src/iter/iter.go - go - Git at Google

 // Copyright 2023 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 /*
 Package iter provides basic definitions and operations related to
 iterators over sequences.

 # Iterators

 An iterator is a function that passes successive elements of a
 sequence to a callback function, conventionally named yield.
 The function stops either when the sequence is finished or
 when yield returns false, indicating to stop the iteration early.
 This package defines [Seq] and [Seq2]
 (pronounced like seek—the first syllable of sequence)
 as shorthands for iterators that pass 1 or 2 values per sequence element
 to yield:

 	type (
 		Seq[V any]     func(yield func(V) bool)
 		Seq2[K, V any] func(yield func(K, V) bool)
 	)

 Seq2 represents a sequence of paired values, conventionally key-value
 or index-value pairs.

 Yield returns true if the iterator should continue with the next
 element in the sequence, false if it should stop.

 For instance, [maps.Keys] returns an iterator that produces the sequence
 of keys of the map m, implemented as follows:

 	func Keys[Map ~map[K]V, K comparable, V any](m Map) iter.Seq[K] {
 		return func(yield func(K) bool) {
 			for k := range m {
 				if !yield(k) {
 					return
 				}
 			}
 		}
 	}

 Further examples can be found in [The Go Blog: Range Over Function Types].

 Iterator functions are most often called by a [range loop], as in:

 	func PrintAll[V any](seq iter.Seq[V]) {
 		for v := range seq {
 			fmt.Println(v)
 		}
 	}

 # Naming Conventions

 Iterator functions and methods are named for the sequence being walked:

 	// All returns an iterator over all elements in s.
 	func (s *Set[V]) All() iter.Seq[V]

 The iterator method on a collection type is conventionally named All,
 because it iterates a sequence of all the values in the collection.

 For a type containing multiple possible sequences, the iterator's name
 can indicate which sequence is being provided:

 	// Cities returns an iterator over the major cities in the country.
 	func (c *Country) Cities() iter.Seq[*City]

 	// Languages returns an iterator over the official spoken languages of the country.
 	func (c *Country) Languages() iter.Seq[string]

 If an iterator requires additional configuration, the constructor function
 can take additional configuration arguments:

 	// Scan returns an iterator over key-value pairs with min ≤ key ≤ max.
 	func (m *Map[K, V]) Scan(min, max K) iter.Seq2[K, V]

 	// Split returns an iterator over the (possibly-empty) substrings of s
 	// separated by sep.
 	func Split(s, sep string) iter.Seq[string]

 When there are multiple possible iteration orders, the method name may
 indicate that order:

 	// All returns an iterator over the list from head to tail.
 	func (l *List[V]) All() iter.Seq[V]

 	// Backward returns an iterator over the list from tail to head.
 	func (l *List[V]) Backward() iter.Seq[V]

 	// Preorder returns an iterator over all nodes of the syntax tree
 	// beneath (and including) the specified root, in depth-first preorder,
 	// visiting a parent node before its children.
 	func Preorder(root Node) iter.Seq[Node]

 # Single-Use Iterators

 Most iterators provide the ability to walk an entire sequence:
 when called, the iterator does any setup necessary to start the
 sequence, then calls yield on successive elements of the sequence,
 and then cleans up before returning. Calling the iterator again
 walks the sequence again.

 Some iterators break that convention, providing the ability to walk a
 sequence only once. These “single-use iterators” typically report values
 from a data stream that cannot be rewound to start over.
 Calling the iterator again after stopping early may continue the
 stream, but calling it again after the sequence is finished will yield
 no values at all. Doc comments for functions or methods that return
 single-use iterators should document this fact:

 	// Lines returns an iterator over lines read from r.
 	// It returns a single-use iterator.
 	func (r *Reader) Lines() iter.Seq[string]

 # Pulling Values

 Functions and methods that accept or return iterators
 should use the standard [Seq] or [Seq2] types, to ensure
 compatibility with range loops and other iterator adapters.
 The standard iterators can be thought of as “push iterators”, which
 push values to the yield function.

