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

Gor Gyolchanyan gor at gyolchanyan.com
Sat Jun 10 08:06:37 CDT 2017 

Yeah, that's why I mentioned a big **if** at the end. I love the `do { }` construct or variable isolation purposes and logical grouping, but unfortunately, Swift 4 has made it a lot uglier, by making it an error in its current form:

do {
	let a = "123"
	print(a)
} // error: missing `while`, also use `repeat` instead

The workaround is to do this:

do {
	let a = "123"
	print(a)
};

It might seem like a little change, but this really really bugs me for some reason. I always felt like semicolons in Swift should never be mandatory and should only be used for writing multiple statements on the same line.

Overall, I agree that this isn't a big enough reason to change the syntax for. Let's just make the `do { }` great again instead.

> On Jun 10, 2017, at 3:33 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> 
> _Every_ addition to the basic syntax of the language is, by definition, high cost. The bar for additions to the standard library is already very high; the bar for additions to control flow syntax would be extraordinarily high.
> 
> The proposed use case here is far from the original topic of repeat {} while, which is unique because the condition lexically follows the loop.
> 
> For those loops in Swift where it is possible to declare variables in the condition, these live in a magical middle scope that is intuitive to use but also an exception to the rule of thumb that scopes are surrounded by braces. As I wrote earlier, it is possible to manually create an analogous scope by surrounding any loop with do {}. Any addition to the language would have to be vastly superior to this currently possible alternative, and I seriously doubt it is possible to invent such a thing because anything shorter than the four letters in “do {}” would also obscure the existence of the middle scope being created.
> 
> 
> On Sat, Jun 10, 2017 at 08:05 Goffredo Marocchi via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> 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 <mailto: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 <mailto: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 <mailto: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 <mailto: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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