Swift Lens

Lens.swift

import UIKit



protocol LensProtocol {
    func lget(data: Any?) -> Any?;
    func lset(data: Any?, val: Any?) -> Any?;
    
}

struct KeyLens<KeyType> : LensProtocol where KeyType : Hashable {
    let key : KeyType;
    func lget(data: Any?) -> Any? {

        if let d = data as? Dictionary<KeyType, Any> {
            return d[key];
        } else {
            print("oops!")
        }
        return nil;
    }
    func lset(data: Any?, val: Any?) -> Any?{
        if let d = data as? Dictionary<KeyType, Any> {
            var newd = d;
            newd[key] = val;
            return newd;
        } else {
            print("oops!")
        }
        return nil;
    }
}

struct IndexLens : LensProtocol {
    let index : Int;
    func lget(data: Any?) -> Any? {
        
        if let arr = data as? Array<Any?> {
            return arr[index];
        } else {
            print("oops!")
        }
        return nil;
    }
    func lset(data: Any?, val: Any?) -> Any?{
        if let arr = data as? Array<Any?> {
            var newArr = arr;
            newArr[index] = val;
            return newArr;
        } else {
            print("oops!")
        }
        return nil;
    }
}


struct ComposeLens : LensProtocol{
    let lenses: [LensProtocol];
    func lget(data: Any?) -> Any? {
        var retData = data;
        for lens in lenses {
            retData = lens.lget(data: retData);
        }
        return retData;
    }
    func lset(data: Any?, val: Any?) -> Any? {
        var retData = data;
        var original_data : [(LensProtocol, Any?)] = [];
        let outer_lenses = lenses[0..<lenses.count - 1];
        for lens in outer_lenses{
            original_data.append((lens, retData));
            retData = lens.lget(data: retData);
        }
        
        retData = lenses.last?.lset(data: retData, val:val);
        for (lens, original_datum) in original_data.reversed(){
            retData = lens.lset(data: original_datum, val: retData);
        }
        return retData;

    }
};

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


print (todos);

let first_home_todo_completed_lens = ComposeLens(lenses: [KeyLens(key: "home"),
                                                IndexLens(index: 0),
                                                KeyLens(key: "completed")])
print(first_home_todo_completed_lens.lset(data: todos, val: true   ));