 Sometimes a range loop is not the most natural way to consume values
 of the sequence. In this case, [Pull] converts a standard push iterator
 to a “pull iterator”, which can be called to pull one value at a time
 from the sequence. [Pull] starts an iterator and returns a pair
 of functions—next and stop—which return the next value from the iterator
 and stop it, respectively.

 For example:

 	// Pairs returns an iterator over successive pairs of values from seq.
 	func Pairs[V any](seq iter.Seq[V]) iter.Seq2[V, V] {
 		return func(yield func(V, V) bool) {
 			next, stop := iter.Pull(seq)
 			defer stop()
 			for {
 				v1, ok1 := next()
 				if !ok1 {
 					return
 				}
 				v2, ok2 := next()
 				// If ok2 is false, v2 should be the
 				// zero value; yield one last pair.
 				if !yield(v1, v2) {
 					return
 				}
 				if !ok2 {
 					return
 				}
 			}
 		}
 	}

 If clients do not consume the sequence to completion, they must call stop,
 which allows the iterator function to finish and return. As shown in
 the example, the conventional way to ensure this is to use defer.

 # Standard Library Usage

 A few packages in the standard library provide iterator-based APIs,
 most notably the [maps] and [slices] packages.
 For example, [maps.Keys] returns an iterator over the keys of a map,
 while [slices.Sorted] collects the values of an iterator into a slice,
 sorts them, and returns the slice, so to iterate over the sorted keys of a map:

 	for _, key := range slices.Sorted(maps.Keys(m)) {
 		...
 	}

 # Mutation

 Iterators provide only the values of the sequence, not any direct way
 to modify it. If an iterator wishes to provide a mechanism for modifying
 a sequence during iteration, the usual approach is to define a position type
 with the extra operations and then provide an iterator over positions.

 For example, a tree implementation might provide:

 	// Positions returns an iterator over positions in the sequence.
 	func (t *Tree[V]) Positions() iter.Seq[*Pos]

 	// A Pos represents a position in the sequence.
 	// It is only valid during the yield call it is passed to.
 	type Pos[V any] struct { ... }

 	// Pos returns the value at the cursor.
 	func (p *Pos[V]) Value() V

 	// Delete deletes the value at this point in the iteration.
 	func (p *Pos[V]) Delete()

 	// Set changes the value v at the cursor.
 	func (p *Pos[V]) Set(v V)

 And then a client could delete boring values from the tree using:

 	for p := range t.Positions() {
 		if boring(p.Value()) {
 			p.Delete()
 		}
 	}

 [The Go Blog: Range Over Function Types]: https://go.dev/blog/range-functions
 [range loop]: https://go.dev/ref/spec#For_range
 */
 package iter

 import (
 	"internal/race"
 	"runtime"
 	"unsafe"
 )

 // Seq is an iterator over sequences of individual values.
 // When called as seq(yield), seq calls yield(v) for each value v in the sequence,
 // stopping early if yield returns false.
 // See the [iter] package documentation for more details.
 type Seq[V any] func(yield func(V) bool)

 // Seq2 is an iterator over sequences of pairs of values, most commonly key-value pairs.
 // When called as seq(yield), seq calls yield(k, v) for each pair (k, v) in the sequence,
 // stopping early if yield returns false.
 // See the [iter] package documentation for more details.
 type Seq2[K, V any] func(yield func(K, V) bool)

 type coro struct{}

 //go:linkname newcoro runtime.newcoro
 func newcoro(func(*coro)) *coro

 //go:linkname coroswitch runtime.coroswitch
 func coroswitch(*coro)

