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

[Xiaodi Wu]

On Sat, Jun 11, 2016 at 5:03 PM, Thorsten Seitz <tseitz42 at icloud.com> wrote:

>
>
> Am 11.06.2016 um 23:45 schrieb Xiaodi Wu <xiaodi.wu at gmail.com>:
>
>
>
> On Sat, Jun 11, 2016 at 3:37 PM, Thorsten Seitz <tseitz42 at icloud.com>
> wrote:
>
>>
>>
>> Am 11.06.2016 um 22:29 schrieb L. Mihalkovic <
>> laurent.mihalkovic at gmail.com>:
>>
>>
>>
>> On Jun 11, 2016, at 9:53 PM, Thorsten Seitz via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>>
>>
>> Am 10.06.2016 um 18:28 schrieb Xiaodi Wu via swift-evolution <
>> swift-evolution at swift.org>:
>>
>> On Fri, Jun 10, 2016 at 6:10 AM, Karl <razielim at gmail.com> wrote:
>>
>>> -1
>>>
>>> * Swift is explicitly a C-family language. In most or all other C-family
>>> languages, for loop statements allow specification of conditions for
>>> exiting the loop but not for filtering. Therefore, Swift's use of `where`
>>> is unprecedented and needs to be learned anew by every user of Swift.
>>>
>>>
>>> When was this decided? I distinctly remember some bloke under Craig
>>> Federighi’s hair saying that it was time to “move beyond” C and essentially
>>> ditch legacy conventions which no longer make sense.
>>>
>>
>> I think you misunderstood my argument here. I don't mean that we should
>> yoke ourselves to C conventions, and we should absolutely ditch C
>> convention when it doesn't make sense. The big-picture argument here is
>> that `where` doesn't pass the bar of correcting a C convention that no
>> longer makes sense.
>>
>> FWIW, on the topic of syntax choices, here is what Chris Lattner had to
>> say on this list:
>>
>> Kevin got it exa*c*tly right, but I’d expand that last bit a bit to:
>>> “… pi*c*king the one that is most familiar to programmers in the
>>> extended *C* *family* is a good idea.["]
>>> The extended *C* *family* of language (whi*c*h in*c*ludes *C*, *C*++,
>>> Obj*C*, but also *C*#, Java, Javas*c*ript, and more) is
>>> an extremely popular and widely used set of languages that have a lot of
>>> surfa*c*e-level similarity. I
>>> don’t *c*laim to know the design rationale of all of these languages,
>>> but I surmise that this is not an
>>> a*c**c*ident: programmers move around and work in different languages,
>>> and this allows a non-expert in the
>>> language to understand what is going on. While there are things about
>>> *C* that are really unfortunate IMO
>>> (e.g. the de*c*larator/de*c*laration spe*c*ifier part of the grammar)
>>> there is a lot of goodness in the basi
>>> *c*operator set, fo*c*us on dot syntax, and more.
>>> I do agree that there are some benefits to dit*c*hing bra*c*es and
>>> relying on indentation instead, but there are
>>> also downsides. Deviating from the *C* *family* in this respe*c*t would
>>> have to provide **overwhelmingly** large
>>> advantages for us to take su*c*h a plunge, and they simply don’t exist.
>>
>>
>>
>>> As I understand it, Swift is a new language with new conventions. It is
>>> desirable to align as many of those as possible with existing conventions
>>> so as to be easily learned, but if you limit Swift to other languages
>>> conventions you deny it any identity. Did Python ask anybody’s opinion
>>> before dropping curly-braces? Did people learn whatever Perl is supposed to
>>> be? Look at C’s hieroglyphic for loops!
>>>
>>
>> I don't think we disagree here.
>>
>>
>>>
>>> Realistically, “for … in … while” is not going to cause incredible
>>> confusion. Removing it would cause a lot of frustration. You can’t on the
>>> one hand say our users are comfortable with the axioms of C’s hieroglyphic
>>> loops, and on the other hand say “for x in y while" is confusing.
>>>
>>> Again, as I said, once you've mastered something, by definition you find
>>> it not confusing. Why should we doom x% of new users to writing a loop
>>> incorrectly at least once when we don't have to?
>>>
>>>
>>> Ah, but if you’re not “doomed” to failing once, how will you ever master
>>> anything? Nobody knew how to write a C for-loop until someone showed them
>>> (and even then…). Nobody is going to just open a REPL and start writing
>>> code, with zero prior understanding of what Swift syntax looks like.
>>>
>>
>> The thought here is along the lines of what Chris said, quoted above, and
>> repeated here: "The extended C family of language [...] is an extremely
>> popular and widely used set[;] programmers move around and work in
>> different languages, and [aligning to expectations arising from other C
>> family languages] allows a non-expert in the language to understand what is
>> going on." By contrast, the `where` clause violates that expectation and I
>> do not see "overwhelmingly large advantages" for doing so.
>>
>>
>> What about C#'s `where` then? As C# is a member of the C family languages
>> `where` is not violating expectations!
>>
>>
>> Where is not exactly a part of c# it belongs to linq
>>
>>
>> And that is not a part of C#??
>>
>
> SQL is a domain-specific language, and LINQ is an internal domain-specific
> language with a language extension for C#. Neither is a general purpose
> language.
>
>
> I don't see how that is relevant especially given the prominence and reach
> of SQL (which is turing complete, btw ;-)
>

It is relevant because the design of every language has to contend with
trade-offs, and the trade-offs that make `where` a powerful and intuitive
keyword in the context of a query language are different from the tradeoffs
we must contend with in a general purpose language. HTML+CSS has also been
proved Turing-complete; it is irrelevant to the question of whether the
idioms of HTML+CSS are appropriate in a general purpose language.


> Your example actually goes to one of Laurent's points. Should the Swift
> core team or an enterprising community member propose a set of similarly
> powerful tools, along with a set of language extensions that add syntactic
> sugar for them, I (and I think Laurent, if I understand him correctly)
> would absolutely be in favor of such an addition. But as it is, `where` is
> an odd duckling. Just as you say, it looks like a component of a query
> language, but it does no such thing. In a for loop, it does some filtering,
> but until recently it functioned like a comma in `while` loops. Look at
> those other keywords which make this sugar possible in C#: in your example,
> `from` and `select`. We don't have any of that intrastructure in Swift.
>
>
> We can simply extend the for-in-where loop for that like Scala does. No
> need to add new syntax if an existing one can simply be generalized.
>

If you want to propose adopting Scala's `if`, you are free to start a
discussion on that topic. Note how Scala also avoids use of the word
`where`.


>
> -Thorsten
>
>
>
>
>> The following is an example from MSDN with `where` clearly beaing a
>> keyword:
>>
>> *var* numQuery =
>>             *from* num *in* numbers
>>             *where* (num % 2) == 0
>>             *select* num;
>>
>>
>> -Thorsten
>>
>>
>
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