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

[Xiaodi Wu]

I did not realize that change occurred with `do {}`! That seems like it
should be a regression, given that previously there was explicitly a fix-it
to rewrite naked `{}` to `do {}`.
On Sat, Jun 10, 2017 at 09:06 Gor Gyolchanyan <gor at gyolchanyan.com> wrote:

> 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> 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> 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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