[swift-evolution] [Pitch] make @noescape the default

[Matthew Johnson]

Sent from my iPad

> On Dec 20, 2015, at 10:21 AM, Félix Cloutier <felixcca at yahoo.ca> wrote:
> 
> @noescape is part of the interface's contract. If you have it for a while (because you had a warning more than because of thoughtful design) then remove it, you're forcing code changes on your clients. I'm not sure it's an appropriate default/warning, at least for public interfaces.
> 

I'm trying to think of examples where you would change this in scenarios that don't already impact clients.  A change from synchronous to asynchronous or vice versa is the main example I can think of and that definitely impacts clients (at least when going from sync to async).

Do you have any examples of where you would reasonably change the escaping behavior without an impact to client code other than the escaping change?


> Félix
> 
>> Le 20 déc. 2015 à 11:14:51, ilya via swift-evolution <swift-evolution at swift.org> a écrit :
>> 
>> > If we don’t change the default maybe we should at least add a warning when you *could* add @noescape but didn’t.  A lot of folks are likely to leave it off when they should really be adding it.
>> 
>> I like this idea. We have a warning for variables that can be made into let, so why not this? 
>> 
>>> On Sun, Dec 20, 2015 at 18:50 Matthew Johnson via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> > On Dec 19, 2015, at 7:51 PM, Kevin Ballard via swift-evolution <swift-evolution at swift.org> wrote:
>>> >
>>> > -1
>>> >
>>> > The standard library is not representative of how closures are commonly used. Notably, the standard library never executes anything asynchronously (an extremely common use of closures in apps), doesn't ever hold closures that are triggered later by a separate event (e.g. NSNotificationCenter observing, or UIAlertView handlers, or any third-party solution for block-based target/action replacement), no timers, no completion handlers, etc. The only constructs in the standard library that come to mind that would need escaping closures are lazy sequence operations and AnySequence (I haven't actually looked to see if there are any others). But nearly all uses of closures in UIKit require escaping, nearly all uses of closures in libdispatch require escaping, and I wager that nearly all uses of closures in application code require escaping.
>>> 
>>> Those are fair points.  The right place to look is in application code so my skimming of the library was probably not that relevant.
>>> 
>>> The fact that the compiler would tell you when you need @escaping but doesn’t tell you when you could add @noescape is part of what prompted my pitch.  If we don’t change the default maybe we should at least add a warning when you *could* add @noescape but didn’t.  A lot of folks are likely to leave it off when they should really be adding it.
>>> 
>>> It would be interesting to look at function arguments in application code written in different styles (OO vs mixed vs functional-leaning) and see what the split is for escaping vs non-escaping function arguments.
>>> 
>>> >
>>> > -Kevin Ballard
>>> >
>>> > On Sat, Dec 19, 2015, at 12:10 PM, Matthew Johnson via swift-evolution wrote:
>>> >> @noescape is safer because it does not require thinking about lifetime issues for captured objects.  My hunch was that @noescape (or @autoclosure with implies @noescape) is also more common.  I had a look through the standard library and this is definitely the case there.
>>> >>
>>> >> What does everyone think about making @noescape the default and introducing @escaping (or something similar) to annotate function arguments that do escape the call stack?
>>> >>
>>> >> Matthew
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