JSONExperiments.swift

protocol Lookuper {
    func get<T: NSObject>(key: String) -> T?
}

struct KeyError {
    var key: String
    var desiredType: NSObject.Type
    var actualType: Any.Type
    
    func simpleDescription() -> String {
        return "key: \(key), desired: \(desiredType.className), actual: \(actualType)"
    }
}

class DictLookuper : Lookuper {
    let dict: Dictionary<String, AnyObject>
    
    var errors = KeyError[]()
    
    init(dict: Dictionary<String, AnyObject>) {
        self.dict = dict
    }
    
    func get<T: NSObject>(key: String) -> T? {
        let value: AnyObject? = dict[key]
        let typedValue = value as? T
        if !typedValue {
            println("with error!");
            errors.append(KeyError(key: key, desiredType: T.self, actualType: value.dynamicType))
        }
        return typedValue
    }
}

func With(dict: Dictionary<String, AnyObject>, block: (Lookuper) -> Void) -> KeyError[] {
    let lookuper = DictLookuper(dict: dict)
    block(lookuper)
    return lookuper.errors
}

let errors = With(jsonDict) { lookuper in
    let a: NSString? = lookuper.get("a")
    let c: NSNumber? = lookuper.get("c")
    let a_: NSNumber? = lookuper.get("a")
    let c_: NSString? = lookuper.get("c")
    println(a, c, a_, c_)
}