So I'm working on an sql database abstraction layer in swift and I want to write it as safe as possible. By that I mean I would prefer to make it impossible for anybody to do anything illegal in the database using my library.
One thing I'm not fully able to solve yet is how to make the conversion from Swift types to SQL data types 100% safe. So I'll explain my current implementation and its flaws:
So in SQL, many data types have parameters. For example, when you want to define a column as a VARCHAR, you need to give it a parameter that represents the max length of the VARCHAR, like VARCHAR(255). You could say that VARCHAR is one of SQL's primitive data-types and VARCHAR(255) is a more specified data-type. To represent that in a type-safe way in swift I created two protocols:
public protocol PrimitiveDataType {
associatedtype Options = Void
}
public protocol SpecifiedDataType {
associatedtype Primitive: PrimitiveDataType
static var options: Primitive.Options { get }
static func from(primitive: Primitive) -> Self
var primitive: Primitive { get }
}
So here's an example of a PrimitiveDataType:
extension String: PrimitiveDataType {
public enum DatabaseStringType {
case char(length: Int), varchar(limit: Int), text
}
public typealias Options = DatabaseStringType
}
And here's an example of a SpecifiedDataType:
struct Name {
let value: String
init?(_ value: String) {
guard case 0...50 = value.characters.count else {
return nil
}
self.value = value
}
}
extension Name: SpecifiedDataType {
static let options = String.DatabaseStringType.varchar(limit: 50)
static func from(primitive: String) -> Name {
return Name(primitive)!
}
var primitive: String {
return value
}
}
Now I can use the information elsewhere in the library to know what kind of database-column is expected whenever a Name type is requested. This gives my library the power to make sure the database columns are always of the correct type.
I haven't written all that code yet, but it might look something like this:
func createColumn<SDT: SpecifiedDataType>(ofType type: SDT) -> String {
switch SDT.options {
case let options as String.DatabaseStringType:
switch options {
case .text:
return "TEXT"
case .char(let length):
return "CHAR(\(length))"
case .varchar(let limit):
return "VARCHAR(\(limit))"
}
case let length as UInt32.Options:
// return something like UNSIGNED INTEGER(\(length)) or whatever
// case etcetera
}
}
There is just one small problem with this. The set of valid primitive data-types is limited, but the set of possible implementations of the PrimitiveDataType protocol is unlimited, because I can't stop anybody from creating their own implementation of this protocol.
So for this reason it would be better to use some kind of enum instead of a number of implementations of a protocol. Because nobody can extend an enum that I create, so I can keep the options limited to whatever I define as valid types. However, an enum would create another problem that doesn't exist in my current implementation.
You see in my current implementation, whenever someone implements SpecifiedDataType with their options defined like so:
static let options = String.DatabaseStringType.varchar(limit: 50)
I can trust that the compiler will force them to make their type compatible with string by implementing the from(primitive: String) method and the primitive: String (computed) variable. AFAIK, if I switch the protocol for an enum I lose this compiler-enforced type-safety.
So in summary, I want to have all of the following:
A way for a developer to declare what sql data-type they want their type to correspond with.
A way to then force that developer to actually make their type compatible with that sql data-type.
To guarantee that the developer using my library can't somehow extend the list of sql-data types, but can only use the ones that I have defined for him / her.
So far I only know how to do either the first two, but not the last one, or the last one, but not the first two. How do I get all three?
Related
I'm trying to create an extension to (non generic) String Type and add a variable of type string to return some format, but also have a constraint so that not every string can simply use it.
Can it be done using a protocol?
What are the preferred ways to go about achieving that basically.
Thanks!
Additional info:
I was considering a type constraint so perhaps something like this:
protocol Formattable {
var formatted: String { get }
}
extension String where Self: Formattable {
var formatted: String {
return self + "123".uppercased()
}
}
But I'm not sure it's even the right way to do it and also I get an error:
'where' clause cannot be applied to a non-generic top-level declaration
Maybe just create a custom class with a formatting function is better? Sorry for the confusion and hope that it makes a bit more sense.
