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iOS Generics in Swift Generics and Protocols Extending Generic Types

Code doesnt run on treehouse, works on playground

protocol PrefixContaining { func hasPrefix(_ prefix: String) -> Bool }

extension String: PrefixContaining { func hasPrefix(_ prefix: String) -> Bool { return String(self.characters.prefix(prefix.count)) == prefix } }

extension Array where Element : PrefixContaining { func filter(byPrefix prefix: String) -> [Element]{ return self.filter{ $0.hasPrefix(prefix)} } }

let test = ["aa", "ba", "ca", "ab"] let result = test.filter(byPrefix: "a") // result = ["aa", "ab"]

generics.swift
protocol PrefixContaining {
    func hasPrefix(_ prefix: String) -> Bool
}

extension String: PrefixContaining {
    func hasPrefix(_ prefix: String) -> Bool {
        return String(self.characters.prefix(prefix.count)) == prefix
    }
}

extension Array where Element : PrefixContaining {
    func filter(byPrefix prefix: String) -> [Element]{
        return self.filter{ $0.hasPrefix(prefix)}
    }
}

// Enter code below