An iterator is a function that passes successive elements of a 12 sequence to a callback function, conventionally named yield. 13 The function stops either when the sequence is finished or 14 when yield returns false, indicating to stop the iteration early. 15 This package defines [Seq] and [Seq2] 16 (pronounced like seek—the first syllable of sequence) 17 as shorthands for iterators that pass 1 or 2 values per sequence element 18 to yield: 19 20 type ( 21 Seq[V any] func(yield func(V) bool) 22 Seq2[K, V any] func(yield func(K, V) bool) 23 ) 24 25 Seq2 represents a sequence of paired values, conventionally key-value 26 or index-value pairs. 27 28 Yield returns true if the iterator should continue with the next 29 element in the sequence, false if it should stop. 30 31 Iterator functions are most often called by a range loop, as in: 32 33 func PrintAll[V any](seq iter.Seq[V]) { 34 for v := range seq { 35 fmt.Println(v) 36 } 37 } 38 39 # Naming Conventions 40 41 Iterator functions and methods are named for the sequence being walked: 42 43 // All returns an iterator over all elements in s. 44 func (s *Set[V]) All() iter.Seq[V] 45 46 The iterator method on a collection type is conventionally named All, 47 because it iterates a sequence of all the values in the collection. 48 49 For a type containing multiple possible sequences, the iterator's name 50 can indicate which sequence is being provided: 51 52 // Cities returns an iterator over the major cities in the country. 53 func (c *Country) Cities() iter.Seq[*City] 54 55 // Languages returns an iterator over the official spoken languages of the country. 56 func (c *Country) Languages() iter.Seq[string] 57 58 If an iterator requires additional configuration, the constructor function 59 can take additional configuration arguments: 60 61 // Scan returns an iterator over key-value pairs with min ≤ key ≤ max. 62 func (m *Map[K, V]) Scan(min, max K) iter.Seq2[K, V] 63 64 // Split returns an iterator over the (possibly-empty) substrings of s 65 // separated by sep. 66 func Split(s, sep string) iter.Seq[string] 67 68 When there are multiple possible iteration orders, the method name may 69 indicate that order: 70 71 // All returns an iterator over the list from head to tail. 72 func (l *List[V]) All() iter.Seq[V] 73 74 // Backward returns an iterator over the list from tail to head. 75 func (l *List[V]) Backward() iter.Seq[V] 76 77 // Preorder returns an iterator over all nodes of the syntax tree 78 // beneath (and including) the specified root, in depth-first preorder, 79 // visiting a parent node before its children. 80 func Preorder(root Node) iter.Seq[Node] 81 82 # Single-Use Iterators 83 84 Most iterators provide the ability to walk an entire sequence: 85 when called, the iterator does any setup necessary to start the 86 sequence, then calls yield on successive elements of the sequence, 87 and then cleans up before returning. Calling the iterator again 88 walks the sequence again. 89 90 Some iterators break that convention, providing the ability to walk a 91 sequence only once. These “single-use iterators” typically report values 92 from a data stream that cannot be rewound to start over. 93 Calling the iterator again after stopping early may continue the 94 stream, but calling it again after the sequence is finished will yield 95 no values at all. Doc comments for functions or methods that return 96 single-use iterators should document this fact: 97 98 // Lines returns an iterator over lines read from r. 99 // It returns a single-use iterator. 100 func (r *Reader) Lines() iter.Seq[string] 101 102 # Pulling Values 103 104 Functions and methods that accept or return iterators 105 should use the standard [Seq] or [Seq2] types, to ensure 106 compatibility with range loops and other iterator adapters. 107 The standard iterators can be thought of as “push iterators”, which 108 push values to the yield function. 109 110 Sometimes a range loop is not the most natural way to consume values 111 of the sequence. In this case, [Pull] converts a standard push iterator 112 to a “pull iterator”, which can be called to pull one value at a time 113 from the sequence. [Pull] starts an iterator and returns a pair 114 of functions—next and stop—which return the next value from the iterator 115 and stop it, respectively. 116 117 For example: 118 119 // Pairs returns an iterator over successive pairs of values from seq. 120 func Pairs[V any](seq iter.Seq[V]) iter.Seq2[V, V] { 121 return func(yield func(V, V) bool) { 122 next, stop := iter.Pull(seq) 123 defer stop() 124 for { 125 v1, ok1 := next() 126 if !ok1 { 127 return 128 } 129 v2, ok2 := next() 130 // If ok2 is false, v2 should be the 131 // zero value; yield one last pair. 132 if !yield(v1, v2) { 133 return 134 } 135 if !ok2 { 136 return 137 } 138 } 139 } 140 } 141 142 If clients do not consume the sequence to completion, they must call stop, 143 which allows the iterator function to finish and return. As shown in 144 the example, the conventional way to ensure this is to use defer. 