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

[Xiaodi Wu]

On Sat, Jun 11, 2016 at 5:07 PM, Thorsten Seitz <tseitz42 at icloud.com> wrote:

>
>
> Am 11.06.2016 um 23:52 schrieb Xiaodi Wu <xiaodi.wu at gmail.com>:
>
> 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.
>
>
> You don't want to tell me in earnest that discovering and understanding
> `where` in for loops are stumbling blocks for beginners in a language
> containing generics, associated types, existentials, value vs. reference
> types etc.?
>

I do. I challenge you to find an introductory text that teaches generics,
associated types, existentials, or even value vs. reference types before it
introduces `for` loops or `if` (and `guard`).


>
> -Thorsten
>
>
>> 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
>>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20160611/a5e52161/attachment.html>