Using `switch` to find a substring.

ContainsSwitch.swift

import Cocoa

struct Contains : ExpressibleByStringLiteral {
    let s: String
    
    static func ~= (l: Contains, r: Contains) -> Bool {
        return r.s.contains(l.s)
    }
    
    init(_ c: String) {
        s = c
    }
    
    init(stringLiteral value: String) {
        s = value
    }
}

extension String {
    var contains: Contains {
        return Contains(self)
    }
}

var str = "Hello, playground"

switch str.contains {
case "str":     print(0)
case "Hello":   print(1)
case "play":    print(2)
default:        print(3)
}

Prints 1.