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

Xiaodi Wu xiaodi.wu at gmail.com
Tue May 10 19:25:17 CDT 2016 

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
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>>>>
>>>>
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>>>>
>>>>
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>>>
>>>
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