protocol Specification {
associatedtype T
func isSatisfied(item : T) -> Bool
}
protocol AndSpecification {
associatedtype T
var arrayOfSpecs : [Specification] {set}
}
The aforementioned code gives the following error
Protocol 'Specification' can only be used as a generic constraint
because it has Self or associated type requirements
I understand that there is this problem because of associated types, which can be solved if I add a where clause. But I am at loss to understand where and how should I use this where clause
You think you need a where clause because you want the array to contain Specifications all with the same T, right? Well, protocols with associated types can't do that either!
What you can do is to have the array contain specifications all of the same type:
protocol AndSpecification {
associatedtype SpecificationType : Specification
var arrayOfSpecs : [SpecificationType] { get }
}
If you really like your T associated type, you can add one nonetheless:
protocol AndSpecification {
associatedtype T
associatedtype SpecificationType : Specification where SpecificationType.T == T
var arrayOfSpecs : [SpecificationType] { get }
}
But this is quite redundant, because you could just say AndSpecification.SpecificationType.T instead.
Related
I'm trying to create an extension on Set that uses a where clause so that it only works on a struct I have that accepts a generic. But I keep running into errors about it the extension wanting the generic to be defined in the struct
In this example I'm getting the following error, with the compiler hint suggesting I use <Any>: Reference to generic type 'Test' requires arguments in <...>
struct Test<T> {
var value : T
func printIt() {
print("value")
}
}
extension Set where Element == Test {
}
However, when I use <Any> in the struct, I'm getting this error: Same-type constraint type 'Test' does not conform to required protocol 'Equatable'
extension Set where Element == Test<Any> {
}
Any suggestions on how to get the where clause to accept the Test struct for any type I'm using in the generic?
Thanks for your help
This is a limitation of Swift's type system. There's no way to talk about generic types without concrete type parameters, even when those type parameters are unrelated to the use of the type. For this particular situation (an extension for all possible type parameters), I don't believe there's any deep problem stopping this. It's a simpler version of parameterized extensions, which is a desired feature. It's just not supported (though there is an implementation in progress).
The standard way to address this today is with a protocol.
First, a little cleanup that's not related to your question (and you probably already know). Your example requires Test to be Hashable:
struct Test<T: Hashable>: Hashable {
var value : T
func printIt() {
print("value")
}
}
Make a protocol that requires whatever pieces you want for the extension, and make Test conform:
protocol PrintItable {
func printIt()
}
extension Test: PrintItable {}
And then use the protocol rather than the type:
extension Set where Element: PrintItable {
func printAll() {
for item in self { item.printIt() }
}
}
let s: Set<Test<Int>> = [Test(value: 1)]
s.printAll() // value
Just one more note on the error messages you're getting. The first error, asking you to add Any is really just complaining that Swift can't talk about unparameterized generics, and suggesting it's fallback type when it doesn't know what type to suggests: Any.
But Set<Any> isn't "any kind of Set." It's a Set where Element == Any. So Any has to be Hashable, which isn't possible. And a Set<Int> isn't a subtype of Set<Any>. There' completely different types. So the errors are a little confusing and take you down an unhelpful road.
This is not possible. The where clause requires a specific data type and simply passing a Test will not work unless I specify something more concrete like Test<String>.
Thank you to Joakim and flanker for answering the question in the comments
If you want to add extension for Set with where clause your Test must confirm to Hashable protocol.
Your Struct must look like this.
struct Test<T: Hashable> : Hashable {
var value : T
func printIt() {
print("value")
}
func hash(into hasher: inout Hasher) {
hasher.combine(value.hashValue)
}
}
So you can't use Any for your extension you must specify type that confirm to Hashable protocol.
I need to work with objects that not only conform to a protocol, but also expose the type of a second protocol which they conform to. (This is for use with NSXPCConnection, where you must configurable not only an object to proxy, but also tell it which protocol should be exposed on that proxied object.)
I tried something like:
protocol Conformer where Self : Conformer.P {
associatedtype P : Protocol
static var interface : P {get}
init(info: String)
}
func exposeOverXPC<T:Conformer>(_ _: T.Type) {
let c : NSXPCConnection = …
c.exportedInterface = NSXPCInterface(with: T.interface)
c.exportedObject = T(info:"foo")
}
But it results in an error:
Associated type 'P' can only be used with a concrete type or generic parameter base
Specifically I want exposeOverXPC to only accept objects that:
Are initializeable in a particular way
Have a static property interface which references a protocol
Are themselves conformant to said interface
It's the last step that I'm getting stuck on, is there any way I can accomplish it?
