Generic parameter 'T' could not be inferred on extension

August 23, 2022

I’m trying to make an extension for Array that will format it. The issue is that since I’m using generics, I need to define the type on the calling Array.

public struct ASValue<T: Hashable>: Hashable {
    private let id = UUID()
    var item: T
}

public extension Array {
    func asCast<T: Hashable>() -> [ASValue<T>]{
        self.map({
            ASValue(item: $0 as! T)
        })
    }
}

// How can I call this without having to define the generic type?
@State private var numbers = [32, 78, 38, 28, 38].asCast() // Generic parameter 'T' could not be inferred

>Solution :

You actually don’t need a generic for this, just constraint the extension to only work with arrays of Hashable:

extension Array where Element: Hashable {
    func asCast() -> [ASValue<Element>]{
        self.map({
            ASValue(item: $0)
        })
    }
}