In my Quiz app I initialize quizzes, and the providing class does not know the format of the questions before being provided them (although they are constrained by a QuestionProtocol):
public protocol QuestionProtocol {
init?(fields: [String] )
var description: String {get}
var question: String {get}
var solution: String {get}
var explainAnswer: String {get}
var answered: Int {get set}
var qa: String {get}
var qb: String {get}
var qc: String {get}
var qd: String {get}
}
And I can initialize the quizzes and return them easily enough through a method with the signature
public func initializeQuizzes<T: QuestionProtocol>(with type: T.Type, withCompletionHandler completion: ((Result<[Quiz<T>], Error>) -> Void)?)
However to provide these quizzes is expensive (an API call or a SQL retrieval) so I want to store these quizzes and retrieve them separately from a suitable function with signature
public func getNextQFromSet<T: QuestionProtocol>(with type: T.Type) -> (question: T, answers: [String])?
The problem I have is storing these questions which are of type T.
They are linked to a Quiz object:
public class Quiz<T> {
private let questions : [T]
private let name : String
init(name: String, questions: [T]) {
self.name = name
self.questions = questions
}
public func getQuestions() -> [T] {
return questions
}
func getName() -> String {
return name
}
}
So I'm able to store them as quizzes that conform to the QuestionProtocol
private var quizzes = [Quiz<QuestionProtocol>]()
But then I lose the extra information I want to store in the question.
I can store Any, but I believe that is bad practice
private var anyquizzes = [Quiz<Any>]()
Ideally I would like to store T i.e.
Quiz<T>
but that seems to be impossible in Swift.
Because these classes are in a pod they have no way of knowing about the internal workings of a Question, and are provided these at runtime hence the use of generics and the difficulties in storing these questions.
I can't think of a way to improve the design of the App (more specifically the Pod) - I want to initialize the quizzes once and once only and then run functions like getNextQFromSet() to retrieve a relevant question - which obviously depends on me knowing the type of the question (which I do not know before runtime).
For clarity here is a link to the Pod: https://github.com/stevencurtis/QuizManager
How can I store an array containing these questions without knowing the type?
To be short, I think it makes sense to remove QuestionProtocol and replace it with plain data structure struct Question.
Before I explain my point of view, I want to note that even though I looked at the pod, I still do not know all the requirements, so I might be wrong.
Let's try to have a look at the problem from design perspective instead of programming language perspective.
What is the reason of having QuestionProtocol? Could it be replaced with, let's say, object instead? Why do those properties should be polymorphic? Of course implementation details should be hidden, but hiding data is not about protocols or additional function layers, is about abstractions.
Let's convert QuestionProtocol to Question object for now and think about an abstraction. If there is a real abstraction, then there should an object that hides the data (details) and expose functions that manipulate that data. But there is no functions in Question object and it means that there is no real abstraction behind.
Finally, It means that Question entity most likely is a plain data structure with public properties and could be defined as struct Question.
Having this Question struct now, you can define quizzes as Quiz<Question> and use it to save and retrieve the data.
In addition, I think it worth to point out two things which could simplify and potentially improve design and implementation:
Why does SQLiteManager knows something about concrete question (depends on QuestionProtocol)? I think it makes sense to introduce some generic DBObject or at least plain dictionary [String: Any] which SQLiteManager would know how process and then insert. Then Repository could transform Question data structure into DBObject on some level of composition and pass it to SQLiteManager.
When using generics, in most cases there is no need to define additional type: T.Type parameter. Once generic is defined you can use it as [T], T.init, etc. If you still need a metatype (T.Type) you can get by T.self.
Hope this helps!
Update:
There is great example of Quiz app created with TDD and modular design: Quiz App. There is also a video series explaining design and creation process step by step.
How can I store an array containing these questions without knowing
the type?
To my knowledge you can't. As rraphael pointed out in his comment generics aren't resolved at runtime. Furthermore Arrays in swift are designed to hold a single type:
Specifically, you use the Array type to hold elements of a single type, the array’s Element type.
