ExpressibleByStringLiteral

KeyPath.swift

struct KeyPath {
    let key: String

    init(_ key: String) {
        self.key = key
    }
}

extension KeyPath: ExpressibleByStringLiteral {

    init(unicodeScalarLiteral value: String) {
        self.key = value
    }

    init(extendedGraphemeClusterLiteral value: String) {
        self.key = value
    }

    init(stringLiteral value: String) {
        self.key = value
    }
}

func findBy(keyPath: KeyPath) {
    print("OK")
}

findBy(keyPath: KeyPath("Key")) //OK
findBy(keyPath: "StringLiteral") //OK

let strVar: String = "strVar"
findBy(keyPath: strVar) //NG

ExpressibleByStringLiteral なのでStringLiteralじゃないとだめという例