I have a simple protocol with an associated type, and a protocol extension that returns an array of this type.
protocol Foo {
associatedtype Unit
}
extension Foo {
var allTheFoos: [Unit] {
return []
}
}
I then have a struct which returns some Foo in a computed property, and another computed property that returns the allTheFoos array.
struct FakeFoo: Foo {
typealias Unit = Int
}
struct FooFactory {
var myFoo: some Foo {
return FakeFoo()
}
/* WHICH RETURN TYPE WILL
PLEASE THE SWIFT GODS?!
*/
var allTheFoos: [Foo.Unit] {
return myFoo.allTheFoos
}
}
The return type of allTheFoos matches Xcode's autocomplete type suggestion for the myFoo.allTheFoos call, but understandably, this yields a:
// var allTheFoos: [Foo.Unit] {}
ERROR: Associated type 'Unit' can only be used with a concrete type or generic parameter base
My question is: What return type will make Xcode happy?
Below are my attempts, and their corresponding errors
// var allTheFoos: [some Foo.Unit] {}
ERROR: 'some' types are only implemented for the declared type of properties and subscripts and the return type of functions
// func allTheFoos() -> some [Foo.Unit]
ERROR: Associated type 'Unit' can only be used with a concrete type or generic parameter base
// func allTheFoos<U: Foo.Unit>() -> [U]
ERROR: Associated type 'Unit' can only be used with a concrete type or generic parameter base
ERROR: Cannot convert return expression of type '[(some Foo).Unit]' to return type '[U]'
// func allTheFoos<U>() -> [U] where U: (some Foo).Unit
ERROR: 'some' types are only implemented for the declared type of properties and subscripts and the return type of functions
FYI: The reason I'm doing this in a computed property in the first place is to keep things clean in some SwiftUI code.
Thanks for any help you can give!
=========== UPDATE ===========
I missed some important stuff in my sample code, so to give some context: the code is used in a unit conversion app, so something that can turn Celsius into Kelvin, Kg into lbs, and anything else into anything else.
protocol Unit: Equatable {
var suffix: String { get }
}
struct Value<UnitType: Unit> {
let amount: Double
let unit: UnitType
var description: String {
let formatted = String(format: "%.2f", amount)
return "\(formatted)\(unit.suffix)"
}
}
Value is constrained to a unit type, so that it's not possible to convert Celsius into Litres.
Therefore, we have a Conversion protocol that stores all similar units together:
protocol Conversion {
associatedtype UnitType: Unit
var allUnits: [UnitType] { get }
func convert(value: Value<UnitType>, to unit: UnitType) -> Value<UnitType>
}
extension Conversion {
func allConversions(for value: Value<UnitType>) -> [Value<UnitType>] {
let units = self.allUnits.filter { $0 != value.unit }
return units.map { convert(value: value, to: $0) }
}
}
So an example of a conversion for Temperature would be:
struct Temperature: Conversion {
enum Units: String, Unit, CaseIterable {
case celsius, farenheit, kelvin
var suffix: String {
switch self {
case .celsius: return "˚C"
case .farenheit: return "˚F"
case .kelvin: return "K"
}
}
}
var allUnits: [Units] { return Units.allCases }
func convert(value: Value<Units>, to unit: Units) -> Value<Units> {
/* ... */
}
}
Finally, the actual app code where the problem occurs is here:
struct MyApp {
var current: some Conversion {
return Temperature()
}
// ERROR: Associated type 'UnitType' can only be used with a concrete type or generic parameter base
var allConversions: [Value<Conversion.UnitType>] {
// This value is grabbed from the UI
let amount = 100.0
let unit = current.allUnits.first!
let value = Value(amount: amount, unit: unit)
return current.allConversions(for: value)
}
}
Looking at how you've implemented AnyValue, I think what you want here is just:
var allConversions: [String] {
let units = self.allUnits.filter { $0 != value.unit }
return units.map { convert(value: value, to: $0) }.description
}
Or something like that. All the algorithms that match what you're describing are just "conversion -> string." If that's the case, all you really want is CustomStringConvertible.
