Range over function in Go

Go 1.23 lets a function be ranged over, which is how iterators work now.

func Countdown(from int) iter.Seq[int] {
    return func(yield func(int) bool) {
        for n := from; n > 0; n-- {
            if !yield(n) {
                return
            }
        }
    }
}

func Enumerate[T any](values []T) iter.Seq2[int, T] {
    return func(yield func(int, T) bool) {
        for i, value := range values {
            if !yield(i, value) {
                return
            }
        }
    }
}

func use() (int, string) {
    sum := 0
    for n := range Countdown(4) {
        sum += n
    }

    var last string
    for _, value := range Enumerate([]string{"a", "b"}) {
        last = value
    }
    return sum, last
}

How it works

1. An iterator is a func taking a yield callback.
2. Returning false from yield stops the iteration.
3. iter.Seq and iter.Seq2 name the two shapes.

Keywords and builtins used here

• any
• bool
• for
• func
• if
• int
• range
• return
• string
• var

The run, in numbers

Lines

32

Characters to type

479

Tokens

160

Three-star pace

110 tpm

At the three-star pace of 110 tokens a minute, this run takes about 87 seconds.

Type this snippet

Step 2 of 2 in API shapes, step 18 of 19 in Interfaces & generics.

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