### Iterators

The new [iter] package provides the basic definitions for working with
user-defined iterators.

The [slices] package adds several functions that work with iterators:
- [All](/pkg/slices#All) returns an iterator over slice indexes and values.
- [Values](/pkg/slices#Values) returns an iterator over slice elements.
- [Backward](/pkg/slices#Backward) returns an iterator that loops over
  a slice backward.

pkg maps, func Collect[$0 comparable, $1 interface{}](iter.Seq2[$0, $1]) map[$0]$1 #61900
pkg maps, func Insert[$0 interface{ ~map[$1]$2 }, $1 comparable, $2 interface{}]($0, iter.Seq2[$1, $2]) #61900
pkg maps, func Keys[$0 interface{ ~map[$1]$2 }, $1 comparable, $2 interface{}]($0) iter.Seq[$1] #61900
pkg maps, func Values[$0 interface{ ~map[$1]$2 }, $1 comparable, $2 interface{}]($0) iter.Seq[$2] #61900

// license that can be found in the LICENSE file.

// Linkname coroswitch is not allowed, even if iter.Pull
// is instantiated in the same package.

package main

import (
	"iter"
	"unsafe"
)

func seq(yield func(int) bool) {
	yield(123)
}

func main() {
	next, stop := iter.Pull(seq)
	next()
	stop()
	coroswitch(nil)
}

//go:linkname coroswitch runtime.coroswitch

<!-- see ../../3-iter.md -->...

<!-- see ../../3-iter.md -->...

<!-- see ../../3-iter.md -->...

// license that can be found in the LICENSE file.

// Use of public API is ok.

package main

import (
	"iter"
	"unique"
)

func seq(yield func(int) bool) {
	yield(123)
}

var s = "hello"

func main() {
	h := unique.Make(s)
	next, stop := iter.Pull(seq)
	defer stop()
	println(h.Value())
	println(next())
	println(next())

<!-- see ../../3-iter.md -->...

// If a name is in this map, it is allowed only in listed packages,
// even if it has a linknamed definition.
var blockedLinknames = map[string][]string{
	// coroutines
	"runtime.coroswitch": {"iter"},
	"runtime.newcoro":    {"iter"},
	// weak references
	"internal/weak.runtime_registerWeakPointer": {"internal/weak"},
	"internal/weak.runtime_makeStrongFromWeak":  {"internal/weak"},
}

				runTests: "SystemRoots",
				env:      []string{"GOOS=ios", "CGO_ENABLED=1"},
				pkg:      "crypto/x509",
			})
	}

	// GOEXPERIMENT=rangefunc tests
	if !t.compileOnly {
		for _, pkg := range []string{"iter", "slices", "maps"} {
			t.registerTest("GOEXPERIMENT=rangefunc",
				&goTest{
					variant: pkg,
					short:   t.short,
					env:     []string{"GOEXPERIMENT=rangefunc"},
					pkg:     pkg,
				})
		}