Managed to solve this issue using some Type Erasure:
struct AnyValue {
let description: String
init<U: Unit>(_ value: Value<U>) {
self.description = value.description
}
}
allowing for:
var allConversions: [AnyValue] {
// This value is grabbed from the UI
let amount = 100.0
let unit = current.allUnits.first!
let value = Value(amount: amount, unit: unit)
return current.allConversions(for: value).map(AnyValue.init)
}
However, this feels like a clunky solution (and one that opaque return types was introduced to avoid). Is there a better way?
Related
I have a structure that I simplified like this:
protocol Protocol {
associatedtype T
var common: T { get }
}
class Superclass<T>: Protocol {
let common: T
init(common: T) { self.common = common }
}
class IntClass<T>: Superclass<T> {
let int = 5
}
class StringClass<T>: Superclass<T> {
let string = "String"
}
class Example<P: Protocol> {
let object: P
init(object: P) { self.object = object }
func common() -> P.T { object.common }
func int() -> Int where P == IntClass<Any> { object.int }
func string() -> String where P == StringClass<Any> { object.string }
}
I would like to create objects of the generic class Example where some of them contain an object of the also generic IntClass while others have a generic StringClass object. Now I’d like to add accessors on Example for IntClass and StringClass specific properties (so I don’t have to access them directly). They would need to be constrained to the respective class. These would be int() and string() in my example.
My example doesn’t work like intended though:
let intExample = Example(object: IntClass(common: Double(1)))
// 👍 (= expected)
intExample.common() // Double 1
// 👍 (= expected)
intExample.string() // Instance method 'string()' requires the types 'IntClass<Float>' and 'StringClass<Any>' be equivalent
// 👎 (= not expected)
intExample.int() // Instance method 'int()' requires the types 'IntClass<Float>' and 'IntClass<Any>' be equivalent
I also tried:
func int() -> Int where P == IntClass<P.T> { object.int }
With these compiler complaints:
- Generic class 'Example' requires that 'P' conform to 'Protocol'
- Same-type constraint 'P' == 'IntClass<P.T>' is recursive
- Value of type 'P' has no member 'int'
And I tried:
func string<T>() -> String where P == StringClass<T> { object.string }
which, when using like intExample.string() results in Generic parameter 'T' could not be inferred (next to Instance method 'string()' requires the types 'IntClass<Double>' and 'StringClass<T>' be equivalent).
I don’t want string() to appear on an Example<IntClass> object in code completion.
Is there a syntax to accomplish what I want (anything with typealias?) or would I have to navigate around that problem?
Since the properties you are trying to access here doesn't depend on the type parameter T of IntClass or StringClass, you can write two non generic protocols HasInt and HasString:
protocol HasInt {
var int: Int { get }
}
protocol HasString {
var string: String { get }
}
extension IntClass: HasInt { }
extension StringClass: HasString { }
Then you can constrain to the protocols, and access int and string through the protocol instead:
func int() -> Int where P: HasInt { object.int }
func string() -> String where P: HasString { object.string }
I am trying to make a dispatch function which can take Payload as an argument. The Payload can be Int, String or any other type. I tried the following approaches but inside the dispatch function the payload.value is always T and not Int or String. Casting is an option but I thought that was the whole point of generics.
struct Payload<T> {
let value: T
}
func dispatch<T>(payload: Payload<T>) {
print(payload.value) // get the value as Int, String or some other type
}
let payload = Payload<Int>(value: 100)
let payload2 = Payload<String>(value: "FOO")
dispatch(payload: payload)
dispatch(payload: payload2)
As you already know T is unconstrained so it can be any type. Your only option is to cast the value to the types you are expecting. You can simply switch the value:
switch payload.value {
case let value as Int:
print("Int:", value)
case let value as String:
print("String:", value)
default:
print("some other type", payload.value)
}
Depends on what you want to get inside your dispatch, you can create a protocol and requires T to be conformed to that protocol.
protocol Printable {
func printValue() -> String
}
struct Payload<T> {
let value: T
}
func dispatch<T: Printable>(payload: Payload<T>) {
print(payload.value.printValue()) // get the value as Int, String or some other type
}
extension String: Printable {
func printValue() -> String {
return self
}
}
extension Int: Printable {
func printValue() -> String {
return "\(self)"
}
}
I am trying to achieve a design where I can have a base class that has a generic property that I can change values on by conforming to a protocol.
