Cannot use protocol to define common initializers - swift

Question

I'm trying to create a generic class that is able to convert models to their associated view models. Unfortunately the compiler does not allow it and fails with the following error:
Cannot convert value of type 'Model' (generic parameter of generic class 'Store') to expected argument type 'ViewModel.Model' (associated type of protocol 'StoreViewModel')
I'm using the following simplified code:
protocol StoreViewModel {
associatedtype Model
init(model: Model)
}
class Store<Model, ViewModel: StoreViewModel> {
var models = [Model]()
var results = [ViewModel]()
func update() {
results = models.map {
ViewModel(model: $0)
// Cannot convert value of type 'Model' (generic parameter of generic class 'Store') to expected argument type 'ViewModel.Model' (associated type of protocol 'StoreViewModel')
}
}
}
class Foo: NSManagedObject {}
class FooViewModel: StoreViewModel {
var model: Foo
required init(model: Foo) {
self.model = model
}
}
let store = Store<Foo, FooViewModel>()
I've already read about type erasure, but I'm wondering of there is no simpler solution for this problem.

Answer 1

As it is, your current code allows things like:
let store = Store<Int, FooViewModel>()
Clearly that makes no sense, but your code allows it because there is no constraint on Model. Model can be anything, can't it?
Let's put a constraint on:
class Store<Model, ViewModel: StoreViewModel> where Model == ViewModel.Model {
Now we see that there is no point of having two generic parameters! Why use Model, when we can just use ViewModel.Model?
class Store<ViewModel: StoreViewModel> {
var models = [ViewModel.Model]()
or, if you hate long type names, use a typealias:
typealias Model = ViewModel.Model
var models = [Model]()