proposal: Go 2: iterators

[Zaba505]

This proposal is for the addition of a new builtin type, iterator, and a new package, iter:

type iterator interface{
  Next() (item interface{}, more bool)
}

// Under package iter

// This struct is for extending the basic builtin iterator to support
// a broader and higher level method set.
//
type Iterator struct {
  // internals would all be private
}

// An example of a method that this type could provide
func (i Iterator) Map(f func(item interface{}) interface{}) Iterator {}

Why

builtin iterator type

This follows a similar reason as to why error is a single method interface and not a struct because it allows any implementation to provide the desired behaviour. The behaviour then comes from the definition of an iterator: a producer of items; hence, the single method, Next. A direct benefit (all be it difficult to document) is that existing builtin types that are iterable can automatically implement the iterator interface. Thus no wrapper types would have to be created to accommodate the use of, for example, []int in a function which looks like this func sum(nums iterator) int.

package iter

Again much like iterator follows error, iter follows the errors package in the sense that its purpose is to provide utilities for working with iterators. I won't go in to everything that can be built on top of the basic iterator interface because a quick google search can bring you up to speed on some very common methods like map, reduce, filter, etc. The main reason for including this package is to provide all users with a consistent and convenient package for simplifying their work with iterators.

Some Thoughts

• What about errors? - Should iterator actually look like Next() (interface{}, error) where no more items would be identified with io.EOF or possibly iter.EOI.
• Is this future proof in regards to generics? - Some initial thinking leads me to believe so since ideally it would amount to a change looking like this: Next() (item T, more bool)
• How useful is this to the greater community really? - I think fairly useful since my (probably wrong) view of the community is one rich with "data processing" (broader sense than just pipelines/aggregation) usage which are workloads that iterators really excel at

For reference, this proposal has been heavily influenced by Rust's implementation of iterators.

[martisch]

"is that existing builtin types that are iterable can automatically implement the iterator interface"

I think it needs to be spelled out what this means. How can []int implement the iterator interface? Does that mean []int would have a Next method which is a fairly invasive language change for all types now to have methods. Next requires a state. How and when is this intialized/reset for a value like []int{1,2,3} or map[string]int{"foo":1,...} and where is it stored?

Please give some full examples how usage would look like.

What does an iterator on a channel do? How do iterators deal with the slice or maps being modified (also in the number of entries)?

Would e.g. range use that iterator interface? Why not? If yes how?

To change the signature of next with generics would be backwards incomaptible. It seems it would be good to see how generics will turn out first. Using interface{} values is also very cumbersome as it will require type assertions in the calling code of next.

Its also good to think about implementation. Using all these interfaces will cause a lot of allocations and performance impact.

[mvdan]

Please note that you should fill https://github.com/golang/proposal/blob/master/go2-language-changes.md when proposing a language change.

[icholy]

The generics proposal will enable writing your own iterator interface outside of the stdlib.

type Iterator[type T] interface {
    Next() bool
    Value() T
}

This gives you everything except the range behavior (which I don't think adds too much value anyway).

for x := range it {
    fmt.Println(x)
}

// vs

for it.Next() {
    fmt.Println(it.Value())
}

[leaxoy]

To get an usable and convenient version of Iterator, I think we need above features.

1. interface default method. To chain function like a.Map(func (int) string {}).Filter(func(string)bool{}).Collect() while a is any struct that implement Iterator interface. This design borrow from rust.
2. Method type parameter. With this feature, we can write Map method with another type parameter inside Iterator interface.
3. Type specialization. With this feature, we can get Sum method only type parameter implement Summable or Addable.

Further，we can add simple lambda expression, describe at #21498. Enum for Option, describe at #19412.

[leaxoy]

And any type implement Iterator should be passed to for-loop statement.

[elissa2333]

Why don't you try to implement iterators using channels

package main

import "fmt"

func main() {
	for i := range iterator() {
		if i > 5 { // You can exit at any time
			break
		}
		fmt.Println(i)
	}
}

func iterator() <-chan int {
	ch := make(chan int)
	go func(ch chan int) {
		defer close(ch)
		for i := 0; i < 10; i++ {
			ch <- i
		}

	}(ch)

	return ch
}

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