Composable.swift

public protocol ComposableCache {
    
    associatedtype Key
    associatedtype Value
    
    var getC: (Key) -> Value? { get set }
    var setC: (Key, Value) -> Void { get set }
    
    init(getC: @escaping (Key) -> Value?, setC: @escaping ((Key, Value) -> Void))
    
    func get(_ key: Key) -> Value?
    func set(_ key: Key, _ value: Value)
    
}

precedencegroup ComposePredecesor {
    associativity: left
}

infix operator |&: ComposePredecesor

func |&<A: ComposableCache, B: ComposableCache>(_ left: A, _ right: B) -> A where A.Key == B.Key, A.Value == B.Value {
    return left.compose(right)
}

extension ComposableCache {
    func compose<A: ComposableCache>(_ second: A) -> Self where A.Key == Self.Key, A.Value == Self.Value {
        return Self.init(getC: { key in
            if let value = self.getC(key) {
                return value
            } else if let deepValue = second.getC(key) {
                self.setC(key, deepValue)
                return deepValue
            }
            return nil
        }, setC: {  key, value in
            self.setC(key, value)
            second.setC(key, value)
        })
    }
    
    func get(_ key: Key) -> Value? {
        return getC(key)
    }
    
    func set(_ key: Key, _ value: Value) {
        setC(key, value)
    }
    
}

struct MemoryComposableCache<K, V>: ComposableCache {
    var getC: (K) -> V?
    var setC: (K, V) -> Void
    typealias Key = K
    typealias Value = V
}

struct DiscComposableCache<K, V>: ComposableCache {
    var getC: (K) -> V?
    var setC: (K, V) -> Void
    typealias Key = K
    typealias Value = V
}

var memory = MemoryComposableCache<String, String>(getC: { key in print("getting value for \(key) in memory"); return nil}, setC: { key, value in print("setting \(value) for \(key) in memory") })
var disc = MemoryComposableCache<String, String>(getC: { key in print("getting value for \(key) in disc"); return "default"}, setC: { key, value in print("setting \(value) for \(key) in disc") })


let composed = memory |& disc
composed.get("sample key")