So whatever you do you'll have either an array of Any or maybe QuestionProtocol but nothing more dynamic than that : the type will be resolved at compilation time
You may be able to redesign your QuestionProtocol to suit your needs but without any information on the different types of question it's a bit difficult to help you more since it is an architecture matter.
You can use enum with associated values for describing types. For example:
struct QuestionTypeA { }
struct QuestionTypeB { }
struct QuestionTypeC { }
enum Question {
case typeA(question: QuestionTypeA)
case typeB(question: QuestionTypeB)
case typeC(question: QuestionTypeC)
}
And then:
public class Quiz {
private let questions : Question
private let name : String
...
And store an array of Quiz without generic
private var anyquizzes = [Quiz]()
You wouldn't be able to store a Quiz<T> and a Quiz<U> in the same array using either of those types. They're just not the same type.
If you have an Array<QuizProtocol>, you can just match against your known types in a switch-case statement:
var quizzes: [QuizProtocol] = ...
for quiz in quizzes {
switch quiz {
case let someQuiz as SomeQuiz:
...
case let someOtherQuiz as SomeOtherQuiz:
...
default:
... // couldn't cast to any known type; do some fallback logic
....
}
}
where SomeQuiz and SomeOtherQuiz conform to QuizProtocol (though strictly speaking, you could match against any type).
Situation
I have a two generic classes which will fetch data either from api and database, lets say APIDataSource<I, O> and DBDataSource<I, O> respectively
I will inject any of two class in view-model when creating it and view-model will use that class to fetch data it needed. I want view-model to work exactly same with both class. So I don't want different generic constraints for the classes
// sudo code
ViewModel(APIDataSource <InputModel, ResponseModel>(...))
// I want to change the datasource in future like
ViewModel(DBDataSource <InputModel, ResponseModel>(...))
To fetch data from api ResponseModel need to confirms to "Decodable" because I want to create that object from JSON. To fetch data from realm database it need to inherit from Object
Inside ViewModel I want to get response like
// sudo code
self.dataSource.request("param1", "param2")
If developer tries to fetch api data from database or vice-versa it will check for correct type and throws proper error.
Stripped out version of code for playground
Following is stripped out version of code which shows what I want to achieve or where I am stuck (casting un-constrained generic type to generic type that confirms to Decodable)
import Foundation
// Just to test functions below
class DummyModel: Decodable {
}
// Stripped out version of function which will convert json to object of type T
func decode<T:Decodable>(_ type: T.Type){
print(type)
}
// This doesn't give compilation error
// Ignore the inp
func testDecode<T:Decodable> (_ inp: T) {
decode(T.self)
}
// This gives compilation error
// Ignore the inp
func testDecode2<T>(_ inp: T){
if(T.self is Decodable){
// ??????????
// How can we cast T at runtime after checking T confirms to Decodable??
decode(T.self as! Decodable.Type)
}
}
testDecode(DummyModel())
Any help or explanation that this could not work would be appreciated. Thanks in advance :)
As #matt suggests, moving my various comments over to an answer in the form "your problem has no good solution and you need to redesign your problem."
What you're trying to do is at best fragile, and at worst impossible. Matt's approach is a good solution when you're trying to improve performance, but it breaks in surprising ways if it impacts behavior. For example:
protocol P {}
func doSomething<T>(x: T) -> String {
if x is P {
return "\(x) simple, but it's really P"
}
return "\(x) simple"
}
func doSomething<T: P>(x: T) -> String {
return "\(x) is P"
}
struct S: P {}
doSomething(x: S()) // S() is P
So that works just like we expect. But we can lose the type information this way:
func wrapper<T>(x: T) -> String {
return doSomething(x: x)
}
wrapper(x: S()) // S() simple, but it's really P!
So you can't solve this with generics.
Going back to your approach, which at least has the possibility of being robust, it's still not going to work. Swift's type system just doesn't have a way to express what you're trying to say. But I don't think you should be trying to say this anyway.
