Monoid.swift

protocol Monoid {
  func unit() -> Self
  func append(b: Self) -> Self
}

operator infix |+| { associativity left precedence 140 }
func |+|<A: Monoid>(a: A, b: A) -> A {
  return a.append(b)
}

extension Int: Monoid {
  func unit() -> Int { return 0 }
  func append(b: Int) -> Int {
    return self + b
  }
}

println("2 |+| 3 = \(2 |+| 3)")