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

[Xiaodi Wu]

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.


> 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
>
>
>
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20160510/a8c65d16/attachment.html>