example of protocol extension

protocolExtension.swift

//: Playground - noun: a place where people can play

import UIKit

var str = "Hello, playground"

/*

class (Eq a, Show a) => Num a where
(+) :: a -> a -> a
(-) :: a -> a -> a
(*) :: a -> a -> a
negate :: a -> a
abs :: a -> a
signum :: a -> a
fromInteger :: Integer -> a
fromInt :: Int -> a

*/

protocol Num: CustomStringConvertible, Equatable {
    func +(left: Self, right: Self) -> Self
    func -(left: Self, right: Self) -> Self
    func *(left: Self, right: Self) -> Self
    var isNegative: Bool { get }
    var negate: Self { get }
    var abs: Self { get }
    static func fromInt(intValue: Int) -> Self
}

extension Num {
    var negate: Self {
        return isNegative ? self : Self.fromInt(-1) * self
    }
    var abs: Self {
        return !isNegative ? self : Self.fromInt(-1) * self
    }
}

/*
instance Num Int
*/

extension Int: Num {
    var isNegative: Bool { return self < 0 }
    static func fromInt(intValue: Int) -> Int {
        return intValue
    }
}