[swift-evolution] Enforce non-nil assignment to an implicitly unwrapped optional property?

[Glen Huang]

Anyone has any thoughts on this?

> On 16 Jul 2017, at 3:46 PM, Glen Huang via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I want to define a property for a struct/class that is nil after initialization, but later is guaranteed to have a value after I manually assign to it. I think implicitly unwrapped optional property is designed specifically for that.
> 
> However, the problem arises when it comes to how do I enforce that I never mistakenly assign nil to this property?
> 
> struct Foo {
>    var name: String!
> }
> var foo = Foo()
> 
> I can do foo.name = nil and it will compile just fine.
> 
> I guess I could do a run-time check with something like:
> 
> struct Foo {
>    var name: String! {
>        willSet {
>            if newValue == nil {
>                fatalError("nil isn't allowed")
>            }
>        }
>    }
> }
> 
> But it feels ugly, and seems to be something checkable at compile time.
> 
> I could define a new setter method:
> 
> struct Foo {
>    private(set) var name: String!
>    mutating func setName(_ name: String) {
>        self.name = name
>    }
> }
> 
> But it feel tedious. Enforcing non-nil assignment probably fits 90% of the use cases of IUOs, since that’s basically their definition. Having to deviate from direct assignment syntax in usage and also define a new setter method for the majority cases seems unfortunate.
> 
> I wonder if it’s desirable to define an attribute or something to ask the compiler to only allow non-optionals to be assigned to IUO properties?
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