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

[Vladimir.S]

IMO `for-in` is a special kind of loop to manipulate(iterate) a collection, 
and was introduced because we so *often* needs to iterate collections that 
we need a *sugar* for this (you can do all the same with `while` loop).

*Filtering* is another important operation we often need during the 
iteration of collection. This is why many of us want to keep `where` for 
`for-in` loop. Yes, as sugar, because it really simplifies our every day 
coding(processing of collections) and makes the code more readable(IMO) and 
understandable(IMO).

Personally I don't insist on `where` keyword, probably we can find another 
word, so it will not be mixed with `where` in other places of the language.

Why not `filter` instead of `where` ?

for item in collection filter item < 10 {..}

we have .filter for collections, it is clear what it means, `filter` in 
`for-in` mimics the same behavior, etc

I'd even suggest to make `for-in` loop more powerful with adding suggested 
'while' but as under another name. So, `for-in` will be a powerful 
construct to iterate, filter and break processing of collections - the only 
one purpose why we need `for-in` at all.

for item in collection until item > 100 {..}
or
for item in collection break item > 100 {..}
or
for item in collection breakif item > 100 {..}
or
for item in collection limit item > 100 {..}
or
for item in collection stop item > 100 {..}
or other keyword.


On 13.06.2016 16:36, Xiaodi Wu via swift-evolution wrote:
> On Mon, Jun 13, 2016 at 8:16 AM, Brandon Knope <bknope at me.com
> <mailto:bknope at me.com>> wrote:
>
>     Are you really surprised that some people don't want this taken away?
>
>
> Nope, that's to be expected.
>
>
>     The burden should be on those that want it taken out of the language
>     and not those that want it kept. After all something is being removed
>     and it should be a delicate process.
>
>
> Agreed. We who think it's better to take this syntax out have advanced an
> argument with several prongs. Namely, that the `where` clause serves no
> independent purpose; that a more general solution has already been added to
> the language (as well as another in the stdlib); that the `where` clause is
> not necessary for progressive disclosure to new users before they're ready
> for the general solution; that it is, at present, rarely used in practice;
> that it has no analog in other commonly used general purpose languages in
> the C family; that it is the remnant of a direction in which the core team
> later decided not to pursue; and that, given its lack of utility, lack of
> use, and vestigial state, being the cause of confusion even among a small
> number of users (if their number be small) is grounds to conclude that it
> is harmful to the language and therefore ought to be removed.
>
>
>     Don't be surprised when the defenders say it is more readable to them.
>     That is a *sound* argument in my opinion.
>
>
> IMO, it cannot stand on its own as a complete argument for saving a feature
> in the face of the arguments we've advanced. Couldn't you say the same for
> `++` or `for;;` loops? I'd say our case is at least as strong as that for
> `for;;` loops. By comparison, if I recall, the `for;;` loop was argued to
> be ill-fitting the rest of the language and lacking in usage, but it
> certainly had utility independent of `for...in` loops and was well
> precedented in C languages.
>
>
>
>     Brandon
>
>     Sent from my iPad
>
>     On Jun 13, 2016, at 8:33 AM, Xiaodi Wu via swift-evolution
>     <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>
>>     This is not a sound argument. If your filtering can be expressed as a
>>     where clause, then you would only have to read one line into the loop
>>     to see it in the form of a guard clause.
>>
>>     Moreover, if what you're arguing is that you shouldn't ever have to
>>     *read* inside the loop to know if a sequence is filtered, how do you
>>     propose that we do that? Remove the continue keyword?
>>
>>     On Mon, Jun 13, 2016 at 6:16 AM Jean-Daniel Dupas via swift-evolution
>>     <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>
>>         -1 for the removal.
>>
>>         When I read code, I find it far more visible that a loop is over
>>         a filter list when the filter clause is on the same line, than
>>         when the filter clause is inside the loop.
>>
>>         Having to read the full content of the loop to determine if the
>>         list is filtered or not is not an improvement IMHO.
>>
>>         Moreover, I find it far cleaner to use the where clause that
>>         having to remember than I have to use the lazy accessor to avoid
>>         a performance hit.
>>
>>>         Le 13 juin 2016 à 06:39, Charlie Monroe via swift-evolution
>>>         <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> a
>>>         écrit :
>>>
>>>>         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.
>>>>
>>>>         -- E
>>>
>>>         Not to undermine this fact, but I believe the fact that `where`
>>>         can be used in a for loop is not widely known. I didn't know
>>>         about it until about a month ago (haven't really read much docs,
>>>         but most people don't either).
>>>
>>>         But after I found out about it, I started using it and it IMHO
>>>         improved readability of my code. Not by much, but it's the
>>>         little things that make you smile, right?
>>>
>>>         Many people here argument that `where` is a Swift speciality and
>>>         needs to be learned by the developer - the alternative is to
>>>         teach the person what's the proper alternative - that using
>>>         .filter can have performance impact and that the *correct* way
>>>         is to use guard within the for loop. And that's IMHO much worse
>>>         than teaching a person about using `where` within a for loop.
>>>
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