Swift Functor and Monad

1-Functor-and-Monad.md

Copy and paste the swift code below into a playground to experiment.

This is a very close emulation of Functor and Monad typeclasses in swift. However, it is very fragile (i.e. easy to crash the compiler).

For example, instance methods of fmap will run fine, but attempting to use a globally defined fmap that acts on Functor types will cause a crash. Similarly for bind. Unfortunately this means we cannot define the nice infix operator versions of these functions.

2-Functor-and-Monad.swift

import Foundation

public struct K<A> {}

public protocol Functor {
  typealias _A
  typealias _B
  typealias _FB = K<_B>

  func fmap (_A -> _B) -> _FB
}

/**
 Using this global fmap usually crashes :(
 */
public func fmap <F: Functor> (f: F._A -> F._B) -> F -> F._FB {
  return { $0.fmap(f) }
}

public protocol Monad : Functor {
  class func unit (f: _A) -> Self
  func bind (f : _A -> _FB) -> _FB
  func >>= (x: Self, f : _A -> _FB) -> _FB
}

/**
 Using these global binds usually crashes :(
 */
infix operator >>= {associativity left}
public func >>= <M: Monad> (x: M, f: M._A -> M._FB) -> M._FB {
  return x.bind(f)
}
public func bind <M: Monad> (x: M, f: M._A -> M._FB) -> M._FB {
  return x.bind(f)
}

/**
 Make Array a functor
*/
extension Array : Functor {
  typealias _A = T
  typealias _B = Any
  typealias _FB = [_B]

  public func fmap <_B> (f: _A -> _B) -> [_B] {
    return self.map(f)
  }
}

/**
 Make Array a monad
*/
extension Array : Monad {
  public static func unit (x: _A) -> [_A] {
    return [x]
  }

  public func bind <_B> (f: _A -> [_B]) -> [_B] {
    return self.map(f).reduce([], +)
  }
}

/**
  Make optional a functor
*/
extension Optional : Functor {
  typealias _A = T
  typealias _B = Any
  typealias _FB = _B?

  public func fmap <_B> (f: _A -> _B) -> _B? {
    switch self {
    case let .Some(value):
      value
      return f(value)
    case .None:
      return .None
    }
  }
}

/**
Make optional a monad
*/
extension Optional : Monad {
  public static func unit (x: _A) -> _A? {
    return Optional<_A>.Some(x)
  }

  public func bind <_B> (f: _A -> _B?) -> _B? {
    switch self {
    case let .Some(value):
      return f(value)
    case .None:
      return .None
    }
  }
}

func square (x: Double) -> Double {
  return x * x
}

func invert (x: Double) -> Double? {
  if (x != 0.0) {
    return 1.0 / x
  }
  return nil
}

func squareRoot (x: Double) -> Double? {
  if (x < 0.0) {
    return nil
  }
  return sqrt(x)
}

func test (x: Double) -> String {
  return "test: \(x)"
}

/**
 Let's take Functor and Monad out for a spin...
 */

let xs = [2.0, 3.0, 5.0, 7.0, 11.0, 13.0, 17.0]
xs.fmap(square)
// fmap(square)(xs) // crash!

let optional2: Double? = 2
optional2.fmap(test)
optional2.bind(squareRoot)
// optional2 >>= squareRoot // crash!




"done"