CODEWITHSUNDEEP
swiftgenerics
David James
David James

Reputation: 2466

Swift custom flatMap implementation causes error Generic parameter could not be inferred

I'm trying to implement a custom flatMap on a struct that has some generic items. The flatMap would just Swift.flatMap over the items using the provided transform function. The code here is a bit contrived, but it boggles my mind why the standard library version does not cause an error, but mine does.

Is there something I'm missing in my implementation of flatMap?

struct Wrapper<T:NSObject> {

    let items:[T]

    func flatMap<U>(_ transform:(T) -> U?) -> Wrapper<U> {
        let newItems = items.flatMap { transform($0) }
        return Wrapper<U>(items: newItems)
    }
}

protocol Something {}

let wrapper = Wrapper<UIView>(items: [UIView()])

// Using standard library flatMap
wrapper
    .items
    .flatMap { $0 as? Something } // ✅ No error
    // ..

// Using custom flatMap
wrapper
    .flatMap { $0 as? Something } // 🛑 ERROR: Generic parameter 'U' could not be inferred
    // ..

Upvotes: 0

Views: 609

Answers (1)

Palle
Palle

Reputation: 12109

Something does not conform to NSObject.

Swift 4 automatically infers constraints for the generic parameter U used in your flatMap function by looking at all parameters which include U. U is used as a generic parameter to Wrapper which requires U: NSObject. Something is not a subclass of NSObject.

The error message is just misleading.

Upvotes: 2

Related Questions