 // Pull converts the “push-style” iterator sequence seq
 // into a “pull-style” iterator accessed by the two functions
 // next and stop.
 //
 // Next returns the next value in the sequence
 // and a boolean indicating whether the value is valid.
 // When the sequence is over, next returns the zero V and false.
 // It is valid to call next after reaching the end of the sequence
 // or after calling stop. These calls will continue
 // to return the zero V and false.
 //
 // Stop ends the iteration. It must be called when the caller is
 // no longer interested in next values and next has not yet
 // signaled that the sequence is over (with a false boolean return).
 // It is valid to call stop multiple times and when next has
 // already returned false. Typically, callers should “defer stop()”.
 //
 // It is an error to call next or stop from multiple goroutines
 // simultaneously.
 //
 // If the iterator panics during a call to next (or stop),
 // then next (or stop) itself panics with the same value.
 func Pull[V any](seq Seq[V]) (next func() (V, bool), stop func()) {
 	var (
 		v          V
 		ok         bool
 		done       bool
 		yieldNext  bool
 		racer      int
 		panicValue any
 		seqDone    bool // to detect Goexit
 	)
 	c := newcoro(func(c *coro) {
 		race.Acquire(unsafe.Pointer(&racer))
 		if done {
 			race.Release(unsafe.Pointer(&racer))
 			return
 		}
 		yield := func(v1 V) bool {
 			if done {
 				return false
 			}
 			if !yieldNext {
 				panic("iter.Pull: yield called again before next")
 			}
 			yieldNext = false
 			v, ok = v1, true
 			race.Release(unsafe.Pointer(&racer))
 			coroswitch(c)
 			race.Acquire(unsafe.Pointer(&racer))
 			return !done
 		}
 		// Recover and propagate panics from seq.
 		defer func() {
 			if p := recover(); p != nil {
 				panicValue = p
 			} else if !seqDone {
 				panicValue = goexitPanicValue
 			}
 			done = true // Invalidate iterator
 			race.Release(unsafe.Pointer(&racer))
 		}()
 		seq(yield)
 		var v0 V
 		v, ok = v0, false
 		seqDone = true
 	})
 	next = func() (v1 V, ok1 bool) {
 		race.Write(unsafe.Pointer(&racer)) // detect races

 		if done {
 			return
 		}
 		if yieldNext {
 			panic("iter.Pull: next called again before yield")
 		}
 		yieldNext = true
 		race.Release(unsafe.Pointer(&racer))
 		coroswitch(c)
 		race.Acquire(unsafe.Pointer(&racer))

 		// Propagate panics and goexits from seq.
 		if panicValue != nil {
 			if panicValue == goexitPanicValue {
 				// Propagate runtime.Goexit from seq.
 				runtime.Goexit()
 			} else {
 				panic(panicValue)
 			}
 		}
 		return v, ok
 	}
 	stop = func() {
 		race.Write(unsafe.Pointer(&racer)) // detect races

 		if !done {
 			done = true
 			race.Release(unsafe.Pointer(&racer))
 			coroswitch(c)
 			race.Acquire(unsafe.Pointer(&racer))

 			// Propagate panics and goexits from seq.
 			if panicValue != nil {
 				if panicValue == goexitPanicValue {
 					// Propagate runtime.Goexit from seq.
 					runtime.Goexit()
 				} else {
 					panic(panicValue)
 				}
 			}
 		}
 	}
 	return next, stop
 }

 // Pull2 converts the “push-style” iterator sequence seq
 // into a “pull-style” iterator accessed by the two functions
 // next and stop.
 //
 // Next returns the next pair in the sequence
 // and a boolean indicating whether the pair is valid.
 // When the sequence is over, next returns a pair of zero values and false.
 // It is valid to call next after reaching the end of the sequence
 // or after calling stop. These calls will continue
 // to return a pair of zero values and false.
 //
 // Stop ends the iteration. It must be called when the caller is
 // no longer interested in next values and next has not yet
 // signaled that the sequence is over (with a false boolean return).
 // It is valid to call stop multiple times and when next has
 // already returned false. Typically, callers should “defer stop()”.
 //
 // It is an error to call next or stop from multiple goroutines
 // simultaneously.
 //
 // If the iterator panics during a call to next (or stop),
 // then next (or stop) itself panics with the same value.
 func Pull2[K, V any](seq Seq2[K, V]) (next func() (K, V, bool), stop func()) {
 	var (
 		k          K
 		v          V
 		ok         bool
 		done       bool
 		yieldNext  bool
 		racer      int
 		panicValue any
 		seqDone    bool
 	)
 	c := newcoro(func(c *coro) {
 		race.Acquire(unsafe.Pointer(&racer))
 		if done {
 			race.Release(unsafe.Pointer(&racer))
 			return
 		}
 		yield := func(k1 K, v1 V) bool {
 			if done {
 				return false
 			}
 			if !yieldNext {
 				panic("iter.Pull2: yield called again before next")
 			}
 			yieldNext = false
 			k, v, ok = k1, v1, true
 			race.Release(unsafe.Pointer(&racer))
 			coroswitch(c)
 			race.Acquire(unsafe.Pointer(&racer))
 			return !done
 		}
 		// Recover and propagate panics from seq.
 		defer func() {
 			if p := recover(); p != nil {
 				panicValue = p
 			} else if !seqDone {
 				panicValue = goexitPanicValue
 			}
 			done = true // Invalidate iterator.
 			race.Release(unsafe.Pointer(&racer))
 		}()
 		seq(yield)
 		var k0 K
 		var v0 V
 		k, v, ok = k0, v0, false
 		seqDone = true
 	})
 	next = func() (k1 K, v1 V, ok1 bool) {
 		race.Write(unsafe.Pointer(&racer)) // detect races

