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

[Xiaodi Wu]

On Tue, Jun 14, 2016 at 12:37 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:

> On Tue, Jun 14, 2016 at 12:16 PM, David Waite <
> david at alkaline-solutions.com> wrote:
>
>> I’m a bit late to this conversation, and I don’t totally understand the
>> goal.
>>
>> There are a *lot* of things you can do in for…in loop with pattern
>> matching that also would supposedly go against this interpretation of
>> approachability. Pattern matching in general might be considered to go
>> against this interpretation.
>>
>> Is this pitch saying statements such as:
>>
>> for i in 1..<100 where i%2 == 1 {…}
>>
>> should be disallowed, while statements like
>>
>> for case let view? in views { … }
>>
>> are still approachable enough to warrant being supported in the language?
>>
>
> Language design has to weigh many factors simultaneously, I think you'd
> agree. The argument, essentially, is that `where` is not approachable *for
> the functionality that it provides* (namely, as an alternative for a
> trivial `guard...continue` statement). Pattern matching is daunting no
> doubt, but it offers functionality not conducive to much simpler syntax.
> (Or could it be much simpler? If so, then I would support a proposal to
> that effect.)
>
> Put simply, `where` is a less-than-straightforward expression of a very
> straightforward concept (filtering an array), whereas pattern matching is
> an advanced concept with a commensurately difficult syntax. Others have
> brought up generics, for example, but again that's an advanced *concept*;
> filtering an array is not.
>
>
>>
>> FWIW, I wouldn’t support removing where based on current arguments
>> without either the keyword “where" being eliminated completely from the
>> language
>>
>
I should add, if this proposal is adopted along with the enclosed
suggestion to replace `where` with `if` in `case` and `catch`, the `where`
keyword would be completely eliminated except in the context of generics.
If the enclosed suggestion is not adopted here, alternatives will follow on
shortly.


> and/or adding equivalent intuitive functionality to Sequence with
>> same-class performance, e.g. a .where(...) equivalent to .lazy.filter(…).
>>
>>
> I feel bad sending clearly passionate people over to crush another
> conversation, but I think you'll find in the Swift repository the
> beginnings of some explorations by a certain member of the core team to
> rename `.filter()` to `.where()` :D
>
> As to whether certain methods should be lazy or eager by default, that's a
> discussion certainly appropriate for this list.
>
>
>> I’ve known about and used the feature since it was first added to Swift
>> (learned via the language book), and don’t fully understand the confusion
>> that some developers may have - especially since ‘while’ is already a
>> keyword and could have been used if that was the actual semantics.
>>
>
> One source of confusion was that `while...where` was supported and had
> breaking semantics. Now that's gone with SE-0099. Still, the point is that
> `where` is favored by some *because* you don't have to write explicitly
> what happens when something doesn't pass the filter, whereas the
> counterpoint argument is that not writing explicitly what happens when a
> rejected element is encountered *is* the very source of confusion.
>
>
>>
>> -DW
>>
>> On Jun 14, 2016, at 10:32 AM, Xiaodi Wu via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>> And from the WWDC Platforms SOTU: "Swift is super simple and
>> approachable.... It's great as a first language. And in fact, we think this
>> is so important that when we designed Swift this was an explicit design
>> goal."
>>
>> I would be absolutely against adding any more sugar to the for loop. In
>> that sense, `where` sets a terrible example that certain features of
>> sequences deserve contextual sugar. (And before someone points it out
>> again, I've already argued why `for...in` holds its own weight, namely
>> difficulty of writing a correct `while` replacement and progressive
>> disclosure to the learner so that the concept of iterators can be learned
>> afterwards.)
>>
>> In short, I would very much be opposed to adding keywords "for fun."
>>
>>
>>
>
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