Finally tagless in Swift 3

expression-problem.swift

protocol Expr {
  static func lit(_ x: Int) -> Self
  static func add(_ lhs: Self, _ rhs: Self) -> Self
}

extension Int : Expr {
    static func lit(_ x : Int) -> Int {
        return x;
    }
    static func add(_ lhs: Int, _ rhs: Int) -> Int {
        return lhs + rhs
    }
}

extension String : Expr {
    static func lit(_ x : Int) -> String {
        return "\(x)";
    }
    static func add(_ lhs: String, _ rhs: String) -> String {
        return "(\(lhs) + \(rhs))"
    }
}

protocol Mul : Expr {
  static func mul(_ lhs: Self, _ rhs: Self) -> Self
}

extension Int : Mul {
    static func mul(_ lhs: Int, _ rhs: Int) -> Int {
        return lhs * rhs
    }
}

extension String : Mul {
    static func mul(_ lhs: String, _ rhs: String) -> String {
        return "(\(lhs) * \(rhs))"
    }
}

extension Mul {
  static var test: Self {
    return add(lit(1), mul(lit(2), lit(3)))
  }
}

print(Int.test) // 7
print(String.test) // (1 + (2 * 3))

func test2<A : Mul>() -> A {
    return A.add(A.lit(1), A.mul(A.lit(2), A.lit(3)))
}

print(test2() as Int)
print(test2() as String)