 		if done {
 			return
 		}
 		if yieldNext {
 			panic("iter.Pull2: next called again before yield")
 		}
 		yieldNext = true
 		race.Release(unsafe.Pointer(&racer))
 		coroswitch(c)
 		race.Acquire(unsafe.Pointer(&racer))

 		// Propagate panics and goexits from seq.
 		if panicValue != nil {
 			if panicValue == goexitPanicValue {
 				// Propagate runtime.Goexit from seq.
 				runtime.Goexit()
 			} else {
 				panic(panicValue)
 			}
 		}
 		return k, v, ok
 	}
 	stop = func() {
 		race.Write(unsafe.Pointer(&racer)) // detect races

 		if !done {
 			done = true
 			race.Release(unsafe.Pointer(&racer))
 			coroswitch(c)
 			race.Acquire(unsafe.Pointer(&racer))

 			// Propagate panics and goexits from seq.
 			if panicValue != nil {
 				if panicValue == goexitPanicValue {
 					// Propagate runtime.Goexit from seq.
 					runtime.Goexit()
 				} else {
 					panic(panicValue)
 				}
 			}
 		}
 	}
 	return next, stop
 }

 // goexitPanicValue is a sentinel value indicating that an iterator
 // exited via runtime.Goexit.
 var goexitPanicValue any = new(int)
	// Copyright 2023 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	/*
	Package iter provides basic definitions and operations related to
	iterators over sequences.

	# Iterators

	An iterator is a function that passes successive elements of a
	sequence to a callback function, conventionally named yield.
	The function stops either when the sequence is finished or
	when yield returns false, indicating to stop the iteration early.
	This package defines [Seq] and [Seq2]
	(pronounced like seek—the first syllable of sequence)
	as shorthands for iterators that pass 1 or 2 values per sequence element
	to yield:

	type (
	Seq[V any] func(yield func(V) bool)
	Seq2[K, V any] func(yield func(K, V) bool)
	)

	Seq2 represents a sequence of paired values, conventionally key-value
	or index-value pairs.

	Yield returns true if the iterator should continue with the next
	element in the sequence, false if it should stop.

	For instance, [maps.Keys] returns an iterator that produces the sequence
	of keys of the map m, implemented as follows:

	func Keys[Map ~map[K]V, K comparable, V any](m Map) iter.Seq[K] {
	return func(yield func(K) bool) {
	for k := range m {
	if !yield(k) {
	return
	}
	}
	}
	}

	Further examples can be found in [The Go Blog: Range Over Function Types].

	Iterator functions are most often called by a [range loop], as in:

	func PrintAll[V any](seq iter.Seq[V]) {
	for v := range seq {
	fmt.Println(v)
	}
	}

	# Naming Conventions

	Iterator functions and methods are named for the sequence being walked:

	// All returns an iterator over all elements in s.
	func (s *Set[V]) All() iter.Seq[V]

	The iterator method on a collection type is conventionally named All,
	because it iterates a sequence of all the values in the collection.

