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

Mon Jun 13 10:55:57 CDT 2016

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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