[swift-users] Optional variables from a non-optional source with type inference

Kevin Lundberg kevin at klundberg.com
Thu Jan 28 22:11:09 CST 2016

I wrote a small library a while back that handles this problem without
needing a new closure:

...which should be SwiftPM compatible on top of the more established
distribution methods.

On 1/26/2016 10:39 PM, Jordan Rose via swift-users wrote:
>> On Jan 25, 2016, at 16:16 , Michael Henson via swift-users
>> <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>> While reading the summary of a newsletter's notification about the
>> acceptance of SE-0021's new syntax, a thought occurred to me:
>>     let fn = someView.insertSubview(_:at:) |let fn1 =
>>     someView.insertSubview(_:aboveSubview:)
>>     ||let fn2 = someView.insertSubview(_:belowSubview:)|
>> That code will create variables fn, fn1, fn2 of the correct type to
>> hold each of the named functions.
>> As I understand it, the following things are true:
>> 1. Member functions on instances of a class type are curried
>> functions that strongly capture 'self'.
>> 2. Any strong reference to those member functions will also prevent
>> that instance from ever reaching ref count 0.
>> 3. This isn't a new problem, just one that will become more important
>> now that this use case is easier to... use.
>> The easy fix for this situation would be to declare fn, fn1, and fn2
>> as weakened optionals so they don't keep someView around if it should
>> have been dallocated. The question is... how? Is there existing
>> syntax that means "infer the type from the right-hand-side
>> production, except as an optional / weakened optional"?
> Hey, Mike. There's no existing syntax for this today; the best you can
> get is a custom closure. The type can still be inferred, though:
>     let fn3 = { [weak someView] in someView?.insertSubview($0,
>     belowSubview: $1) }
> This works because the closure is a single expression, so the body can
> be type-checked together with the enclosing statement. If the body has
> actual statements in it then it is type-checked separately.
> We don't currently have any other syntax for this, but personally I
> think it's not common enough to warrant it. I'd actually rather
> consider /disallowing/ binding member functions like this unless the
> resulting closure is @noescape. That also keeps you from accidentally
> introducing retain cycles. But any serious language change here would
> have to go through the Swift Evolution Process
> <https://github.com/apple/swift-evolution/blob/master/process.md>.
> Best,
> Jordan
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