proposal: Go 2: expand power of range for slices/arrays

[pjebs]

This proposal wishes to expand the power of the range keyword when applied to slices and arrays.

Note: If you don't want to use the features, your code will LOOK EXACTLY THE SAME.

Currently:

s := []string{}

for idx, v := range s {}

Proposal:

1. Iterate forwards (with superfluous +)

for idx, v := range +s {}

*Equivalent:
for idx := 0; idx < len(s); idx++ {
	v := s[idx]
} 

2. Iterate backwards

for idx, v := range -s {}

*Equivalent:
for idx := len(s) - 1; idx >= 0; idx-- {
	v := s[idx]
} 

3. Iterate from middle outwards

for idx, v := range ±s {}

*Equivalent:
mid := (len(s) - 1) >> 1
for j := 0; j < len(s); j++ {
	var idx int
	if j%2 == 0 {
		idx = mid - j/2
	} else {
		idx = mid + (j/2 + 1)
	}

	v := s[idx]
}

[pjebs]

Additional Notes:

1. ± is used to start from middle and go to right initially
2. ∓ is used to start from middle and go to the left initially
3. Iterating from middle outwards has (rare) but important use cases such as in divide and conquer algorithms.
4. I found use for it in my strconv.ParseComplex PR.
5. The use case is rare. But when a developer needs it, they will find the character for it, despite it not being readily available on the keyboard.

[ianlancetaylor]

Would you mind filling out the template in https://go.googlesource.com/proposal/+/bd3ac287ccbebb2d12a386f1f1447876dd74b54d/go2-language-changes.md ?

Thanks.

[pjebs]

where do I save it?

[ianlancetaylor]

Add it as a comment to this issue. Thanks.

[dpinela]

Iterating from the middle seems like too rare a use case to bake into the language - I've personally never encountered it. Iterating in reverse is something I've needed a few times myself; it may be worth adding, if only to help avoid off-by-one errors.

I think the syntax you're proposing is too cryptic, though. I would rather implement this as a reversed built-in (not a keyword, because that'd be a breaking change) instead. Also, if we had generics plus some way to implement custom iterators that work with range, it wouldn't even have to be in the language itself.

[deanveloper]

While this is clever, I'm not sure if I like it. Go has very few symbols that aren't universally understood (:= and <- are the only ones off the top of my head) so adding -slice doesn't seem right. I sortof like @dpinela's idea of a reversed(slice) builtin though.

[ianlancetaylor]

The middle-out range is rarely useful, and the syntax is cryptic. That doesn't seem like a good addition to the language.

How often does one want to iterate backward? It's already possible to do so, in a way that is clear and easy to understand. The single - character seems small for something that will be rarely used.

Also, the - will only be meaningful for slices and arrays, not for maps or channels.

Iterating backward can be done with the usual range, with just one extra line.

    for i := range s {
        v := s[len(s) - 1 - i] // reverse iteration
        ...
    }

This proposal does not seem compelling.