Functions as far as the eye can see.swift

//: A UIKit based Playground for presenting user interface
  
import UIKit
import PlaygroundSupport

typealias Anim = (UIView) -> Void
typealias Func = () -> Void

precedencegroup ForwardComposition
{
    associativity: left
    higherThan: AssignmentPrecedence
}

infix operator >>>: ForwardComposition

func >>> (a: @escaping (Func?) -> Void,
          b: @escaping (Func?) -> Void) -> (Func?) -> Void
{
    return { c in
        a { b(c) }
    }
}

func + (anim1: @escaping Anim,
        anim2: @escaping Anim) -> Anim
{
    return { view in
        anim1(view)
        anim2(view)
    }
}

func transform(_ transform: CGAffineTransform) -> (UIView) -> Void
{
    return { view in
        view.transform = transform
    }
}

func fade(to alpha: CGFloat) -> (UIView) -> Void
{
    return { view in
        view.alpha = alpha
    }
}

func animate(duration: TimeInterval,
             animations: @escaping Func) -> (Func?) -> Void
{
    return { completion in
        let animator = UIViewPropertyAnimator(duration: duration, curve: .linear, animations: animations)
            
        animator.addCompletion({ _ in completion?() })
        animator.startAnimation()
    }
}

func animator(_ views: [UIView],
              _ animations: @escaping Anim,
              _ duration: TimeInterval) -> (Func?) -> Void
{
    return animate(duration: duration, animations: {
        views.forEach { animations($0) }
    })
}

class MyViewController : UIViewController
{
    override func loadView()
    {
        let view = UIView()
        view.backgroundColor = .white
        
        let viewOne = UIView()
        viewOne.frame = CGRect(x: 30, y: 30, width: 30, height: 30)
        viewOne.backgroundColor = .black
        
        let viewTwo = UIView()
        viewTwo.frame = CGRect(x: 120, y: 30, width: 30, height: 30)
        viewTwo.backgroundColor = .black
        
        view.addSubview(viewOne)
        view.addSubview(viewTwo)
        
        self.view = view
        
        let transformIn = transform(CGAffineTransform(scaleX: 2.0, y: 2.0))
        let transformOut = transform(CGAffineTransform(scaleX: 0.5, y: 0.5))
        
        let fadeIn = fade(to: 1)
        let fadeOut = fade(to: 0)
        
        let fadeInAnimations = fadeIn + transformIn
        let fadeOutAnimations = fadeOut + transformOut
        
        let initial = {
            viewOne.alpha = 0
            viewTwo.alpha = 0
        }
        
        initial()
        
        view.setNeedsDisplay()
        
        let f1 = animator([viewOne, viewTwo], fadeInAnimations, 0.5)
        let f2 = animator([viewOne, viewTwo], fadeOutAnimations, 0.5)

        let f3 = f1 >>> f2 >>> f1 >>> f2 >>> f1 >>> f2
        f3(nil)
    }
}

// Present the view controller in the Live View window
PlaygroundPage.current.liveView = MyViewController()