You cannot restrict who conforms to a protocol, that goes against the concept of having protocols in the first place if you think about it. However you can use composed types, Swift4 feature, in your generic parameter in exposeOverXPC.
protocol Interface {
}
protocol XPCExposable {
associatedtype P: Interface
init(info: String)
static var interface: P { get }
}
func exposeOverXPC<T: XPCExposable & Interface>(_ : T.Type) {
// 1: T is initializeable in a particular way
// 2: Has a static property interface which references a protocol
// 3: Are themselves conformant to said interface
}
Yes this constraints T to conform to Interface and not P, your best bet is to make exposeOverXPC private/internal and provide APIs that expect Interface subtype. wherever you have access to the Interface subtype expose that api. e.g:
Solution 1
protocol InterfaceSubType: Interface {
fun test()
}
/// Create as many `API`s as the number of `Interface` subtypes you have.
func exposeOverXPC<T: XPCExposable & InterfaceSubType>(_ : T.Type) {
exposeOverXPC(T.self)
}
/// set to private, you only want to expose the APIs with `Interface` subtype.
private func exposeOverXPC<T: XPCExposable & Interface>(_ : T.Type) {
// Impl.
}
Solution 2
An alternative solution to have a function with parameters whose type is the associated type is to add that api (as static function if you wish) by extending the protocol. You must know all the expected subtypes of Interface in this extension.
extension XPCExposable {
static func exposeOverXPC<T>(_ interface: P, _ xpcType: T.Type) where T: XPCExposable {
// Expected subtype Interface
if let subInterface = interface as? InterfaceSubType {
subInterface.test()
}
// Other subtypes here.
}
}
Can be called as:
let impl = Impl(info: "")
Impl.exposeOverXPC(Impl.interface, Impl.self)
Its an extension on XPCExposable so you constrain the caller to be a conformer and the parameter requires XPCExposable.P so you're all set.
Downsides of this solution are:
You have two parameters instead of one.
It uses if conditions, I don't know if thats worth mentioning as downside other than that I'd like to push the first solution as favourite.
I want to define a protocol that defines a variable "sequence" that is of type "Collection". I want that, because i have several conforming types, that will all hold an array of different types. Example:
protocol BioSequence {
associatedtype T
var sequence: Collection { get }
// Will also implement conformance to Collection and redirect all Collection functions to the property "sequence". This is because my types that conform to BioSequence are just wrappers for an array of Nucleotide or AminoAcid with some extra functionality
}
struct DNASequence: BioSequence {
// Will hold "sequence" of type [Nucleotide]
}
struct AminoSequence: BioSequence {
// Will hold "sequence" of type [AminoAcid]
}
Why do i want this? Because i need to implement the conformance to "Collection" only once in BioSequence and all conforming type inherit it automatically. Plus i can freely add extra functionality on the conforming types.
Now, when i try this like the code above, the compiler says: "Protocol Collection can only be used as a generic constraint". Yes, i googled what this error means, but how can i actually fix it to make my code work, like i want. Or is it not even possible to do what i want?
Thank you.
You can easily achieve this by using an associatedtype in your protocol, which can be constrained to Collection, allowing conforming types to satisfy the requirement with a concrete type when they adopt the protocol.
For example:
protocol CollectionWrapper : Collection {
associatedtype Base : Collection
var base: Base { get }
}
extension CollectionWrapper {
var startIndex: Base.Index {
return base.startIndex
}
var endIndex: Base.Index {
return base.endIndex
}
func index(after i: Base.Index) -> Base.Index {
return base.index(after: i)
}
subscript(index: Base.Index) -> Base.Iterator.Element {
return base[index]
}
// Note that Collection has default implementations for the rest of the
// requirements. You may want to explicitly implement them and forward them to
// the base collection though, as it's possible the base collection implements
// them in a more efficient manner (e.g being non random access and having
// a stored count).
}
// S adopts CollectionWrapper and satisfies the 'Base' associatedtype with [String].
struct S: CollectionWrapper {
var base: [String]
}
let s = S(base: ["foo", "bar", "baz"])
print(s[1]) // "bar"
Regarding your comment:
If I want to use this like that: let a: CollectionWrapper = S() [...] this leaves me with "Protocol can only be used as a generic constraint" again.
The problem is that you cannot currently talk in terms of a protocol with associated type requirements, as the types which are used to satisfy those requirements are unknown to the compiler (this isn't a technical limitation though). You can solve this by using Swift's AnyCollection type eraser to wrap an arbitrary Collection with a given element type.
For example:
let a = AnyCollection(S(base: ["foo", "bar", "baz"]))
If you need to work with additional protocol requirements from CollectionWrapper, you'll have to implement your own type eraser to do so. See Rob's answer here for an idea of how to go about this.