	For a type containing multiple possible sequences, the iterator's name
	can indicate which sequence is being provided:

	// Cities returns an iterator over the major cities in the country.
	func (c Country) Cities() iter.Seq[City]

	// Languages returns an iterator over the official spoken languages of the country.
	func (c *Country) Languages() iter.Seq[string]

	If an iterator requires additional configuration, the constructor function
	can take additional configuration arguments:

	// Scan returns an iterator over key-value pairs with min ≤ key ≤ max.
	func (m *Map[K, V]) Scan(min, max K) iter.Seq2[K, V]

	// Split returns an iterator over the (possibly-empty) substrings of s
	// separated by sep.
	func Split(s, sep string) iter.Seq[string]

	When there are multiple possible iteration orders, the method name may
	indicate that order:

	// All returns an iterator over the list from head to tail.
	func (l *List[V]) All() iter.Seq[V]

	// Backward returns an iterator over the list from tail to head.
	func (l *List[V]) Backward() iter.Seq[V]

	// Preorder returns an iterator over all nodes of the syntax tree
	// beneath (and including) the specified root, in depth-first preorder,
	// visiting a parent node before its children.
	func Preorder(root Node) iter.Seq[Node]

	# Single-Use Iterators

	Most iterators provide the ability to walk an entire sequence:
	when called, the iterator does any setup necessary to start the
	sequence, then calls yield on successive elements of the sequence,
	and then cleans up before returning. Calling the iterator again
	walks the sequence again.

	Some iterators break that convention, providing the ability to walk a
	sequence only once. These “single-use iterators” typically report values
	from a data stream that cannot be rewound to start over.
	Calling the iterator again after stopping early may continue the
	stream, but calling it again after the sequence is finished will yield
	no values at all. Doc comments for functions or methods that return
	single-use iterators should document this fact:

	// Lines returns an iterator over lines read from r.
	// It returns a single-use iterator.
	func (r *Reader) Lines() iter.Seq[string]

	# Pulling Values

	Functions and methods that accept or return iterators
	should use the standard [Seq] or [Seq2] types, to ensure
	compatibility with range loops and other iterator adapters.
	The standard iterators can be thought of as “push iterators”, which
	push values to the yield function.

	Sometimes a range loop is not the most natural way to consume values
	of the sequence. In this case, [Pull] converts a standard push iterator
	to a “pull iterator”, which can be called to pull one value at a time
	from the sequence. [Pull] starts an iterator and returns a pair
	of functions—next and stop—which return the next value from the iterator
	and stop it, respectively.

	For example:

	// Pairs returns an iterator over successive pairs of values from seq.
	func Pairs[V any](seq iter.Seq[V]) iter.Seq2[V, V] {
	return func(yield func(V, V) bool) {
	next, stop := iter.Pull(seq)
	defer stop()
	for {
	v1, ok1 := next()
	if !ok1 {
	return
	}
	v2, ok2 := next()
	// If ok2 is false, v2 should be the
	// zero value; yield one last pair.
	if !yield(v1, v2) {
	return
	}
	if !ok2 {
	return
	}
	}
	}
	}

	If clients do not consume the sequence to completion, they must call stop,
	which allows the iterator function to finish and return. As shown in
	the example, the conventional way to ensure this is to use defer.

	# Standard Library Usage

	A few packages in the standard library provide iterator-based APIs,
	most notably the [maps] and [slices] packages.
	For example, [maps.Keys] returns an iterator over the keys of a map,
	while [slices.Sorted] collects the values of an iterator into a slice,
	sorts them, and returns the slice, so to iterate over the sorted keys of a map:

	for _, key := range slices.Sorted(maps.Keys(m)) {
	...
	}

	# Mutation

	Iterators provide only the values of the sequence, not any direct way
	to modify it. If an iterator wishes to provide a mechanism for modifying
	a sequence during iteration, the usual approach is to define a position type
	with the extra operations and then provide an iterator over positions.

