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

Mon Jun 13 11:04:30 CDT 2016

Moreover, I should add, if your goal is to eliminate the possibility of
continuing and breaking from inside the loop, `.forEach()` does that
exactly, so your argument would be for the elimination of `for..in`
altogether.

On Mon, Jun 13, 2016 at 10:55 Xiaodi Wu <xiaodi.wu at gmail.com> wrote:

> On Mon, Jun 13, 2016 at 10:44 AM, let var go <letvargo at gmail.com> wrote:
>
>> I think we must be reading different discussions.
>>
>> What I have seen in this discussion is the following:
>>
>> a) The need to filter a for-in loop doesn't arise that often; but,
>> b) When it does arise, everyone who has chimed in on this thread (except
>> the two people who are proposing the change) thinks that the "where" clause
>> is the clearest, most expressive way to do it.
>>
>> Something that would help me get on board with this change is more
>> evidence about what kind of problems it is actually creating.
>>
>> As best I can tell, this proposal got started because "somewhere" some
>> new programmers (no one knows how many) expressed some confusion (no one
>> knows how seriously they were confused, or how long it took them to figure
>> it out) about how the where clause worked in a for-in loop. For all we
>> know, once they learned the way it works, they may have said, "Hey that's
>> cool! I'm gonna use that from now on!"
>>
>> In other words, you seem to be talking about removing a feature that is
>> liked by *a lot* people, based on some unsubstantiated reports of user
>> error that may or may not have been totally unsubstantial.
>>
>> I don't want new programmers to be confused, either, but the "where"
>> clause is such a basic programming construct - the keyword is new, but the
>> idea itself is as old as programming - that I don't mind expecting new
>> programmers to learn how to use it. The learning curve should be incredibly
>> short - it is nothing more than a filter operation.
>>
>> There's something else here that is really important to me, though I
>> don't know how others feel about it.
>>
>> Using the guard...continue approach that you are promoting is a code
>> smell. It puts control-flow logic inside the for-in loop. That is something
>> I have always tried to avoid. I know that the language allows for it, but I
>> believe it is bad programming practice. In fact, if you get rid of the
>> `where` keyword, I'm still not going to use guard...continue. I'll just
>> filter the collection first and then loop it.
>>
>
> This is quite the statement. It sounds like you'd be for the elimination
> of `continue`?
>
>
>>
>> It is a code smell for the same reason that messing with the index inside
>> a for;; loop was a code smell. I was always taught never to do this:
>>
>> for var i = 0; i < array.count, i++ {
>>   if iWantThisToLoopAnExtraTime {
>>     i--
>>   }
>> }
>>
>> Why? Because code like that is confusing. It becomes difficult to know
>> how many times the loop will execute, what the looping logic is, etc. Sure,
>> I might get away with it most of the time, but it is bad practice and there
>> is always a better way to do what you want to do. The only thing that keeps
>> you from the better way is laziness.
>>
>> The same is true (albeit to a lesser degree) for the guard...continue. It
>> may not be as extreme, but it is still a code smell. It divides the
>> control-flow logic into two parts - one outside the loop, and one inside
>> the loop, and it suddenly becomes twice as easy to miss something.
>>
>> Using for-in-where, all of the control-flow logic is on one single line,
>> and once it is known that "where" operates as a filter operation, it all
>> works together in a single, harmonious statement that declares exactly what
>> is going to happen in a way that is totally unambiguous.
>>
>> So by getting rid of the "where" clause, I believe that you are actually
>> encouraging bad programming practice. Instead of encouraging the new user
>> to learn this very simple construct that will ultimately make their code
>> safer and more expressive without dividing their control-flow logic
>> unnecessarily into two separate parts, you are encouraging them to just "do
>> what they know". I think that is terrible, and you are doing them a
>> disservice.
>>
>> And from a personal standpoint, you are telling me that I have to write
>> smelly code, even though there is this perfectly good non-smelly option
>> sitting right there, because you don't want someone else to have to learn
>> something.
>>
>> On Mon, Jun 13, 2016 at 5:29 AM Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>
>>> I think this discussion has made it pretty plain that what is claimed to
>>> be 'so useful' is barely ever used. Moreover, it provides no independent
