swift-lazy-query-array.swift

struct LazyQueryArray<T> {
    var cond: ((T) -> Bool)?
    var _update: ((inout T) -> Void)?
    var base: Array<T>
    
    func resolve() -> Array<T> {
        return base.map { x in
            var x = x
            if cond?(x) ?? true {
                _update?(&x)
            }
            return x
        }
    }
    
    mutating func update(_ f: @escaping (inout T) -> Void) {
        _update = f
    }
}

extension Array {
    subscript(`where` wh: @escaping (Element) -> Bool) -> LazyQueryArray<Element> {
        get {
            return LazyQueryArray(cond: wh, _update: nil, base: self)
        }
        set {
            self = newValue.resolve()
        }
    }
}

var s = [0, 1, 2, 3, 4]
print(s)
s[`where`: { $0 % 2 == 0 }].update { (x: inout Int) -> Void in x += 100  }
print(s)



/*
[0, 1, 2, 3, 4]
[100, 1, 102, 3, 104]
*/