Where "where" may be used? http://blog.krzyzanowskim.com/2015/11/13/where-where-may-be-used/

where-where.swift

import Cocoa

// for-in
func checkForIn(array: [Int], dict: [Int: String]) {
    for num in array where dict[num] != nil {
        num
    }
}

checkForIn([1,2,3,4], dict: [1:"one", 2:"two"])

// do-catch

enum ResponseError: ErrorType {
    case HTTP(Int)
}

func errorProne() throws {
    throw ResponseError.HTTP(404)
}

do {
    try errorProne()
} catch ResponseError.HTTP(let code) where code >= 400 && code % 2 == 0 {
    print("400 Bad Request") // match
} catch ResponseError.HTTP(let code) where code >= 500 && code < 600  {
    print("Internal Server Error")
}

// while

func checkWhile(inout array: [Int]?) {
    while let arr = array where arr.count < 5 {
        array?.append(0)
    }
}

var mutableArray:[Int]? = []
checkWhile(&mutableArray)

// if

func checkIf(string: String?) {
    if let str = string where str == "checkmate" {
        print("game over")
        return
    }
    print("let's play")
}

checkIf("checkmate")

// guard

func checkGuard(string: String?) {
    guard let str = string where str != "checkmate" else {
        print("game over")
        return
    }
    print("let's play")
}

checkGuard("checkmate")

// switch-case

var value = (1,2)
switch value {
    case let (x, y) where x == 1:
        // match 1
    break
    case let (x, y) where x / 5 == 1:
        // not-match
    break

    default:
    break
}

// generics

func genericFunction<S where S: StringLiteralConvertible>(string: S) {
    print(string)
}

genericFunction("lambada")

@krzyzanowskim another way to write noop could look like this default: ()