Extension to Array with generic elements with custom initializer

Question

Disclaimer

Please note this is only a simplified example. Actual problem is a bit more complex so I need actual Array extension rather than other solution. I know that

let mapped: [MyGeneric<Int>] = items.map { .init(someVariable: $0) }

will work, but I'm looking for a way to write generic array extension not fix this simplified code

Problem

I'm trying to write a generic extension to an array/collection as below:

struct MyGeneric<T: Hashable> {
    let someVariable: T
}

Expected output

Code below is not working because I'm missing Array extension, but ideally my output would be something like this:

let items: [Int] = [1, 2, 3, 4, 5]
let mapped: [MyGeneric<Int>] = .init(items)

So what I need is:

extension [MyGeneric<Hashable>] {
    init(someArray: [some Hashable]) {
        self = someArray.map { MyGeneric(someVariable: $0)}
    }
}

but of course this is not compiling because generic parameter from extension is not passed to init generic parameter. How can I pass the type from extension to functions in that. Can I use somehow Element.Type in this one and extract Generic type from there? Or maybe there is an extension Array where ... syntax that I'm not familiar with?

See stackoverflow.com/a/62253008/1187415, the same approach should work for your problem. — Martin R
– Martin R, Commented Feb 9, 2023 at 21:26

Joakim Danielson · Accepted Answer · 2023-02-10 08:31:04Z

2

Inspired by the answer provided by @MartinR I came up with the following solution

extension Array {
    init<T>(_ array: [T]) where Element == MyGeneric<T> {
        self.init(array.map(MyGeneric.init))
    }
}

edited Feb 10, 2023 at 8:31

answered Feb 9, 2023 at 22:20

Joakim Danielson

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2 Comments

Martin R Over a year ago

The constraint on T is not needed, apparently the compiler infers it from the definition of MyGeneric. So init<T>(_ array: [T]) where Element == MyGeneric<T> is sufficient.

Joakim Danielson Over a year ago

@MartinR Thanks, I didn't even consider that.

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