ios - How to make generics in collection type constraint?

Question

I have been trying to extract non-nil values from the String array. Like below. But, my senior wants it to be able to extract non-nil values from other types too.

I read, generics could help me for handling different types. How can I use generics so that I get to use following like extension to work with other types too?

getNonNil must return the extracted non-nil values of the specific type (i.e. if array is [String?] it must return [String], returns [Int] if [Int?])

Because I have to do further calculations.

What I have tried is below:

import Foundation
// Extended the collection-type so that collectiontype is constrained to having element with optional strings
extension CollectionType where Self.Generator.Element == Optional<String>{
    func getNonNil() -> [String] {
        // filter out all nil elements and forcefully unwrap them using map
        return self.filter({$0 != nil}).map({$0!})
    }
}

// Usage
let x: [String?] = ["Er", "Err", nil, "errr"]

x.getNonNil().forEach { (str) in
    print(str)
}

Answer 1

For getNonNil you could simply use

x.flatMap { $0 }
// returns ["Er", "Err", "errr"] which is [String]

---

For the original question, typically you could introduce a protocol to the Optional type (e.g. via the muukii/OptionalProtocol package):

protocol OptionalProtocol {
    associatedtype Wrapped
    var value: Wrapped? { get }
}

extension Optional: OptionalProtocol {
    public var value: Wrapped? { return self }
}

extension CollectionType where Self.Generator.Element: OptionalProtocol {
    func getNonNil() -> [Self.Generator.Element.Wrapped] {
        ...
    }
}