Very simple key path based Auto Layout helpers

AutoLayoutHelpers.swift

import UIKit

typealias Constraint = (UIView, UIView) -> NSLayoutConstraint

func equal<L, Axis>(_ to: KeyPath<UIView, L>, constant: CGFloat = 0) -> Constraint where L: NSLayoutAnchor<Axis> {
    return equal(to, to, constant: constant)
}

func equal<L, Axis>(_ from: KeyPath<UIView, L>, _ to: KeyPath<UIView, L>, constant: CGFloat = 0) -> Constraint where L: NSLayoutAnchor<Axis> {
    return { view1, view2 in
        view1[keyPath: from].constraint(equalTo: view2[keyPath: to], constant: constant)
    }
}

func equal<L>(_ keyPath: KeyPath<UIView, L>, constant: CGFloat) -> Constraint where L: NSLayoutDimension {
    return { view1, _ in
        view1[keyPath: keyPath].constraint(equalToConstant: constant)
    }
}

extension UIView {
    func addSubview(_ other: UIView, constraints: [Constraint]) {
        other.translatesAutoresizingMaskIntoConstraints = false
        addSubview(other)
        addConstraints(constraints.map { $0(other, self) })
    }
}

let v1 = UIView()
v1.backgroundColor = .red
v1.frame = CGRect(x: 0, y: 0, width: 200, height: 200)
let v2 = UIView()
v2.backgroundColor = .green

v1.addSubview(v2, constraints:[
    equal(\.centerYAnchor, \.bottomAnchor),
    equal(\.leftAnchor, constant: 10), equal(\.rightAnchor, constant: -10),
    equal(\.heightAnchor, constant: 100)
])