The type of a struct that conforms to a generic protocol with an associated type that is also a protocol

Question

This is very hard to put into words but I've created a minimal example.

Here is a gist if you'd prefer... https://gist.github.com/anonymous/67d83fb2f286cf84539b58be96a971d3

The "data item" protocol

I have a protocol which defines Sortable objects with a property number like so.

protocol Sortable: Comparable {
    var number: Int {get}

    static func < (lhs:Self, rhs: Self) -> Bool
    static func == (lhs:Self, rhs: Self) -> Bool
}

struct BasicSortable: Sortable {
    let number: Int

    static func < (lhs:BasicSortable, rhs: BasicSortable) -> Bool {
        return lhs.number < rhs.number
    }

    static func == (lhs:BasicSortable, rhs: BasicSortable) -> Bool {
        return lhs.number == rhs.number
    }
}

The "worker" protocol

Then I have a protocol that can do something with these Sortable types. But because it has a Self requirement it needs to be defined as a protocol with an associated type and in the structs as generic property...

protocol Sorter {
    associatedtype Item: Sortable

    func sort(items: [Item]) -> [Item]
}

// Two different sorters
struct AscendingSorter<T:Sortable>: Sorter {
    typealias Item = T

    func sort(items: [T]) -> [T] {
        return items.sorted()
    }
}

struct DescendingSorter<T:Sortable>: Sorter {
    typealias Item = T

    func sort(items: [T]) -> [T] {
        return items.sorted{$0 > $1}
    }
}

The handler

Finally a struct that pulls everything together...

struct DataHandler<T: Sortable> {
    let items: [T]
    let sortedItems: [T]

    init(unsortedItems: [T]) {
        items = unsortedItems

        let sorter = AscendingSorter<T>()
        sortedItems = sorter.sort(items: unsortedItems)
    }
}

Making it all work

This all works.

let array = [
    BasicSortable(number: 1),
    BasicSortable(number: 8),
    BasicSortable(number: 13),
    BasicSortable(number: 3),
    BasicSortable(number: 4),
    BasicSortable(number: 14),
    BasicSortable(number: 5),
    BasicSortable(number: 12),
    BasicSortable(number: 3),
]

let handler = DataHandler(unsortedItems: array)

handler.sortedItems

This prints out the array of items in the correct order depending on what type of sorter I create in the Handler

The problem

What I'm trying to do now is to find a property declaration for this sorter struct that can take ANY Sorter type into it but everything I have tried in doing that has failed so far.

Is there a way to do this?

In the struct I'd like to have...

let sorter: SomeTypeHere

And then in the init set it like...

sorter = AscendingSorter()

but no combination I have tried in doing this has worked.

Thanks

Answer 1

You could use type erasure to implement your own AnySorter.

Starting with your own code from above:

protocol Sortable: Comparable {
    var number: Int {get}

    /* as Hamish mentions in his answer: 
       < and == already blueprinted in Comparable and Equatable */
}

protocol Sorter {
    associatedtype Item: Sortable

    func sort(items: [Item]) -> [Item]
}

Construct an AnySorter:

struct AnySorter<Item: Sortable>: Sorter {
    private let _sort: ([Item]) -> [Item]

    init<S: Sorter where S.Item == Item>(_ sorter: S) {
        _sort = sorter.sort
    }

    func sort(items: [Item]) -> [Item] {
        return _sort(items)
    }
}

Which you make use of e.g. as an argument to the initializer in your DataHandler:

struct DataHandler<T: Sortable> {
    let items: [T]
    let sortedItems: [T]

    init(unsortedItems: [T], sorter: AnySorter<T>) {
        items = unsortedItems
        sortedItems = sorter.sort(items: unsortedItems)
    }
}

Your handler can now be used with a type erased AnySorter applied to your Sortable types. E.g., for the two simple sorters you've supplied in your question:

struct AscendingSorter<T:Sortable>: Sorter {
    typealias Item = T

    func sort(items: [T]) -> [T] {
        return items.sorted()
    }
}

struct DescendingSorter<T:Sortable>: Sorter {
    typealias Item = T

    func sort(items: [T]) -> [T] {
        return items.sorted{$0 > $1}
    }
}

/* example usage */ 
extension Int: Sortable {
    var number: Int { return self }
} 

let arr = [1, 4, 2, 8, 3]

let dataHandlerDesc = DataHandler(unsortedItems: arr, sorter: AnySorter(DescendingSorter()))
print(dataHandlerDesc.sortedItems) // [8, 4, 3, 2, 1]

let dataHandlerAsc = DataHandler(unsortedItems: arr, sorter: AnySorter(AscendingSorter()))
print(dataHandlerAsc.sortedItems) // [1, 2, 3, 4, 8]

---

Edit addition to answer your comment:

Is it possible to take the input parameter and store it in a property? Would I just use AnySorter<T> as the type of the property?

