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

Mon Jun 13 11:29:23 CDT 2016

See, the key difference between for...in and .forEach() is that one allows
for continue and break and the other doesn't. Swift is not a
live-and-let-live language: if you truly believe that using continue leads
to bad code, then propose its removal or the removal of for...in altogether.

On Mon, Jun 13, 2016 at 11:21 let var go <letvargo at gmail.com> wrote:

> No, I wouldn't eliminate 'continue'. Even though I consider it a
> sub-optimal solution, I would keep it in the language. Why? A couple of
> reasons:
>
> 1) I don't like it, but even 'continue' may be the best available solution
> in the context of a particular problem. I will look for other options
> first, but I don't rule out the possibility that there might come a time
> when it is the right tool for the job.
>
> 2) Some people like it. Not everyone feels the same way about it as me.
> Some of the people who like it are better programmers than me. I have a lot
> to learn, and someday I might discover that I love 'continue' after all.
> Until then, live-and-let-live is what I say. Everyone should control their
> own flow :) Keep your hands off my 'where' and I'll keep my hands off your
> 'continue' :)
>
> On Mon, Jun 13, 2016 at 8:56 AM 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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