protocol EnumProtocol {
static var startValue: Self { get }
func nextValue() -> Self
}
enum FooState: EnumProtocol {
case foo1, foo2
static var startValue: FooState { return .foo1 }
func nextValue() -> FooState {
switch self {
case .foo1:
return .foo2
case .foo2:
return .foo1
}
}
}
enum BarState: EnumProtocol {
case bar
static var startValue: BarState { return .bar }
func nextValue() -> BarState {
return .bar
}
}
class BaseClass<T: EnumProtocol> {
var state = T.startValue
}
class FooClass: BaseClass<FooState> {
}
class BarClass: BaseClass<BarState> {
}
Is it possible to end up with a solution similar to this where the element type is unknown and the value relies on the nextValue() method.
let foo = FooClass()
let bar = BarClass()
if let test = bar as? BaseClass {
test.state = test.state.nextValue()
}
This works but BarState will be unknown in my case and a lot of classes will be subclasses of BaseClass and have different state types.
let bar = BarClass()
if let test = bar as? BaseClass<BarState> {
test.state = test.state.nextValue()
}
This is a simplified example. In my case I will get a SKNode subclass that has a state property that is an enum with a nextvalue method that have defined rules to decide what the next value will be. I am trying to have a generic implementation of this that only relies on what is returned from the nextValue method. Is there a better pattern to achieve this?
This will not work for this exact scenario because EnumProtocol can not be used as concrete type since it has a Self type requirement, however, in order to achieve this type of behavior in other cases you can create a protocol that the base class conforms to and try to cast objects to that type when you are trying to determine if an object is some subclass of that type.
Consider the following example
class Bitcoin { }
class Ethereum { }
class Wallet<T> {
var usdValue: Double = 0
}
class BitcoinWallet: Wallet<Bitcoin> {
}
class EthereumWallet: Wallet<Ethereum> {
}
let bitcoinWallet = BitcoinWallet() as Any
if let wallet = bitcoinWallet as? Wallet {
print(wallet.usdValue)
}
This will not work, due to the same error that you are referring to:
error: generic parameter 'T' could not be inferred in cast to 'Wallet<_>'
However, if you add the following protocol
protocol WalletType {
var usdValue: Double { get set }
}
and make Wallet conform to that
class Wallet<T>: WalletType
then you can cast values to that protocol and use it as expected:
if let wallet = bitcoinWallet as? WalletType {
print(wallet.usdValue)
}
I have created 2 protocols with associated types. A type conforming to Reader should be able to produce an instance of a type conforming to Value.
The layer of complexity comes from a type conforming to Manager should be able to produce a concrete Reader instance which produces a specific type of Value (either Value1 or Value2).
With my concrete implementation of Manager1 I'd like it to always produce Reader1 which in turn produces instances of Value1.
Could someone explain why
"Reader1 is not convertible to ManagedReaderType?"
When the erroneous line is changed to (for now) return nil it all compiles just fine but now I can't instantiate either Reader1 or Reader2.
The following can be pasted into a Playground to see the error:
import Foundation
protocol Value {
var value: Int { get }
}
protocol Reader {
typealias ReaderValueType: Value
func value() -> ReaderValueType
}
protocol Manager {
typealias ManagerValueType: Value
func read<ManagerReaderType: Reader where ManagerReaderType.ReaderValueType == ManagerValueType>() -> ManagerReaderType?
}
struct Value1: Value {
let value: Int = 1
}
struct Value2: Value {
let value: Int = 2
}
struct Reader1: Reader {
func value() -> Value1 {
return Value1()
}
}
struct Reader2: Reader {
func value() -> Value2 {
return Value2()
}
}
class Manager1: Manager {
typealias ManagerValueType = Value1
let v = ManagerValueType()
func read<ManagerReaderType: Reader where ManagerReaderType.ReaderValueType == ManagerValueType>() -> ManagerReaderType? {
return Reader1()// Error: "Reader1 is not convertible to ManagedReaderType?" Try swapping to return nil which does compile.
