[swift-evolution] Change `repeat` to loop indefinitely if no while clause is present

Xiaodi Wu xiaodi.wu at gmail.com
Tue May 10 19:29:36 CDT 2016 

I was speaking of what I meant in my written English prose. What `repeat 1`
means is a topic for another day.
On Tue, May 10, 2016 at 19:27 Tyler Cloutier <cloutiertyler at aol.com> wrote:

> What would repeat 1 { } mean then? Repeat N? Would it run N or N + 1
> times?
>
> That sounds a massive source of bugs.
>
>
> On May 10, 2016, at 5:25 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>
> FWIW, repeat once means do twice.
> On Tue, May 10, 2016 at 19:16 Tyler Cloutier <cloutiertyler at aol.com>
> wrote:
>
>> On May 10, 2016, at 4:07 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>
>>
>>
>> On Tue, May 10, 2016 at 6:02 PM, Tyler Cloutier <cloutiertyler at aol.com>
>> wrote:
>>
>>>
>>> On May 10, 2016, at 3:59 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>>
>>> On Tue, May 10, 2016 at 5:56 PM, Tyler Cloutier <cloutiertyler at aol.com>
>>> wrote:
>>>
>>>>
>>>> On May 10, 2016, at 3:13 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>>>
>>>> On Tue, May 10, 2016 at 3:30 PM, Tyler Cloutier via swift-evolution <
>>>> swift-evolution at swift.org> wrote:
>>>>
>>>>> I’d actually say that I’m strongly in favor of allowing just a repeat
>>>>> keyword, although I wouldn’t support making 'while true’.
>>>>>
>>>>> Firstly it reduces clutter
>>>>>
>>>>
>>>> Can you explain what clutter you see? Unless I misunderstand what
>>>> you're referring to, reducing the 10 letters in `while true` to the six
>>>> letters in `repeat` is hardly "reducing clutter."
>>>>
>>>>
>>>>> and makes it very clear that the the code is just supposed to repeat.
>>>>>
>>>>
>>>> I disagree here also. It is not very clear at all that the code is
>>>> supposed to repeat indefinitely, not to any audience.
>>>>
>>>> First, it would not be clear to users who are experienced in Swift and
>>>> aware of this proposal. Code is meant to be read, and allowing the omission
>>>> of a trailing clause to produce two very different behaviors means that it
>>>> is not clear what `repeat {` means until you encounter the closing brace
>>>> and check for what follows. Moreover, what follows could be the keyword
>>>> `while` on the following line, and in that case you cannot know whether the
>>>> expression that follows `while` is the beginning of a new while loop until
>>>> you encounter or don't encounter a new opening brace. By contrast, `while
>>>> true {` cannot be anything other than the beginning of an infinite loop.
>>>> You already know that fact after reading 12 letters.
>>>>
>>>> Second, it would not be clear to users migrating from another C-family
>>>> language. `while true { }` is immediately understood by users of any other
>>>> related language.
>>>>
>>>> Third, it would not be clear based on a knowledge of English. In common
>>>> use, "repeat" does not mean repeat forever; it means to repeat once (i.e.
>>>> do something twice). If I ask you to repeat something you just said, I
>>>> should hope that you do not keep reciting it over and over until I tell you
>>>> to stop.
>>>>
>>>>
>>>>> Secondly it’s a very simple way of introducing new programmers to
>>>>> loops. It’s IMHO more clear to a new programmer that repeat will just
>>>>> repeat indefinitely vs while true.
>>>>>
>>>>
>>>> I can speak to this a little bit, having introduced a new programmer to
>>>> loops very recently and having done so in the past as well. I have not
>>>> encountered anyone who has trouble with the *concept* of looping--i.e. the
>>>> idea that the same code can be run over and over.
>>>>
>>>> Where things get tricky is the difficulty of mastering the syntax of
>>>> the while loop and, more problematic, the syntax of the classic for;; loop.
>>>> Introducing a simple way to make something repeat forever does not solve
>>>> this learning hurdle, because students will continue to have to contend
>>>> with these other types of loops in order to be productive in the language.
>>>> A special syntax for repeating forever is especially unhelpful because it
>>>> is just functional enough that a discouraged student may choose to avoid
>>>> learning other types of loops and instead combine the infinite loop with
>>>> if, continue, and break.
>>>>
>>>>
>>>> I’d also like to point out Chris’ comments on the
>>>>
>>>> repeat X {
>>>>
>>>> }
>>>>
>>>> discussion.
>>>>
>>>> “
>>>>
>>>> This is a very valid use case.
>>>>
>>>> FWIW, “repeat N {}” was originally designed and scoped into the Swift 2 implementation of the feature, but was cut due to schedule limitations.  There is precedent for this sort of feature in many teaching oriented languages (e.g. Logo).
>>>>
>>>> I’d say that the pro’s and con’s of this are:
>>>>
>>>> + Makes a simple case very simple, particularly important in teaching.
>>>> + Even if you aren’t familiar with it, you can tell at first glance what the behavior is.
>>>> - It is “just syntactic sugar”, which makes the language more complex.
