KeyPath

KP.swift

import UIKit


precedencegroup KeyPathPrecedence {
    associativity: right
    higherThan: MultiplicationPrecedence
}

infix operator ...: KeyPathPrecedence


func magic<T,E>(obj: inout T, path: ReferenceWritableKeyPath<T, E>, value: E) -> T {
    obj[keyPath: path] = value
    return obj
}

class KPObject<T,E> {
    var obj: T
    var path: ReferenceWritableKeyPath<T, E>
    
    init(obj: T, path: ReferenceWritableKeyPath<T, E>) {
        self.obj = obj
        self.path = path
    }
}

func ...<T,E>(obj: T, path: ReferenceWritableKeyPath<T, E>) -> KPObject<T, E> {
    return KPObject(obj: obj, path: path)
}

func ...<T,E>(obj: KPObject<T,E>, path: ReferenceWritableKeyPath<T, E>)  -> KPObject<T, E> {
    obj.path = path
    return obj
}

func +<T,E>(obj: KPObject<T,E>, value: E) -> T {
    var o = obj.obj
    magic(obj: &o, path: obj.path, value: value)
    return o
}

var v = UIView()

let view: UIView = v ... \.backgroundColor + .red 


((v ... \.layer.cornerRadius + 3)
    ... \.backgroundColor + .blue)
    ... \.layer.borderWidth + 6

print(v.backgroundColor)

error: the compiler is unable to type-check this expression in reasonable time; try breaking up the expression into distinct sub-expressions
dumb + .string - "Hello!" + .double - 3.14 + .array - [1, 43, 6, true]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~