Iterators in Go: Range Over Functions

Since Go 1.23, for range can loop over functions. This gives Go a standard way to write iterators for custom data structures, database rows or paginated APIs.

The iterator types

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

An iterator calls yield for each value. If yield returns false (the loop hit break), the iterator must stop.

Writing one

import "iter"

// Evens yields even numbers up to max
func Evens(max int) iter.Seq[int] {
    return func(yield func(int) bool) {
        for n := 0; n <= max; n += 2 {
            if !yield(n) {
                return
            }
        }
    }
}

for n := range Evens(10) {
    fmt.Println(n) // 0 2 4 6 8 10
}

A paginated API as one stream

func AllOrders(ctx context.Context, c *Client) iter.Seq2[Order, error] {
    return func(yield func(Order, error) bool) {
        cursor := ""
        for {
            page, err := c.ListOrders(ctx, cursor)
            if err != nil {
                yield(Order{}, err)
                return
            }
            for _, o := range page.Orders {
                if !yield(o, nil) {
                    return
                }
            }
            if page.NextCursor == "" {
                return
            }
            cursor = page.NextCursor
        }
    }
}

for order, err := range AllOrders(ctx, client) {
    if err != nil {
        return err
    }
    process(order)
}

Standard library support

keys := slices.Sorted(maps.Keys(ages))      // maps.Keys returns an iterator
evens := slices.Collect(Evens(20))          // iterator -> slice
for i, v := range slices.All(names) { ... }  // index and value
for v := range slices.Values(names) { ... }

Also new in Go 1.22: for i := range 10 loops over integers 0 to 9.