Yes, you can keep a property in DataHandler with type AnySorter. E.g., for a contrived example, we can let sortedItems be a computed property that makes use of an AnySorter instance to sort a stored list of items (of course in reality we don't want to do this re-sorting for each call, but for this example only!):

struct DataHandler<T: Sortable> {
    let items: [T]
    var sortedItems: [T] { return sorter.sort(items: items) }
    var sorter: AnySorter<T>

    init(unsortedItems: [T], sorter: AnySorter<T>) {
        items = unsortedItems
        self.sorter = sorter
    }

    mutating func changeSorter(newSorter: AnySorter<T>) {
        sorter = newSorter
    }
}

/* example usage */ 
extension Int: Sortable {
    var number: Int { return self }
} 

let arr = [1, 4, 2, 8, 3]

var dataHandler = DataHandler(unsortedItems: arr, sorter: AnySorter(DescendingSorter()))
print(dataHandler.sortedItems) // [8, 4, 3, 2, 1]

dataHandler.changeSorter(newSorter: AnySorter(AscendingSorter()))
print(dataHandler.sortedItems) // [1, 2, 3, 4, 8]

Answer 2

If an instance of a given type that conforms to Sorter can deal with any homogenous array of elements that conform to Sortable (if it's restricted to a single concrete type, then @dfri's answer has got you covered) – then Sorter need not have an associatedtype in the first place. You could simply make the sort(items:) method generic instead, which would allow you to use Sorter as a type.

Also if your sort(items:) method doesn't utilise any instance state (it doesn't in your example code), then you could make it static – and simply pass around the types of sorters, instead of instances.

For example, your Sortable protocol, and BasicSortable implementation:

protocol Sortable : Comparable {
    var number : Int { get }

    // note that you don't need to re-define the < and == operator requirements,
    // as they're already defined by Comparable and Equatable
}

struct BasicSortable : Sortable {

    let number : Int

    static func < (lhs:BasicSortable, rhs: BasicSortable) -> Bool {
        return lhs.number < rhs.number
    }

    static func == (lhs:BasicSortable, rhs: BasicSortable) -> Bool {
        return lhs.number == rhs.number
    }
}

Your Sorter protocol, and different sorter implementations:

protocol Sorter {

    // A sort function that can take any homogenous array of a given
    // Sortable element (meaning that an instance of a type that conforms to
    // Sorter isn't restricted to a single concrete type of Sortable).
    // As the function doesn't rely on any instance state, it's static.
    static func sort<T:Sortable>(items: [T]) -> [T]
}

// Two different sorters
enum AscendingSorter : Sorter {
    static func sort<T:Sortable>(items: [T]) -> [T] {
        return items.sorted(by: <)
    }
}

enum DescendingSorter : Sorter {
    static func sort<T:Sortable>(items: [T]) -> [T] {
        return items.sorted(by: >)
    }
}

And finally, your DataHandler with an example usage:

struct DataHandler<T: Sortable> {

    let items: [T]
    private(set) var sortedItems: [T]

    var sorter : Sorter.Type { // simply hold a given type of sorter
        willSet {
            if sorter != newValue {
                // re-sort items upon (different) sorter being set
                sortedItems = newValue.sort(items: items)
            }
        }
    }

    init(unsortedItems: [T], sorter: Sorter.Type) {
        items = unsortedItems
        self.sorter = sorter
        sortedItems = sorter.sort(items: unsortedItems)
    }
}

let items = [BasicSortable(number: 2), BasicSortable(number: 4), BasicSortable(number: 6),
             BasicSortable(number: 1), BasicSortable(number: 4)]

var handler = DataHandler(unsortedItems: items, sorter: AscendingSorter.self)
print(handler.sortedItems)

// [BasicSortable(number: 1), BasicSortable(number: 2), BasicSortable(number: 4),
//  BasicSortable(number: 4), BasicSortable(number: 6)]

handler.sorter = DescendingSorter.self
print(handler.sortedItems)

// [BasicSortable(number: 6), BasicSortable(number: 4), BasicSortable(number: 4),
//  BasicSortable(number: 2), BasicSortable(number: 1)]