Let custom iterators return multiple values

[Casper64]

Describe the feature

Let custom iterators return multiple values. Since there are no tuples in V custom iterators could be used to handle data that would otherwise be in a tuple.

Use Case

Lets say we have 2 arrays with a different datatype and we want to iterate over those 2 arrays simultaneously.

Right now this could be done by creating a for loop and keeping track of the index, but this code has to be written every time
and is not reusable.

module main

fn main() {
	arr1 := ['a', 'b', 'c']
	arr2 := [1, 2, 3]
	
	mut index := 0
	for {
		if index >= arr1.len || index >= arr2.len {
			break
		}
		
		letter := arr1[index]
		number := arr2[index]
		println('${letter}: ${number}')
	}
}

Or

module main

fn zip[T, U](a []T, b []U, iterator fn (T, U)) {
	mut idx := 0
	for idx < a.len && idx < b.len {
		iterator(a[idx], b[idx])
		idx++
	}
}

fn main() {
	arr1 := ['a', 'b', 'c']
	arr2 := [1, 2, 3]

	zip(arr1, arr2, fn(letter string, number int) {
		println('${letter}: ${number}')
	})
}

The second example is much better but it is not better than with a custom iterator in my opinion as it involves a callback function.

It would be nice to have a generic function for this like pythons zip function. An implementation for zip would look something like this:

module main

struct ZipIterator[T, U] {
	arr1 []T
	arr2 []U
mut:
	idx int
}

fn zip[T, U](array1 []T, array2 []U) ZipIterator[T, U] {
	return ZipIterator[T, U]{
		arr1: array1
		arr2: array2
	}
}

fn (mut iter ZipIterator[T, U]) next[T, U]() ?(T, U) {
	if iter.idx >= iter.arr1.len || iter.idx >= iter.arr2.len {
		return none
	}
	defer {
		iter.idx++
	}
	return iter.arr1[iter.idx], iter.arr2[iter.idx]
}

fn main() {
	arr1 := ['a', 'b', 'c']
	arr2 := [1, 2, 3]

	for letter, index in zip(arr1, arr2) {
		println('${letter}: ${index}')
	}
}

At this moment the code above generates an error

error: iterator method `next()` must not return multiple values

Itertools

I noticed that VSL has some extra iteration tools like cycle, but it is only for f64 and I think a small iterators utility module in the standard library would be good to have.

Proposed Solution

Let iterators return multiple values.

Other Information

No response

Acknowledgements

• I may be able to implement this feature request
• This feature might incur a breaking change

Version used

V 0.3.3 4e498b4.e738d67

Environment details (OS name and version, etc.)

V full version: V 0.3.3 4e498b4.e738d67
OS: linux, Kali GNU/Linux Rolling (WSL 2)
Processor: 24 cpus, 64bit, little endian, 12th Gen Intel(R) Core(TM) i9-12900K

getwd: /home/casper/code/v/issues
vexe: /opt/v/v
vexe mtime: 2023-04-29 14:35:19

vroot: OK, value: /opt/v
VMODULES: OK, value: /home/casper/.vmodules
VTMP: OK, value: /tmp/v_1000

Git version: git version 2.39.2
Git vroot status: weekly.2023.15-82-ge738d671
.git/config present: true

CC version: cc (Debian 12.2.0-14) 12.2.0
thirdparty/tcc status: thirdparty-linux-amd64 12f392c3