Showing off using errors to speed up JSON decoding

ErrorDecoding.swift

import Foundation

enum JSONDecodingError: ErrorType {
    case NilValueForProperty
    case TypeMismatch
}

extension Dictionary where Key: StringLiteralConvertible, Value: AnyObject {

    // Adds a Swift-y way to get a non-optional type from a key
    func getValue<TypedReturn>(key: Key) throws -> TypedReturn {
        guard let valueObject = self[key] else {
            throw JSONDecodingError.NilValueForProperty
        }
        guard let value = valueObject as? TypedReturn else {
            throw JSONDecodingError.TypeMismatch
        }
        return value
    }


    // Adds a Swift-y way to get a optional type from a key
    func getOptionalValue<TypedReturn>(key: Key) throws -> TypedReturn? {
        if let valueObject = self[key] where valueObject !== NSNull() {
            guard let value = valueObject as? TypedReturn else {
                throw JSONDecodingError.TypeMismatch
            }
            return value
        }
        return nil
    }

}

let values: [String: AnyObject] = [
    "a": 2,
    "b" :"Test",
    "c": 4
]

do {
    let a: Int = try values.getValue("a")
}
catch {
    error
}

do {
    let b: Int = try values.getValue("b")
}
catch {
    error
}

do {
    let q: Int = try values.getValue("q")
}
catch {
    error
}

do {
    let q: Int? = try values.getOptionalValue("q")
}
catch {
    error
}

do {
    let b: Int? = try values.getOptionalValue("b")
}
catch {
    error
}