	For example, a tree implementation might provide:

	// Positions returns an iterator over positions in the sequence.
	func (t Tree[V]) Positions() iter.Seq[Pos]

	// A Pos represents a position in the sequence.
	// It is only valid during the yield call it is passed to.
	type Pos[V any] struct { ... }

	// Pos returns the value at the cursor.
	func (p *Pos[V]) Value() V

	// Delete deletes the value at this point in the iteration.
	func (p *Pos[V]) Delete()

	// Set changes the value v at the cursor.
	func (p *Pos[V]) Set(v V)

	And then a client could delete boring values from the tree using:

	for p := range t.Positions() {
	if boring(p.Value()) {
	p.Delete()
	}
	}

	[The Go Blog: Range Over Function Types]: https://go.dev/blog/range-functions
	[range loop]: https://go.dev/ref/spec#For_range
	*/
	package iter

	import (
	"internal/race"
	"runtime"
	"unsafe"
	)

	// Seq is an iterator over sequences of individual values.
	// When called as seq(yield), seq calls yield(v) for each value v in the sequence,
	// stopping early if yield returns false.
	// See the [iter] package documentation for more details.
	type Seq[V any] func(yield func(V) bool)

	// Seq2 is an iterator over sequences of pairs of values, most commonly key-value pairs.
	// When called as seq(yield), seq calls yield(k, v) for each pair (k, v) in the sequence,
	// stopping early if yield returns false.
	// See the [iter] package documentation for more details.
	type Seq2[K, V any] func(yield func(K, V) bool)

	type coro struct{}

	//go:linkname newcoro runtime.newcoro
	func newcoro(func(coro)) coro

	//go:linkname coroswitch runtime.coroswitch
	func coroswitch(*coro)

	// Pull converts the “push-style” iterator sequence seq
	// into a “pull-style” iterator accessed by the two functions
	// next and stop.
	//
	// Next returns the next value in the sequence
	// and a boolean indicating whether the value is valid.
	// When the sequence is over, next returns the zero V and false.
	// It is valid to call next after reaching the end of the sequence
	// or after calling stop. These calls will continue
	// to return the zero V and false.
	//
	// Stop ends the iteration. It must be called when the caller is
	// no longer interested in next values and next has not yet
	// signaled that the sequence is over (with a false boolean return).
	// It is valid to call stop multiple times and when next has
	// already returned false. Typically, callers should “defer stop()”.
	//
	// It is an error to call next or stop from multiple goroutines
	// simultaneously.
	//
	// If the iterator panics during a call to next (or stop),
	// then next (or stop) itself panics with the same value.
	func Pull[V any](seq Seq[V]) (next func() (V, bool), stop func()) {
	var (
	v V
	ok bool
	done bool
	yieldNext bool
	racer int
	panicValue any
	seqDone bool // to detect Goexit
	)
	c := newcoro(func(c *coro) {
	race.Acquire(unsafe.Pointer(&racer))
	if done {
	race.Release(unsafe.Pointer(&racer))
	return
	}
	yield := func(v1 V) bool {
	if done {
	return false
	}
	if !yieldNext {
	panic("iter.Pull: yield called again before next")
	}
	yieldNext = false
	v, ok = v1, true
	race.Release(unsafe.Pointer(&racer))
	coroswitch(c)
	race.Acquire(unsafe.Pointer(&racer))
	return !done
	}
	// Recover and propagate panics from seq.
	defer func() {
	if p := recover(); p != nil {
	panicValue = p
	} else if !seqDone {
	panicValue = goexitPanicValue
	}
	done = true // Invalidate iterator
	race.Release(unsafe.Pointer(&racer))
	}()
	seq(yield)
	var v0 V
	v, ok = v0, false
	seqDone = true
	})
	next = func() (v1 V, ok1 bool) {
	race.Write(unsafe.Pointer(&racer)) // detect races

	if done {
	return
	}
	if yieldNext {
	panic("iter.Pull: next called again before yield")
	}
	yieldNext = true
	race.Release(unsafe.Pointer(&racer))
	coroswitch(c)
	race.Acquire(unsafe.Pointer(&racer))