145 146 # Standard Library Usage 147 148 A few packages in the standard library provide iterator-based APIs, 149 most notably the [maps] and [slices] packages. 150 For example, [maps.Keys] returns an iterator over the keys of a map, 151 while [slices.Sorted] collects the values of an iterator into a slice, 152 sorts them, and returns the slice, so to iterate over the sorted keys of a map: 153 154 for _, key := range slices.Sorted(maps.Keys(m)) { 155 ... 156 } 157 158 # Mutation 159 160 Iterators provide only the values of the sequence, not any direct way 161 to modify it. If an iterator wishes to provide a mechanism for modifying 162 a sequence during iteration, the usual approach is to define a position type 163 with the extra operations and then provide an iterator over positions. 164 165 For example, a tree implementation might provide: 166 167 // Positions returns an iterator over positions in the sequence. 168 func (t *Tree[V]) Positions() iter.Seq[*Pos] 169 170 // A Pos represents a position in the sequence. 171 // It is only valid during the yield call it is passed to. 172 type Pos[V any] struct { ... } 173 174 // Pos returns the value at the cursor. 175 func (p *Pos[V]) Value() V 176 177 // Delete deletes the value at this point in the iteration. 178 func (p *Pos[V]) Delete() 179 180 // Set changes the value v at the cursor. 181 func (p *Pos[V]) Set(v V) 182 183 And then a client could delete boring values from the tree using: 184 185 for p := range t.Positions() { 186 if boring(p.Value()) { 187 p.Delete() 188 } 189 } 190 */ 191 package iter 192 193 import ( 194 "internal/race" 195 "runtime" 196 "unsafe" 197 ) 198 199 // Seq is an iterator over sequences of individual values. 200 // When called as seq(yield), seq calls yield(v) for each value v in the sequence, 201 // stopping early if yield returns false. 202 // See the [iter] package documentation for more details. 203 type Seq[V any] func(yield func(V) bool) 204 205 // Seq2 is an iterator over sequences of pairs of values, most commonly key-value pairs. 206 // When called as seq(yield), seq calls yield(k, v) for each pair (k, v) in the sequence, 207 // stopping early if yield returns false. 208 // See the [iter] package documentation for more details. 209 type Seq2[K, V any] func(yield func(K, V) bool) 210 211 type coro struct{} 212 213 //go:linkname newcoro runtime.newcoro 214 func newcoro(func(*coro)) *coro 215 216 //go:linkname coroswitch runtime.coroswitch 217 func coroswitch(*coro) 218 219 // Pull converts the “push-style” iterator sequence seq 220 // into a “pull-style” iterator accessed by the two functions 221 // next and stop. 222 // 223 // Next returns the next value in the sequence 224 // and a boolean indicating whether the value is valid. 225 // When the sequence is over, next returns the zero V and false. 226 // It is valid to call next after reaching the end of the sequence 227 // or after calling stop. These calls will continue 228 // to return the zero V and false. 229 // 230 // Stop ends the iteration. It must be called when the caller is 231 // no longer interested in next values and next has not yet 232 // signaled that the sequence is over (with a false boolean return). 233 // It is valid to call stop multiple times and when next has 234 // already returned false. Typically, callers should “defer stop()”. 235 // 236 // It is an error to call next or stop from multiple goroutines 237 // simultaneously. 238 // 239 // If the iterator panics during a call to next (or stop), 240 // then next (or stop) itself panics with the same value. 241 func Pull[V any](seq Seq[V]) (next func() (V, bool), stop func()) { 242 var ( 243 v V 244 ok bool 245 done bool 246 yieldNext bool 247 racer int 248 panicValue any 249 seqDone bool // to detect Goexit 250 ) 251 c := newcoro(func(c *coro) { 252 race.Acquire(unsafe.Pointer(&racer)) 253 if done { 254 race.Release(unsafe.Pointer(&racer)) 255 return 256 } 257 yield := func(v1 V) bool { 258 if done { 259 return false 260 } 261 if !yieldNext { 262 panic("iter.Pull: yield called again before next") 263 } 264 yieldNext = false 265 v, ok = v1, true 266 race.Release(unsafe.Pointer(&racer)) 267 coroswitch(c) 268 race.Acquire(unsafe.Pointer(&racer)) 269 return !done 270 } 271 // Recover and propagate panics from seq. 272 defer func() { 273 if p := recover(); p != nil { 274 panicValue = p 275 } else if !seqDone { 276 panicValue = goexitPanicValue 277 } 278 done = true // Invalidate iterator 279 race.Release(unsafe.Pointer(&racer)) 280 }() 281 seq(yield) 282 var v0 V 283 v, ok = v0, false 284 seqDone = true 285 }) 286 next = func() (v1 V, ok1 bool) { 287 race.Write(unsafe.Pointer(&racer)) // detect races 288 289 if done { 290 return 291 } 292 if yieldNext { 293 panic("iter.Pull: next called again before yield") 294 } 295 yieldNext = true 296 race.Release(unsafe.Pointer(&racer)) 297 coroswitch(c) 298 race.Acquire(unsafe.Pointer(&racer)) 299 300 // Propagate panics and goexits from seq. 301 if panicValue != nil { 302 if panicValue == goexitPanicValue { 303 // Propagate runtime.Goexit from seq. 304 runtime.Goexit() 305 } else { 306 panic(panicValue) 307 } 308 } 309 return v, ok 310 } 311 stop = func() { 312 race.Write(unsafe.Pointer(&racer)) // detect races 313 314 if !done { 315 done = true 316 race.Release(unsafe.Pointer(&racer)) 317 coroswitch(c) 318 race.Acquire(unsafe.Pointer(&racer)) 319 320 // Propagate panics and goexits from seq. 321 if panicValue != nil { 322 if panicValue == goexitPanicValue { 323 // Propagate runtime.Goexit from seq. 324 runtime.Goexit() 325 } else { 326 panic(panicValue) 327 } 328 } 329 } 330 } 331 return next, stop 332 } 333 334 // Pull2 converts the “push-style” iterator sequence seq 335 // into a “pull-style” iterator accessed by the two functions 336 // next and stop. 337 // 338 // Next returns the next pair in the sequence 339 // and a boolean indicating whether the pair is valid. 340 // When the sequence is over, next returns a pair of zero values and false. 341 // It is valid to call next after reaching the end of the sequence 342 // or after calling stop. These calls will continue 343 // to return a pair of zero values and false. 344 // 345 // Stop ends the iteration. It must be called when the caller is 346 // no longer interested in next values and next has not yet 347 // signaled that the sequence is over (with a false boolean return). 348 // It is valid to call stop multiple times and when next has 349 // already returned false. Typically, callers should “defer stop()”. 350 // 351 // It is an error to call next or stop from multiple goroutines 352 // simultaneously. 353 // 354 // If the iterator panics during a call to next (or stop), 355 // then next (or stop) itself panics with the same value. 356 func Pull2[K, V any](seq Seq2[K, V]) (next func() (K, V, bool), stop func()) { 357 var ( 358 k K 359 v V 360 ok bool 361 done bool 362 yieldNext bool 363 racer int 364 panicValue any 365 seqDone bool 366 ) 367 c := newcoro(func(c *coro) { 368 race.Acquire(unsafe.Pointer(&racer)) 369 if done { 370 race.Release(unsafe.Pointer(&racer)) 371 return 372 } 373 yield := func(k1 K, v1 V) bool { 374 if done { 375 return false 376 } 377 if !yieldNext { 378 panic("iter.Pull2: yield called again before next") 379 } 380 yieldNext = false 381 k, v, ok = k1, v1, true 382 race.Release(unsafe.Pointer(&racer)) 383 coroswitch(c) 384 race.Acquire(unsafe.Pointer(&racer)) 385 return !done 386 } 387 // Recover and propagate panics from seq. 388 defer func() { 389 if p := recover(); p != nil { 390 panicValue = p 391 } else if !seqDone { 392 panicValue = goexitPanicValue 393 } 394 done = true // Invalidate iterator. 395 race.Release(unsafe.Pointer(&racer)) 396 }() 397 seq(yield) 398 var k0 K 399 var v0 V 400 k, v, ok = k0, v0, false 401 seqDone = true 402 }) 403 next = func() (k1 K, v1 V, ok1 bool) { 404 race.Write(unsafe.Pointer(&racer)) // detect races 405 406 if done { 407 return 408 } 409 if yieldNext { 410 panic("iter.Pull2: next called again before yield") 411 } 412 yieldNext = true 413 race.Release(unsafe.Pointer(&racer)) 414 coroswitch(c) 415 race.Acquire(unsafe.Pointer(&racer)) 416 417 // Propagate panics and goexits from seq. 418 if panicValue != nil { 419 if panicValue == goexitPanicValue { 420 // Propagate runtime.Goexit from seq. 421 runtime.Goexit() 422 } else { 423 panic(panicValue) 424 } 425 } 426 return k, v, ok 427 } 428 stop = func() { 429 race.Write(unsafe.Pointer(&racer)) // detect races 430 431 if !done { 432 done = true 433 race.Release(unsafe.Pointer(&racer)) 434 coroswitch(c) 435 race.Acquire(unsafe.Pointer(&racer)) 436 437 // Propagate panics and goexits from seq. 438 if panicValue != nil { 439 if panicValue == goexitPanicValue { 440 // Propagate runtime.Goexit from seq. 441 runtime.Goexit() 442 } else { 443 panic(panicValue) 444 } 445 } 446 } 447 } 448 return next, stop 449 } 450 451 // goexitPanicValue is a sentinel value indicating that an iterator 452 // exited via runtime.Goexit. 453 var goexitPanicValue any = new(int) 454