In the method that fetch data I will check type of generic type and if it confirms to "Decodable" protocol I will use it to fetch data from api else from database.
If fetching from the API vs the database represents different semantics (rather than just a performance improvement), this is very dangerous even if you could get it to work. Any part of the program can attach Decodable to any type. It can even be done in a separate module. Adding protocol conformance should never change the semantics (outwardly visible behaviors) of the program, only the performance or capabilities.
I have a generic class which will fetch data either from api or database
Perfect. If you already have a class, class inheritance makes a lot of sense here. I might build it like:
class Model {
required init(identifier: String) {}
}
class DatabaseModel {
required init(fromDatabaseWithIdentifier: String) {}
convenience init(identifier: String) { self.init(fromDatabaseWithIdentifier: identifier )}
}
class APIModel {
required init(fromAPIWithIdentifier: String) {}
convenience init(identifier: String) { self.init(fromAPIWithIdentifier: identifier )}
}
class SomeModel: DatabaseModel {
required init(fromDatabaseWithIdentifier identifier: String) {
super.init(fromDatabaseWithIdentifier: identifier)
}
}
Depending on your exact needs, you might rearrange this (and a protocol might also be workable here). But the key point is that the model knows how to fetch itself. That makes it easy to use Decodable inside the class (since it can easily use type(of: self) as the parameter).
Your needs may be different, and if you'll describe them a bit better maybe we'll come to a better solution. But it should not be based on whether something merely conforms to a protocol. In most cases that will be impossible, and if you get it working it will be fragile.
What you'd really like to do here is have two versions of testDecode, one for when T conforms to Decodable, the other for when it doesn't. You would thus overload the function testDecode so that the right one is called depending on the type of T.
Unfortunately, you can't do that, because you can't do a function overload that depends on the resolution of a generic type. But you can work around this by boxing the function inside a generic type, because you can extend the type conditionally.
Thus, just to show the architecture:
protocol P{}
struct Box<T> {
func f() {
print("it doesn't conform to P")
}
}
extension Box where T : P {
func f() {
print("it conforms to P")
}
}
struct S1:P {}
struct S2 {}
let b1 = Box<S1>()
b1.f() // "it conforms to P"
let b2 = Box<S2>()
b2.f() // "it doesn't conform to P"
This proves that the right version of f is being called, depending on whether the type that resolves the generic conforms to the protocol or not.
Ok I have a protocol called Environment
protocol Environment {
var rootURL: String {get}
}
Then two structs:
struct Production: Environment {
var rootURL = "www.api.mybackend.com/v1"
}
struct Development: Environment {
var rootURL = "www.api.mydevelopmentbackend.com/v1"
}
A settings object with a function that retrieves the environment:
class Settings {
func getEnvironment<T>() -> T where T: Environment {
let environmentRaw = self.retreiveEnvironmentFromLocalStore()
switch environmentRaw {
case 0:
return Development() as! T
case 1:
return Production() as! T
default:
return Development() as! T
}
}
func retreiveEnvironmentFromLocalStore() -> Int {
//Realm, SQLLite, Core Date, I don't really care for this example.
//Let's just say it defaults to returning 1
}
}
I really want to keep moving in a Protocol Oriented Programming direction but now when I call this function on a settings object and try to use the rootURL property the compiler complains it can't figure out the type. So, to better my understanding and to maybe find a solution:
1) Why does it care about the type if I am accessing a property defined by a protocol that at the very least it knows the returning type conforms to?
2) Structs don't have inheritance. Should I define a base class and forget about generics?
3) Can I make my getEnvironment function better? I don't like force casting, it seems like a code smell.
4) Am I even using generics correctly here?
EDIT 1:
To be clear, I want one function that returns a struct that I know will have this property.
Am I even using generics correctly here?
