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

[Thorsten Seitz]

> Am 12.06.2016 um 12:02 schrieb L Mihalkovic <laurent.mihalkovic at gmail.com>:
> 
> 
>> On Jun 11, 2016, at 11:45 PM, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> wrote:
>> 
>> 
>> 
>> On Sat, Jun 11, 2016 at 3:37 PM, Thorsten Seitz <tseitz42 at icloud.com <mailto:tseitz42 at icloud.com>> wrote:
>> 
>> 
>> Am 11.06.2016 um 22:29 schrieb L. Mihalkovic <laurent.mihalkovic at gmail.com <mailto:laurent.mihalkovic at gmail.com>>:
>> 
>>> 
>>> 
>>> On Jun 11, 2016, at 9:53 PM, Thorsten Seitz via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>>> 
>>>> 
>>>> Am 10.06.2016 um 18:28 schrieb Xiaodi Wu via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>>:
>>>> 
>>>>> On Fri, Jun 10, 2016 at 6:10 AM, Karl <razielim at gmail.com <mailto: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 exactly right, but I’d expand that last bit a bit to:
>>>>> “… picking the one that is most familiar to programmers in the extended C family is a good idea.["]
>>>>> The extended C family of language (which includes C, C++, ObjC, but also C#, Java, Javascript, and more) is
>>>>> an extremely popular and widely used set of languages that have a lot of surface-level similarity. I
>>>>> don’t claim to know the design rationale of all of these languages, but I surmise that this is not an
>>>>> accident: 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 declarator/declaration specifier part of the grammar) there is a lot of goodness in the basic
>>>>> operator set, focus on dot syntax, and more.
>>>>> I do agree that there are some benefits to ditching braces and relying on indentation instead, but there are
>>>>> also downsides. Deviating from the C family in this respect would have to provide *overwhelmingly* large 
>>>>> advantages for us to take such 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.
>> 
>> 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.
>>  
> 
> IMHO the team has taken an Ockham Axe to the grammar: in the presence of multiple ways to produce the same or an acceptable stand-in (for eg when the only difference is an acceptable temporary perf setback), then the solution requiring the least assumptions on the compiler wins. 
> 
> I would even extend this rule with the corollary that between an assumption materialized as a type checker rule and an assumption materialized as a full blown extra language keyword, there might be a bias to accept the former if it kills the latter. But this is just my personal inference of what their decision heuristic might be based solely on what I saw. My sole interest in trying to understand their decision making process is to try to avoid proposals that have little to no chance to go anywhere, as well as trying to present ones that will align better with where the language is going.
> 
> In this instance, WHERE is a heavy assumption on the compiler for no greater gain than filters can provide. So I think we save the WHERE keyword for an outcome that will be really worth it! Something along the idea of Linq, but with a proper Swift feel to it. What does it look like? I cannot say yet. But the good news is that having taken WHERE out now will make that next step a purely additive process (nothing will be taken out then, but a big thing will be gained). 

I am all for extending `for` to be more like Scala’s `for`-expression or Haskell’s `do`-notation. LINQ might be too focused on querying.
Extending the current `where` to be syntax sugar for `filter` would be quite straightforward, though, so I’m not sure whether removing it first is really necessary. 
That was not object of the pitch to retire `where` from `for`-loops, though. Instead the argument was (and still is) that `where` might be misunderstood there by beginners and should therefore be removed. This argument would apply to an extended `where` unchanged, because I can still write the exact same code as being criticized now and can even do more complicated things which would by even more hard to understand for beginners.
That’s the argument I disagree with.


> 
> 
>> 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;
> 
> 
> Here is food for your thoughts, you think WHERE is a keyword?! then look at this:
> 
> var numbers = new int[]{0,1,23,4,5,6,7,87,9};
> var numQuery = from num in numbers where (num % 2) == 0 select num;
> 
> Program does not compile:
> 
> // Error CS1935: An implementation of `Where' query expression pattern could not be found. Are you missing `System.Linq' using directive or `System.Core.dll' assembly reference? (CS1935) (SessionsFinder)
> 
> using System.Linq;
> var numbers = new int[]{0,1,23,4,5,6,7,87,9};
> var numQuery = from num in numbers where (num % 2) == 0 select num;
> 
> That program does compiles and runs fine. This tells you that “where" is not at all the ordinary keyword that it appears to be. hence my “it is not C# per-se, it is Linq”.

Digging into https://msdn.microsoft.com/en-us/library/bb397909.aspx and https://msdn.microsoft.com/en-us/library/the35c6y.aspx tells me the following:
(1) `from`, `where` etc. are contextual keywords, but they are nonetheless keywords of C# and expressly *not* a specific feature of LINQ: "Although these new features are all used to a degree with LINQ queries, they are not limited to LINQ and can be used in any context where you find them useful."
(2) the compiler translates them into standard method calls (like Scala’s `for`-expression or Haskell’s `do`-notation). That is an integral part of C#.
(3) your error message originates from not having an implementation of a `Where`-method in scope. You could have provided one yourself instead of `using System.Linq`.


> And if you still wonder, then rewrite the code so that “WHERE” no longer looks like a keyword:
> 
> var numQuery2 = numbers.Where( num => num %2 == 0);	// exact same result as numQuery1

That’s exactly what I expected as it is the standard way syntactic language sugar is mapped to customizable behavior. Haskell’s `do`-notation or Scala’s `for`-expression are mapped likewise to functions on the types used.

-Thorsten


> 
> 
> These are other real life examples from a OS X tool I wrote recently: 
> 
> IEnumerable<Task<Session>> asyncOps = from session in sessions.Values select parseSessionDetails(updater, parser, session);
> var ul = node.Descendants().Where(x => x.Name == "ul").First();
> 
> 

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