[swift-evolution] Rekindling: "Extending declaration scope to condition for `repeat { } while ()"

[Goffredo Marocchi]

If it is low cost and people do not come up with regressions/high cost + negative impact scenarios then I would say go full steam ahead. It does address an annoying scenario.

Sent from my iPhone

> On 10 Jun 2017, at 12:04, Gor Gyolchanyan <gor at gyolchanyan.com> wrote:
> 
> Not much, I think. The `where` clause already exists, conditional `let` and `var` binding already exists. It'd take loosening up conditional binding rules a bit and expanding the lexical structure to include `let` and `var` bindings in `repeat`.
> 
>> On Jun 10, 2017, at 2:01 PM, Goffredo Marocchi <panajev at gmail.com> wrote:
>> 
>> Quite interesting :), what impact would it have on the compiler?
>> 
>> Sent from my iPhone
>> 
>>> On 10 Jun 2017, at 11:46, Gor Gyolchanyan via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> I think a better way of achieving this would be to use the already existing `where` keyword in loops. The way it works right now is as follows:
>>> 
>>> let many = [1, 2, 3, 4, 5]
>>> for each in many where each % 2 == 0 {
>>> 	print("found an even number: \(each)")
>>> }
>>> 
>>> Unfortunately, unlike all other conditional scopes, `where` does not allow `let` and `var` bindings in it, so I'd suggest we add ability to do that:
>>> 
>>> let many: [Int?] = [1, 2, nil, 3, 4, nil, 5]
>>> for each in many where let number = each {
>>> 	print("found a non-nil number: \(number)")
>>> }
>>> 
>>> Or, more interestingly:
>>> 
>>> for each in many where let number = each, number % 2 == 0 {
>>> 	print("found a non-nil even number: \(number)")
>>> }
>>> 
>>> And in case of a while loop:
>>> 
>>> var optional: Int? = 1
>>> while let nonoptional = optional {
>>> 	if nonoptional >= 10 {
>>> 		optional = nil
>>> 	}
>>> 	optional = nonoptional + 1
>>> }
>>> 
>>> But this is only for optional unpacking, so another addition would be to allow any `let` and `var` bindings in conditional scopes without them contributing to the condition itself:
>>> 
>>> while let a = 0, a < 10 {
>>> 	a += 1
>>> 	print(a)
>>> }
>>> 
>>> And finally, allow these bindings in `repeat`:
>>> 
>>> repeat let a = 0 {
>>> 	a += 1
>>> 	print(0)
>>> } while a < 10
>>> 
>>> I think **if** the core team would consider this a worthwhile addition, this would be a less invasive and more intuitive way of achieving what you want.
>>> 
>>>> On Jun 10, 2017, at 1:31 PM, Haravikk via swift-evolution <swift-evolution at swift.org> wrote:
>>>> 
>>>> Not sure if my e-mail didn't go through or if discussion just fizzled out; one other benefit if we ever move to a proper message board is we might gain the ability to bump topics. Anyway, I'll resend my message just in case:
>>>> 
>>>> 
>>>> 
>>>> Just to add my thoughts, as I like the idea of adding the variables to the start somehow, but was wondering if might make sense to have a keyword such as "using", but allow it on all block statements, like-so:
>>>> 
>>>> 	// Original use-case of repeat … while
>>>> 	repeat using (var i = 0) {
>>>> 		// Do something
>>>> 	} while (i < 20)
>>>> 
>>>> 	// for … in demonstrating combination of using and where
>>>> 	for eachItem in theItems using (var i = 0) where (i < 20) {
>>>> 		// Do something either until theItems run out or i reaches 20
>>>> 	}
>>>> 
>>>> 	// Standard while loop
>>>> 	while let eachItem = it.next() using (var i = 0) where (i < 20) {
>>>> 		// As above, but with an iterator and a while loop and conditional binding to also stop on nil
>>>> 	}
>>>> 
>>>> 	// Closure with its own captured variable
>>>> 	let myClosure:(Int) -> Int = using (var i = 0) { i += 1; return i * $0 }
>>>> 
>>>> 	// If statements as well
>>>> 	if somethingIsTrue() using (var i = 0) where (i < 20) {
>>>> 		// Do something
>>>> 	}
>>>> 
>>>> 	// Or even a do block; while it does nothing functionally new, I quite like it aesthetically
>>>> 	do using (var i = 0) {
>>>> 		// Do something
>>>> 	}
>>>> 
>>>> Unifying principle here is that anything created in the using clause belongs to the loop, conditional branch etc. only, but exists outside the block itself (thus persisting in the case of loops and closures). I quite like the possible interaction with where clauses here as a means to avoid simple inner conditionals as well.
>>>> 
>>>> Basically the two clauses can work nicely together to avoid some common inner and outer boilerplate, as well as reducing pollution from throwaway variables.
>>>> 
>>>> Only one I'm a bit iffy on is the closure; I'm trying to avoid declaring the captured variable externally, but I'm not convinced that having using on its own is clear enough?
>>>> 
>>>> Anyway, just an idea!
>>>> 
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>>> 
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