No, I don't think so. You are saying that getEnvironment will return T which can be any type that the client code specifies, as long as it implements Environment. However, the implementation of the method doesn't do this. It will only return two kinds of Environments, and which type it returns is not determined by the client code. The type returned depends on what retreiveEnvironmentFromLocalStore returns. Hence, generics is not suitable in this case.
Since the type of environment it returns is decided by the implementation of the method, instead of the client code, you should make use of polymorphism here - make it return an Environment instead:
func getEnvironment() -> Environment {
let environmentRaw = self.retreiveEnvironmentFromLocalStore()
switch environmentRaw {
case 0:
return Development()
case 1:
return Production()
default:
return Development()
}
}
I really want to keep moving in a Protocol Oriented Programming direction
I suggest instead you try to move in a clear, understandable code direction, regardless of what the trendy orientation is.
Here's your function declaration:
func getEnvironment() -> T where T: Environment
This says that getEnvironment() will return an object of some type T, where T is deduced at compile time based on the code that calls getEnvironment().
What types could T be? It could be either Production or Development. For example, you could write:
let e: Production = Settings().getEnvironment()
This lets the compiler deduce that getEnvironment() returns a Production (which is an Environment) at this call site.
But there's a problem: getEnvironment() might try to return a Development anyway, based on the random number generator inside retreiveEnvironmentFromLocalStore. Then you'll get a crash at run time when it fails to cast Development to Production.
Why do you think getEnvironment() needs to be generic at all? Based on the code in your question, it shouldn't be.
import Foundation
protocol Environment {
var rootURL: String {get}
}
struct Production: Environment {
var rootURL = "www.api.mybackend.com/v1"
}
struct Development: Environment {
var rootURL = "www.api.mydevelopmentbackend.com/v1"
}
class Settings {
func getEnvironment() -> Environment {
let environmentRaw = self.retreiveEnvironmentFromLocalStore()
switch environmentRaw {
case 1: return Production()
default: return Development()
}
}
func retreiveEnvironmentFromLocalStore() -> Int {
return Int(arc4random())
}
}
let settings = Settings()
let e = settings.getEnvironment()
Style note: the Swift API Design Guidelines advise us to
Name functions and methods according to their side-effects
Those without side-effects should read as noun phrases, e.g. x.distance(to: y), i.successor().
Unless the methods in Settings have important side effects, better names would be environment() and rawEnvironmentFromLocalStore().
If I have two types
struct PersonFromLibraryA {
let name: String
let age: Int
}
struct PersonFromLibraryB {
let name: String
let age: Int
}
Is there a way to implicitly be able to pass an A into a method which is expecting a B?
func doSomething(withPerson person: PersonFromLibraryA) {
...
}
let person = PersonFromLibraryB(name: "Alice", age: 42)
doSomething(withPerson: person))
I'd expect this to be type safe i.e. if A or B diverge in any way whatsoever, this shouldn't compile.
I have about 20 or so of this situation (I'm mapping between layers of abstraction in a library) and am getting very tired of filling files with boilerplate mapping methods!
I suspect I already know the answer to this question, but I figure I'll ask it here just to be sure!
Agreed with rmaddy that the one way to fix this w/ type-saftey is protocol (without type-safety, you can do it with unsafeBitCast, but please don't do that). This raises a warning flag, though, that you may have over-abstracted something (or more likely: abstracted along the wrong axis) if you have many types that are identical but distinct between layers.
I believe you should think OOP as Swift is an OOP language. Change both into classes. Generally, you can do this by simply changing the word struct to class. Then create a new class.
class PersonFromLibrary {
let name: String
let age: Int
}
Now setup your other classes:
class PersonFromLibraryA: PersonFromLibrary {
}
class PersonFromLibraryB: PersonFromLibrary {
}
Now instead of using PersonFromLibraryA as the parameter type use PersonFromLibrary. Now you can pass all of the PersonFromLibraryA type things as PersonFromLibrary. The only restriction here is that if you add any unique qualities to any of the PersonFromLibraryA level classes you will have to cast it back to PersonFromLibraryA or it's corresponding class in order to use its unique properties.