}
}
let manager = Manager1()
let v = manager.v.value
let a: Reader1? = manager.read()
a.dynamicType
The error occurs because ManagerReaderType in the read function is only a generic placeholder for any type which conforms to Reader and its ReaderValueType is equal to the one of ManagerReaderType. So the actual type of ManagerReaderType is not determined by the function itself, instead the type of the variable which gets assigned declares the type:
let manager = Manager1()
let reader1: Reader1? = manager.read() // ManagerReaderType is of type Reader1
let reader2: Reader2? = manager.read() // ManagerReaderType is of type Reader2
if you return nil it can be converted to any optional type so it always works.
As an alternative you can return a specific type of type Reader:
protocol Manager {
// this is similar to the Generator of a SequenceType which has the Element type
// but it constraints the ManagerReaderType to one specific Reader
typealias ManagerReaderType: Reader
func read() -> ManagerReaderType?
}
class Manager1: Manager {
func read() -> Reader1? {
return Reader1()
}
}
This is the best approach with protocols due to the lack of "true" generics (the following isn't supported (yet)):
// this would perfectly match your requirements
protocol Reader<T: Value> {
fun value() -> T
}
protocol Manager<T: Value> {
func read() -> Reader<T>?
}
class Manager1: Manager<Value1> {
func read() -> Reader<Value1>? {
return Reader1()
}
}
So the best workaround would be to make Reader a generic class and Reader1 and Reader2 subclass a specific generic type of it:
class Reader<T: Value> {
func value() -> T {
// or provide a dummy value
fatalError("implement me")
}
}
// a small change in the function signature
protocol Manager {
typealias ManagerValueType: Value
func read() -> Reader<ManagerValueType>?
}
class Reader1: Reader<Value1> {
override func value() -> Value1 {
return Value1()
}
}
class Reader2: Reader<Value2> {
override func value() -> Value2 {
return Value2()
}
}
class Manager1: Manager {
typealias ManagerValueType = Value1
func read() -> Reader<ManagerValueType>? {
return Reader1()
}
}
let manager = Manager1()
// you have to cast it, otherwise it is of type Reader<Value1>
let a: Reader1? = manager.read() as! Reader1?
This implementation should solve you problem, but the Readers are now reference types and a copy function should be considered.
I have a custom collection that can receive values of any type and converts them to strings. For example:
collection["key"] = 10
let value = collection["key"] // value is a String
Is there a way to do this? I tried implementing two subscript methods but Swift doesn't support write-only subscripts.
subscript(key: String) -> String {
get { ... }
}
// Doesn't compile
subscript(key: String) -> AnyObject {
set { ... }
}
You can use two different subscript implementations and disable the getter for one of them:
subscript(key: String) -> String {
get { return "howdy" } // put real implementation here
}
subscript(key: String) -> AnyObject {
get { fatalError("Oooops") }
set { }
}
However, this still leaves open the question of how to distinguish between these two subscript calls in context. It would be better to give them different signatures through their external parameter names:
subscript(key: String) -> String {
get { return "howdy" } // put real implementation here
}
subscript(# any: String) -> AnyObject {
get { fatalError("Oooops") }
set { }
}
And here's how to use it:
let m = MyClass()
m[any:"thing"] = 1
println(m["thing"]) // "1", presumably
Define subscript to return AnyObject (or Any as needed) and at the point you use the getter cast the result to String. You may already need to deal with subscript returning an optional so the coercion is just all part of extracting your desired value.
if let value = collection["key"] as String { ... }
else {...}
You could also define your own type and make it conform to the IntegerLiteralConvertible and the StringLiteralConvertible protocols.
Technically you could also write an extension for String to make it conform to IntegerLiteralConvertible but that might get confusing, since it will be available in your entire project.
I was facing a similar problem here and I solved it using a generic type for my variable and returning the type I want on its getter. You can try doing something like this:
class StorageClass {
private var _value: String?
public var value: Any? {
set {
if let num = newValue as? Int {
self._value = String(format: "%d",num)
}
}
get {
return self._value
}
}
}
By doing this, it is possible to do something like:
var storage = StorageClass()
storage.value = 10 /* setting value as an Integer */
let aString = storage.value as! String /* receiving a String value back */