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

let var go letvargo at gmail.com
Mon Jun 13 16:36:09 CDT 2016 

On Mon, Jun 13, 2016 at 12:21 PM Xiaodi Wu <xiaodi.wu at gmail.com> wrote:

> On Mon, Jun 13, 2016 at 12:20 PM, let var go <letvargo at gmail.com> wrote:
>
>>
>>
>> On Mon, Jun 13, 2016 at 9:46 AM Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>
>>> On Mon, Jun 13, 2016 at 11:41 AM, let var go <letvargo at gmail.com> wrote:
>>>
>>>>
>>>>
>>>> On Mon, Jun 13, 2016 at 9:04 AM Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>>>
>>>>> 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.
>>>>>
>>>>
>>>> I have no "goal" of eliminating the possibility of continuing or
>>>> breaking from inside the loop. In general, it is not my goal to try and
>>>> control how other people code their programs at all. I don't want to tell
>>>> you that you can't 'continue' if you want to 'continue.'
>>>>
>>>> *I am not trying to make everyone adopt my own personal coding style
>>>> and/or philosophy.*
>>>>
>>>
>>> We're not discussing style.
>>>
>>
>> Yes, we are discussing style, and for a very simple reason: Style is
>> essential to readability and clarity.
>>
>> When we talk about for...in...where vs. guard...continue, all we are
>> talking about is style. In terms of functionality, they do exactly the same
>> thing. In terms of style, however, *one is easier to read than the
>> other. *
>>
>> Some people think that guard...continue is easier to read and understand.
>> Others, like me, think that for...in...where is easier to read and
>> understand. *But that is a disagreement over style, not functionality*.
>>
>> And here is the difficult thing about debates like this: Both sides are
>> right. What I consider to be the clearest, most readable style of coding
>> may be confusing to someone else, and vice versa. Different people find
>> different styles confusing. Not everyone learns the same way. Not everyone
>> reads the same way. Not everyone codes the same way. What is hard for me to
>> understand may be easy for you to understand.
>>
>> So if using 'continue' inside your for...in loops makes sense to you, and
>> it helps you understand your own code, then go for it. It's not my style,
>> not because it is "unfashionable", but because it has the opposite effect
>> on me - it makes the code harder for me to understand.
>>
>
> You misunderstand me. I am not supporting this proposal because I think
> `guard` is superior style. I am supporting it because I have come to the
> conclusion that `where` is harmful. I do happen to think that `guard` is a
> superior style, but that's not the motivation for removing `where`, nor
> could it be a justification.
>

No, I didn't misunderstand you. You are talking about style, even if you
don't realize it. Examine these two pieces of code:

Example 1:
for x in xs where x < 10 {
    print(x)
}

Example 2:
for x in xs {
    guard x < 10 else { continue }
    print(x)
}

Example 1 and Example 2 are functionally equivalent. They do exactly the
same thing. They will produce exactly the same output. *They only differ
with regard to style.*

You think that Example 2 is a better style than Example 1 because you think
that Example 1 is confusing to new programmers. You argue that the
confusion is harmful.

I think that Example 1 is a better style than Example 2 because I think
that it is easier to read and expresses my intent with greater clarity.
Whatever cognitive burden is placed on the new programmer to learn about
the 'where' clause is extremely modest and more than made up for by the
ability to express their intentions in a way that is both concise and
readable. Example 2, by contrast, looks cluttered and splits the
control-flow logic into two parts creating a cognitive burden that I
consider harmful.

These are debates about style, and that is okay, because style is about
clarity, readability, and expressiveness, all of which are essential
goals. *Style
is substance*. Half the battle is getting the computer to follow the
correct instructions. The other half is making the code readable so that
human beings can understand it - and that is a matter of style.

There are times where I will choose to use a 'continue' statement inside a
for...in loop. The reason hardly ever has anything to do with functionality
- I could design the code differently to avoid that particular code smell
if I put my mind to it. No, I would choose the 'continue' statement because
in that particular context it makes my code more accessible to human beings
(including myself, later on). The ability to choose one solution over
another for purely stylistic reasons means that I can maximize both the
functionality and the readability of my code.

This proposal prevents the programmer from writing the best possible code
because it disallows a stylistic choice that improves readability. I don't
like that. It means that I have to write code that is worse than it has to
be. It means that I have to write code that is uglier and more disjointed
than it needs to be. That doesn't make any sense at all.


>
>
>> You made a very strong claim: use of `guard...continue` leads to 'smelly'
>>> code, or in other words bad code. In other words, you claim that
>>> `guard...continue` is harmful. That is grounds for removal, and if you
>>> believe it to be true, I encourage you to propose it to the list and get
>>> feedback on that opinion.
>>>
>>> If you're saying that you simply don't prefer it for reasons of 'style,'
>>> again I emphasize that the topic at hand here is not about personal coding
>>> style. The claim we are making is that `where` is harmful. I too like it
>>> for style, and I'll be sad to see it go. But I have concluded that it must
>>> go.
>>>
>>>
>>>> There is also a big difference between 'break' and 'continue'. I am not
>>>> afraid to use 'break', and I don't consider it a code smell. It allows for
>>>> early exit from a loop based on conditions that are not always known at the
>>>> time the loop is entered. for...in loops are necessary precisely because
>>>> they allow for the early exit.
>>>>
>>>> You don't need for...in if you want to continue - you can use forEach
>>>> for that. Instead of using 'continue', you just use 'return' without doing
>>>> anything and it moves on to the next iteration. So really, the only purpose
>>>> that for...in serves that is not served by forEach is early exit. But
>>>> regardless, *even if for...in served no additional purpose that
>>>> couldn't be served by forEach, I would keep it in the language because
>>>> there are situations where I believe it is easier to read, and
>>>> expressiveness and clarity are important to me.*
>>>>
>>>>
>>>>>
>>>>> 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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>>>>>>>>>>>> swift-evolution at swift.org
>>>>>>>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>>>>>>>>>
>>>>>>>>>>> _______________________________________________
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>>>>>>>>>> _______________________________________________
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>>>>>>>>>
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