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

Haravikk swift-evolution at haravikk.me
Sat Jun 10 05:31:15 CDT 2017

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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