[swift-evolution] [Pitch] Retiring `where` from for-in loops

Vladimir.S svabox at gmail.com
Fri Jun 10 07:49:07 CDT 2016

+1 to Haravikk's opinion, my thoughts exactly the same.

On 10.06.2016 15:18, Haravikk via swift-evolution wrote:
>> On 10 Jun 2016, at 07:25, Xiaodi Wu via swift-evolution
>> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> * Swift is explicitly a C-family language. In most or all other C-family
>> languages, for loop statements allow specification of conditions for
>> exiting the loop but not for filtering. Therefore, Swift's use of `where`
>> is unprecedented and needs to be learned anew by every user of Swift.
> Swift may have some similarities with C, but the last thing anyone should
> want is for it to be bound to C as a language. Besides, the purpose of a
> for in loop is to iterate over elements in a sequence, so filtering is very
> much a useful thing to do so it’s hardly unprecedented, and it’s also a
> fairly common thing to want to do.
>> * The word "where" does not consistently imply `break` or `continue`. In
>> current Swift, `where` implies `break` in the context of a `while` loop
>> and `continue` in the context of a `for` loop. Some users intuitively
>> guess the correct meaning in each context, while others guess the wrong
>> meaning. Therefore, the only way to learn for sure what `where` means in
>> any context is to read the rulebook. That, by definition, means that this
>> is unintuitive.
> This is an argument for renaming the where keyword on for loops to be more
> clear, or to somehow integrate continue/break to be more explicit about
> what the developer intends for it to do.
>> * There are other ways to break from a loop or continue to the next
>> iteration without performance penalty. Nearly all of these serve more
>> general purposes than a `where` clause.
> This isn’t really an argument against the where clause; the where clause is
> useful for common, simple cases, so it’s not surprising if more
> complex/unusual cases can’t (or can’t easily) be handled by it. This is for
> the simple cases where this isn’t an issue.
>> Some of these (such as `if` or `guard`) would already be familiar to a
>> new user before they encounter loops, assuming a typical order for
>> learning a programming language. Many of these (such as filtering methods
>> on collections, or simply `if`) would be familiar to a user of another
>> C-family language. Therefore, the `where` clause provides no independent
>> utility, is not more discoverable than its alternatives, and is not
>> required for progressive disclosure of an important facility to a learner
>> (i.e. a simplified syntax for those who may not be ready for the advanced
>> concepts needed to use a more fully-featured alternative).
> Simplification isn’t just for the new users; all you need to know with
> where is that it’s a shorthand for guard X else { continue }, for many
> people this is intuitive enough, but if there are enough for whom it isn’t
> then again that’s an argument to tweak it to be more clear about what it
> does, rather than remove it entirely.
> The independent utility that it offers is being able to avoid if/guard
> boilerplate at the start of your loop, but instead putting it on the same
> line; in simple cases this can be nice and neat.
>> it has been used incorrectly by at least some users.
> Every feature in every language "has been used incorrectly by at least some
> users", should we just drop all programming languages? It’s not as if users
> can’t make mistakes while using an inline if/guard condition. Again, this
> an argument that the meaning isn’t implicit enough, which is just as well
> served by tweaking the syntax than removing it.
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