swift_lens_playground.swift

public struct Lens<ParentType, ChildType> {
    public let get: (ParentType) -> ChildType
    public let set: (ParentType, ChildType) -> ParentType
}

public struct KeyLens<KeyType, ValueType> where KeyType: Hashable {
    public typealias ParentType = [KeyType: ValueType]
    
    public static func ForKey(_ key: KeyType) -> Lens<ParentType, ValueType> {
        let lens = Lens<ParentType, ValueType>(
            get: { dict in
                return dict[key]!
            },
            set: { (dict, value) in
                var newDict = dict
                newDict[key] = value
                return newDict
            }
        )
        return lens
    }
}

public struct IndexLens<ValueType> {
    public typealias ArrayType = [ValueType]
    
    public static func ForIndex(_ index: Int) -> Lens<ArrayType, ValueType> {
        let lens = Lens<ArrayType, ValueType>(
            get: { $0[index] },
            set: { (arr, value) in arr.enumerated().map{ (i, element) in index == i ? value : element } }
        )
        return lens
    }
}

precedencegroup ComposePrecedence {
    associativity: left
}

infix operator ~> : ComposePrecedence

func ~> <ParentType, MiddleType, ChildType>(lhs: Lens<ParentType, MiddleType>, rhs: Lens<MiddleType, ChildType>) -> Lens<ParentType, ChildType> {
    
    return Lens(
        get: { parent in
            let view = lhs.get(parent)
            let child = rhs.get(view)
            return child
        },
        set: { (parent, child) in
            let initialView = lhs.get(parent)
            let updatedView = rhs.set(initialView, child)
            return lhs.set(parent, updatedView)
        }
    )
}

let todos = ["home": [ ["completed": false, "description": "cook"],
                       ["completed": false, "description": "clean"]],
             "work": [ ["completed": false, "description": "code"],
                       ["completed": true, "description": "drink coffee"]]
];

print("starting todos:\n\(todos)\n")
let homeLens = KeyLens<String, [[String: Any]]>.ForKey("home")
let firstLens = IndexLens<[String: Any]>.ForIndex(0)
let completedLens = KeyLens<String, Any>.ForKey("completed")
let firstHomeCompletedLens = homeLens ~> firstLens ~> completedLens
print("first home item completed status: \(firstHomeCompletedLens.get(todos))\n")
print("after setting:\n\(firstHomeCompletedLens.set(todos, true))\n")