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

[Xiaodi Wu]

On Sat, Jun 11, 2016 at 3:31 PM, Thorsten Seitz <tseitz42 at icloud.com> wrote:

>
>
> Am 11.06.2016 um 21:57 schrieb Xiaodi Wu <xiaodi.wu at gmail.com>:
>
> On Sat, Jun 11, 2016 at 2:50 PM, Thorsten Seitz <tseitz42 at icloud.com>
> wrote:
>
>>
>>
>> Am 10.06.2016 um 17:22 schrieb Erica Sadun via swift-evolution <
>> swift-evolution at swift.org>:
>>
>>
>> On Jun 10, 2016, at 8:02 AM, Xiaodi Wu via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>> On Fri, Jun 10, 2016 at 7:18 AM, Haravikk <swift-evolution at haravikk.me>
>> wrote:
>>
>>> * 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.
>>>
>>
>> Sure: I conclude that the keyword should be *either* removed *or*
>> reformed; both outcomes could address the issue.
>>
>>
>> This is my stance as well and I reserve the right to flit between both
>> choices until we've fully talked it through.
>>
>> One more data point.
>>
>> In the standard library there are just under 950 uses of "for in loops".
>> There are 3 uses of "for in while" :
>>
>>
>> How many of the 950 "for in loops" use a guard with continue? Only these
>> can be compard with the "for in where loops".
>>
>
> That is a different argument, I think, from the one Erica is making. The
> data you're asking about would answer the question, "How often do stdlib
> authors prefer `guard` over `where`?" That is an interesting question,
> certainly.
>
>
> It is the relevant question to ask when deciding whether `where` should be
> better replaced by `guard`. And it should be qualified by asking whether
> the respective author did know about `where`. I wouldn't want a language to
> be designed by looking at usage examples of inexperienced users...
>
>
Here, I disagree with you vehemently. We should absolutely take into
consideration how inexperienced users work with the language. A language
that ignores the struggles and pain points of those who are less
experienced is one that will quickly find itself lacking in users.

>
> I think Erica's point is that what Vladimir earlier called the "simple,
> common" case may be simple but isn't actually common. I did a quick GitHub
> search earlier today (and the tools aren't there, afaik, for an accurate
> regexp search), but what I noticed on a random, not-statistically-sound
> sampling was that uses of `continue` and `break`, when they do occur inside
> a for loop (which isn't always), actually tend to happen after some work
> has been done at the top of the loop. These wouldn't be replaceable by a
> `where` clause. Moreover, I noticed an appreciable share of `return` and
> `fatalError()` calls from inside the loop, which surprised me; I had
> assumed I'd find mostly `break` or `continue`, but even when I did find
> those it was as likely as not to be accompanied by logging. These again
> preclude refactoring into `where`.
>
>
> Logging hints at logic used for filtering out data errors. That's a good
> usage for `guard` as opposed to `where`. The latter is about logic, not
> about error handling.
>
> -Thorsten
>
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