>>>> - It is a very narrow feature that is useful in few practical situations.
>>>>
>>>> -Chris
>>>>
>>>> “
>>>>
>>>> In this case, I would say it’s not making the language any more complex
>>>> given that repeat-while is a current construct. Admittedly it is a very
>>>> narrow feature, but it’s also a small one.
>>>>
>>>
>>> For the reasons I outlined above, I'd be +1 for `repeat N` and -1 for
>>> this case.
>>>
>>>
>>> That’s fair enough. :)
>>>
>>> But surely you’ll admit that if
>>>
>>> repeat N {
>>>
>>> }
>>>
>>> was valid, then repeat { } follows as the logical repeat indefinitely
>>> syntax, no?
>>>
>>
>> No! Not at all! As I wrote above, it could mean repeat once. It currently
>> means repeat until the condition that follows, and if that condition is
>> optional you only find out after you read everything in the loop. So, IMO,
>> it does not follow at all!
>>
>>
>> Let’s talk about this.
>>
>> Could it mean repeat once? No, it hasn’t been run yet. In fact, there is
>> very clear syntax for exactly this: do { }. It means do this once.
>> Currently, repeat-while means that you repeat while the condition is true.
>> Thus if you leave off the condition it can only mean repeat this
>> unconditionally. I really don’t think that this a huge logical leap, and I
>> doubt very much that someone would be confused by it’s meaning.
>>
>> We will have to disagree here. I just see how adding a feature like
>> repeat N, which increases complexity, would be better than extending the
>> repeat syntax to allow you to repeat unconditionally.
>>
>> I’m +1 on this proposal (sans disallowing while true).
>>
>> Tyler
>>
>>
>>
>>>
>>>
>>>
>>>>
>>>>
>>>>
>>>>> Lastly, this isn’t the first time this has been brought up on this
>>>>> list and there was previously discussion about the fact that when people
>>>>> see the repeat keyword that it should naturally repeat indefinitely unless
>>>>> a where clause is specified.
>>>>>
>>>>
>>>> I do believe that this is the first time this suggestion has been
>>>> introduced to the list. I do not recall any previous discussion focused on
>>>> infinite loops; they have been about repeating a finite number of times,
>>>> using proposed syntax such as `repeat 3 times { }` or variations on that
>>>> theme.
>>>>
>>>>
>>>>> I also think the concern that an accidental infinite loop is any
>>>>> greater than it is currently.
>>>>>
>>>>
>>>> Code gets refactored and edited. We're discussing on another thread
>>>> changing the rules about dangling commas in parameter lists for that very
>>>> reason. If you try to move a block of code with a repeat...while loop but
>>>> accidentally leave behind the last line, this syntax will cause you grief.
>>>>
>>>>
>>>>> Tyler
>>>>>
>>>>>
>>>>>
>>>>> On May 10, 2016, at 1:09 PM, Erica Sadun via swift-evolution <
>>>>> swift-evolution at swift.org> wrote:
>>>>>
>>>>> I do not see sufficiently measurable benefits to this proposal to add
>>>>> it to the language.
>>>>> It's easy enough to roll your own `repeatForever` function with
>>>>> trailing closure.
>>>>>
>>>>> I also want to thank you for bring it up on-list. Not every idea is
>>>>> right for Swift but it's
>>>>> always refreshing to see innovative thoughts added to the discussion.
>>>>> Please do not be
>>>>> discouraged by the generally negative feedback on this particular idea.
>>>>>
>>>>> -- Erica
>>>>>
>>>>> On May 10, 2016, at 1:27 AM, Nicholas Maccharoli via swift-evolution <
>>>>> swift-evolution at swift.org> wrote:
>>>>>
>>>>> ​Swift Evolution ​Community,
>>>>>
>>>>> Currently writing an infinite loop in swift looks either something
>>>>> like this:
>>>>>
>>>>>     while true {
>>>>>         if ... { break }
>>>>>         //...
>>>>>     }
>>>>>
>>>>> Or this:
>>>>>
>>>>>     repeat {
>>>>>         if ... { break }
>>>>>         //...
>>>>>     } while true
>>>>>
>>>>> But I think it might be best to change the syntax / behaviour of
>>>>> `repeat` to loop
>>>>> indefinitely if no trailing while clause is present:
>>>>>
>>>>>     repeat {
>>>>>         if ... { break }
>>>>>         //...
>>>>>     }
>>>>>
>>>>> while still allowing a trailing `while` clause as in:
>>>>>
>>>>>     repeat {
>>>>>         foo += bar
>>>>>     } while foo.count < limit
>>>>>
>>>>> I also want to propose that it should be a compile time error to use
>>>>> single `Bool` constants as while loop conditions, so no more `while true {
>>>>> ... }` it would become `repeat { ... }`
>>>>>
>>>>> I was thinking of drafting a short proposal if there was enough
>>>>> positive feedback.
>>>>>
>>>>> How does it sound?
>>>>>
>>>>> - Nick
>>>>> _______________________________________________
>>>>> swift-evolution mailing list
>>>>> swift-evolution at swift.org
>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> swift-evolution mailing list
>>>>> swift-evolution at swift.org
>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
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>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>
>>>>
>
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