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

let var go letvargo at gmail.com
Mon Jun 13 11:50:27 CDT 2016 

No, the key difference between for...in and forEach is that for...in allows
for early exit. They both allow you to 'continue', though in forEach it is
called 'return':

// This prints odd numbers, skipping ("continuing") past the even numbers:
(0..<100).forEach { if $0 % 2 == 0 { return } else { print($0) } }

Early-exit ('break') is a different beast and requires a for...in loop.

You missed my main point. In some situations even 'continue' may prove to
be useful. It is not my preferred method.

Why do you say, "Swift is not a live-and-let-live language?" Where do you
get that from? Are you really saying that there is not room in the Swift
community for people with different coding styles and preferences? I truly
believe that that attitude will be very harmful to the long term future of
the language.

On Mon, Jun 13, 2016 at 9:29 AM Xiaodi Wu <xiaodi.wu at gmail.com> wrote:

> 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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>>>>>>>>>
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