How to Create Lazy Sequences in Go with Iterators

Go does not have built-in lazy sequences or iterators; you must implement them manually using a function that accepts a callback (yield function). Define a function returning a closure that iterates over your data and calls the callback for each item, returning false to stop iteration.

type Seq[T any] func(yield func(T) bool)

func OfSlice[T any](s []T) Seq[T] {
	return func(yield func(T) bool) {
		for _, v := range s {
			if !yield(v) {
				return
			}
		}
	}
}

// Usage
for v := range OfSlice([]int{1, 2, 3}) {
	fmt.Println(v)
}

Note: The standard library does not support for v := range iterator() syntax yet; you must call the iterator function directly with a loop body closure as shown in the OfSlice implementation logic, or use the range keyword only if you are using Go 1.23+ with the specific range-over-func feature enabled for your module version.

Lazy sequences in Go are functions that generate values one by one only when you ask for them, rather than creating a full list in memory at once. You build them by writing a function that takes another function (a callback) and calls it for every item in your data. This is like a conveyor belt that only moves the next item when you are ready to pick it up, saving memory for large datasets.