>>> uses. The point of these pitches is to sound out arguments, not, as far as
>>> I was aware, to take a vote.
>>>
>>> On Mon, Jun 13, 2016 at 1:54 AM Jose Cheyo Jimenez <cheyo at masters3d.com>
>>> wrote:
>>>
>>>> --1
>>>>
>>>> I think it would be a waste of the community's time to do a formal
>>>> review when only two people are in favor of this removal.
>>>>
>>>> 'for in where' is so useful especially since we don't have for;;; loops
>>>> anymore. I'd say leave this alone; the majority doesn't want this changed.
>>>>
>>>>
>>>>
>>>> On Jun 10, 2016, at 10:17 AM, Xiaodi Wu via swift-evolution <
>>>> swift-evolution at swift.org> wrote:
>>>>
>>>> I think this idea--if you don't like it, then you don't have to use
>>>> it--is indicative of a key worry here: it's inessential to the language and
>>>> promotes dialects wherein certain people use it and others wherein they
>>>> don't. This is an anti-goal.
>>>>
>>>> On Fri, Jun 10, 2016 at 12:10 let var go <letvargo at gmail.com> wrote:
>>>>
>>>> Leave it in!
>>>>>
>>>>> It's a great little tool. I don't use it very often, but when I do it
>>>>> is because I've decided that in the context of that piece of code it does
>>>>> exactly what I want it to do with the maximum amount of clarity.
>>>>>
>>>>> If you don't like it, then don't use it, but I can't see how it
>>>>> detracts from the language at all.
>>>>>
>>>>> The *only* argument that I have heard for removing it is that some
>>>>> people don't immediately intuit how to use it. I didn't have any trouble
>>>>> with it at all. It follows one of the most basic programming patterns ever:
>>>>> "For all x in X, if predicate P is true, do something." The use of the
>>>>> keyword "where" makes perfect sense in that context, and when I read it out
>>>>> loud, it sounds natural: "For all x in X where P, do something." That is an
>>>>> elegant, succinct, and clear way of stating exactly what I want my program
>>>>> to do.
>>>>>
>>>>> I don't doubt that it has caused some confusion for some people, but
>>>>> I'm not sold that that is a good enough reason to get rid of it. It seems
>>>>> strange to get rid of a tool because not everyone understands how to use it
>>>>> immediately, without ever having to ask a single question. As long as its
>>>>> not a dangerous tool (and it isn't), then keep it in the workshop for those
>>>>> times when it comes in handy. And even if there is some initial confusion,
>>>>> it doesn't sound like it lasted that long. It's more like, "Does this work
>>>>> like X, or does this work like Y? Let's see...oh, it works like X. Ok."
>>>>> That's the entire learning curve...about 5 seconds of curiosity followed by
>>>>> the blissful feeling of resolution.
>>>>>
>>>>> On Fri, Jun 10, 2016 at 9:32 AM Xiaodi Wu via swift-evolution <
>>>>> swift-evolution at swift.org> wrote:
>>>>>
>>>> On Fri, Jun 10, 2016 at 11:23 AM, Sean Heber via swift-evolution <
>>>>>> swift-evolution at swift.org> wrote:
>>>>>>
>>>>> > And to follow-up to myself once again, I went to my "Cool 3rd Party
>>>>>>> Swift Repos" folder and did the same search. Among the 15 repos in that
>>>>>>> folder, a joint search returned about 650 hits on for-in (again with some
>>>>>>> false positives) and not a single for-in-while use.
>>>>>>>
>>>>>>> Weird. My own Swift projects (not on Github :P) use “where” all the
>>>>>>> time with for loops. I really like it and think it reads *and* writes far
>>>>>>> better as well as makes for nicer one-liners. In one project, by rough
>>>>>>> count, I have about 20 that use “where” vs. 40 in that same project not
>>>>>>> using “where”.
>>>>>>>
>>>>>>> In another smaller test project, there are only 10 for loops, but
>>>>>>> even so one still managed to use where.
>>>>>>>
>>>>>>> Not a lot of data without looking at even more projects, I admit,
>>>>>>> but this seems to suggest that the usage of “where” is going to be very
>>>>>>> developer-dependent. Perhaps there’s some factor of prior background at
>>>>>>> work here? (I’ve done a lot of SQL in another life, for example.)
>>>>>>>
>>>>>> That is worrying if true, because it suggests that it's enabling
>>>>>> 'dialects' of Swift, an explicit anti-goal of the language.
>>>>>>
>>>>>
>>>>>>> I feel like “where” is a more declarative construct and that we
>>>>>>> should be encouraging that way of thinking in general. When using it, it
>>>>>>> feels like “magic” for some reason - even though there’s nothing special
>>>>>>> about it. It feels like I’ve made the language work *for me* a little bit
>>>>>>> rather than me having to contort my solution to the will of the language.
>>>>>>> This may be highly subjective.
>>>>>>>
>>>>>>> l8r
>>>>>>> Sean
>>>>>>>
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