	// Propagate panics and goexits from seq.
	if panicValue != nil {
	if panicValue == goexitPanicValue {
	// Propagate runtime.Goexit from seq.
	runtime.Goexit()
	} else {
	panic(panicValue)
	}
	}
	return v, ok
	}
	stop = func() {
	race.Write(unsafe.Pointer(&racer)) // detect races

	if !done {
	done = true
	race.Release(unsafe.Pointer(&racer))
	coroswitch(c)
	race.Acquire(unsafe.Pointer(&racer))

	// Propagate panics and goexits from seq.
	if panicValue != nil {
	if panicValue == goexitPanicValue {
	// Propagate runtime.Goexit from seq.
	runtime.Goexit()
	} else {
	panic(panicValue)
	}
	}
	}
	}
	return next, stop
	}

	// Pull2 converts the “push-style” iterator sequence seq
	// into a “pull-style” iterator accessed by the two functions
	// next and stop.
	//
	// Next returns the next pair in the sequence
	// and a boolean indicating whether the pair is valid.
	// When the sequence is over, next returns a pair of zero values and false.
	// It is valid to call next after reaching the end of the sequence
	// or after calling stop. These calls will continue
	// to return a pair of zero values and false.
	//
	// Stop ends the iteration. It must be called when the caller is
	// no longer interested in next values and next has not yet
	// signaled that the sequence is over (with a false boolean return).
	// It is valid to call stop multiple times and when next has
	// already returned false. Typically, callers should “defer stop()”.
	//
	// It is an error to call next or stop from multiple goroutines
	// simultaneously.
	//
	// If the iterator panics during a call to next (or stop),
	// then next (or stop) itself panics with the same value.
	func Pull2[K, V any](seq Seq2[K, V]) (next func() (K, V, bool), stop func()) {
	var (
	k K
	v V
	ok bool
	done bool
	yieldNext bool
	racer int
	panicValue any
	seqDone bool
	)
	c := newcoro(func(c *coro) {
	race.Acquire(unsafe.Pointer(&racer))
	if done {
	race.Release(unsafe.Pointer(&racer))
	return
	}
	yield := func(k1 K, v1 V) bool {
	if done {
	return false
	}
	if !yieldNext {
	panic("iter.Pull2: yield called again before next")
	}
	yieldNext = false
	k, v, ok = k1, v1, true
	race.Release(unsafe.Pointer(&racer))
	coroswitch(c)
	race.Acquire(unsafe.Pointer(&racer))
	return !done
	}
	// Recover and propagate panics from seq.
	defer func() {
	if p := recover(); p != nil {
	panicValue = p
	} else if !seqDone {
	panicValue = goexitPanicValue
	}
	done = true // Invalidate iterator.
	race.Release(unsafe.Pointer(&racer))
	}()
	seq(yield)
	var k0 K
	var v0 V
	k, v, ok = k0, v0, false
	seqDone = true
	})
	next = func() (k1 K, v1 V, ok1 bool) {
	race.Write(unsafe.Pointer(&racer)) // detect races

	if done {
	return
	}
	if yieldNext {
	panic("iter.Pull2: next called again before yield")
	}
	yieldNext = true
	race.Release(unsafe.Pointer(&racer))
	coroswitch(c)
	race.Acquire(unsafe.Pointer(&racer))

	// Propagate panics and goexits from seq.
	if panicValue != nil {
	if panicValue == goexitPanicValue {
	// Propagate runtime.Goexit from seq.
	runtime.Goexit()
	} else {
	panic(panicValue)
	}
	}
	return k, v, ok
	}
	stop = func() {
	race.Write(unsafe.Pointer(&racer)) // detect races

	if !done {
	done = true
	race.Release(unsafe.Pointer(&racer))
	coroswitch(c)
	race.Acquire(unsafe.Pointer(&racer))

	// Propagate panics and goexits from seq.
	if panicValue != nil {
	if panicValue == goexitPanicValue {
	// Propagate runtime.Goexit from seq.
	runtime.Goexit()
	} else {
	panic(panicValue)
	}
	}
	}
	}
	return next, stop
	}

	// goexitPanicValue is a sentinel value indicating that an iterator
	// exited via runtime.Goexit.
	var goexitPanicValue any = new(int)