Using associatedtype from generics in function not recognized

Using Swift
, ,
1

I am trying to create af function which takes another instance of the same class and a closure. The clousure should use an associatedtype of the other instance it conforms to as an parameter.

I can't get it to work. I've tried several things and always end up with a closure that is not recognized or doesn't match the signature.

struct Person {
    let name: String
}

let steve = Person(name: "Steve Jobs")

protocol SelectorProtocol {
    associatedtype State
    associatedtype Value
    func map(_ state: State) -> Value
}

class Selector<State, Value> {
    let projector: (State) -> Value
    
    init(keyPath: KeyPath<State, Value>) {
        self.projector = { $0[keyPath: keyPath] }
    }
    
    init<S1: SelectorProtocol>(_ selector1: S1, _ projector: @escaping (S1.Value) -> Value) where S1.State == State {
        self.projector = { projector(selector1.map($0)) }
    }
    
    func map(state: State) -> Value {
        return projector(state)
    }
}

let selectorKeyPath = Selector(keyPath: \Person.name)
print(selectorKeyPath.map(state: steve)) // Prints "Steve Jobs" correctly

let selector1 = Selector(selectorKeyPath, { name in name.capitalized }) // <-- Error here saying "Expression type 'Selector<_, _>' is ambiguous without more context"
print(selector1.map(state: steve))

When I try to specify the generic arguments to the last Selector it says that S1 could not be inferred.

Can anybody help me with this?

2

You forgot to conform Selector to SelectorProtocol, so there's no init that matches the signature.

2 Likes
3

Wow... :man_facepalming: Don't understand how I missed that.

Thank you! :tada:

4

Might still be worthwhile to file your code example at bugs.swift.org for the poor diagnostic message. Seems to me that the compiler error could more clearly state Selection<X, Y> does not conform to SelectorProtocol.

1 Like