Go 1.27 reference · Slices, Maps, Iterators and JSON

Go iter.Seq (range over functions) Type

Write iterators as functions that yield values, and loop over them with for range like slices and maps.

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Syntax

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

Parameters

Parameter	Description
`yield(v)`	Produce a value; returns false when the loop stopped early.
`for v := range seq`	Consume the iterator.

Examples

Example 1

package main

import (
	"fmt"
	"iter"
)

func evens(limit int) iter.Seq[int] {
	return func(yield func(int) bool) {
		for i := 0; i < limit; i += 2 {
			if !yield(i) {
				return
			}
		}
	}
}

func main() {
	for n := range evens(10) {
		fmt.Print(n, " ")
	}
	fmt.Println()
}

Output

0 2 4 6 8

Run this on your computer — see “Get Started” for setup.

Two values and stopping early

package main

import (
	"fmt"
	"iter"
)

func numbered(lines []string) iter.Seq2[int, string] {
	return func(yield func(int, string) bool) {
		for i, l := range lines {
			if !yield(i+1, l) {
				return
			}
		}
	}
}

func main() {
	for i, line := range numbered([]string{"alpha", "beta", "gamma"}) {
		if i == 2 {
			break // stops the iterator early
		}
		fmt.Println(i, line)
	}
}