Num.swift

import Foundation

/**
 Num typeclass
 */
protocol Num {
  func + (lhs: Self, rhs: Self) -> Self
  func - (lhs: Self, rhs: Self) -> Self
  func * (lhs: Self, rhs: Self) -> Self
  func / (lhs: Self, rhs: Self) -> Self
  func % (lhs: Self, rhs: Self) -> Self
  init(_ v: Int)
}

/**
 Partial application of * and +
 */
prefix operator * {}
prefix func * <A: Num> (x: A) -> A -> A {
  return { $0 * x }
}

postfix operator * {}
postfix func * <A: Num> (x: A) -> A -> A {
  return { x * $0 }
}

prefix operator + {}
prefix func + <A: Num> (x: A) -> A -> A {
  return { $0 + x }
}

postfix operator + {}
postfix func + <A: Num> (x: A) -> A -> A {
  return { x + $0 }
}

/**
 Extensions of core data types
 */
extension Double : Num {}
extension Float  : Num {}
extension Int    : Num {}
extension Int8   : Num {}
extension Int16  : Num {}
extension Int32  : Num {}
extension Int64  : Num {}
extension UInt   : Num {}
extension UInt8  : Num {}
extension UInt16 : Num {}
extension UInt32 : Num {}
extension UInt64 : Num {}

test.swift

func square <A: Num> (x: A) -> A {
  return x * x
}

square(5)   //=> 25
square(1.5) //=> 2.25

let xs = [1, 1, 2, 3, 5, 8]
map(xs, *3) //=> [3, 3, 6, 9, 15, 24]