Suppose for example we're talking about elements of type Int (but the question still applies to any type)
I have some functionality which needs to loop over a sequence of Ints. But I don't care if behind the scenes this sequence is implemented as an Array, or a Set or any other exotic kind of structure, the only requirement is that we can loop over them.
Swift standard library defines the protocol SequenceType as "A type that can be iterated with a for...in loop". So my instinct is to define a protocol like this:
protocol HasSequenceOfInts {
var seq : SequenceType<Int> { get }
}
But this doesn't work. SequenceType is not a generic type which can be specialized, it's a protocol. Any particular SequenceType does have a specific type of element, but it's only available as an associated type: SequenceType.Generator.Element
So the question is:
How can we define a protocol which requires a specific type of sequence?
Here's some other things I've tried and why they aren't right:
Fail 1
protocol HasSequenceOfInts {
var seq : SequenceType { get }
}
Protocol 'SequenceType' can only be used as a generic constraint
because it has Self or associated type requirements
Fail 2
protocol HasSequenceOfInts {
var seq : AnySequence<Int> { get }
}
class ArrayOfInts : HasSequenceOfInts {
var seq : [Int] = [0,1,2]
}
I thought this one would work, but when I tried a concrete implementation using an Array we get
Type 'ArrayOfInts' does not conform to protocol 'HasSequenceOfInts'
This is because Array is not AnySequence (to my surprise... my expectation was that AnySequence would match any sequence of Ints)
Fail 3
protocol HasSequenceOfInts {
typealias S : SequenceType
var seq : S { get }
}
Compiles, but there's no obligation that the elements of the sequence seq have type Int
Fail 4
protocol HasSequenceOfInts {
var seq : SequenceType where S.Generator.Element == Int
}
Can't use a where clause there
So now I'm totally out of ideas. I can easily just make my protocol require an Array of Int, but then I'm restricting the implementation for no good reason, and that feels very un-swift.
Update Success
See answer from #rob-napier which explains things very well. My Fail 2 was pretty close. Using AnySequence can work, but in your conforming class you need to make sure you convert from whatever kind of sequence you're using to AnySequence. For example:
protocol HasSequenceOfInts {
var seq : AnySequence<Int> { get }
}
class ArrayOfInts : HasSequenceOfInts {
var _seq : [Int] = [0,1,2]
var seq : AnySequence<Int> {
get {
return AnySequence(self._seq)
}
}
}
There are two sides to this problem:
Accepting an arbitrary sequence of ints
Returning or storing an arbitrary sequence of ints
In the first case, the answer is to use generics. For example:
func iterateOverInts<SeqInt: SequenceType where SeqInt.Generator.Element == Int>(xs: SeqInt) {
for x in xs {
print(x)
}
}
In the second case, you need a type-eraser. A type-eraser is a wrapper that hides the actual type of some underlying implementation and presents only the interface. Swift has several of them in stdlib, mostly prefixed with the word Any. In this case you want AnySequence.
func doubles(xs: [Int]) -> AnySequence<Int> {
return AnySequence( xs.lazy.map { $0 * 2 } )
}
For more on AnySequence and type-erasers in general, see A Little Respect for AnySequence.
If you need it in protocol form (usually you don't; you just need to use a generic as in iterateOverInts), the type eraser is also the tool there:
protocol HasSequenceOfInts {
var seq : AnySequence<Int> { get }
}
But seq must return AnySequence<Int>. It can't return [Int].
There is one more layer deeper you can take this, but sometimes it creates more trouble than it solves. You could define:
protocol HasSequenceOfInts {
typealias SeqInt : IntegerType
var seq: SeqInt { get }
}
But now HasSequenceOfInts has a typealias with all the limitations that implies. SeqInt could be any kind of IntegerType (not just Int), so looks just like a constrained SequenceType, and will generally need its own type eraser. So occasionally this technique is useful, but in your specific case it just gets you basically back where you started. You can't constrain SeqInt to Int here. It has to be to a protocol (of course you could invent a protocol and make Int the only conforming type, but that doesn't change much).
BTW, regarding type-erasers, as you can probably see they're very mechanical. They're just a box that forwards to something else. That suggests that in the future the compiler will be able to auto-generate these type-erasers for us. The compiler has fixed other boxing problems for us over time. For instance, you used to have to create a Box class to hold enums that had generic associated values. Now that's done semi-automatically with indirect. We could imagine a similar mechanism being added to automatically create AnySequence when it's required by the compiler. So I don't think this is a deep "Swift's design doesn't allow it." I think it's just "the Swift compiler doesn't handle it yet."
(Tested and working in Swift 4, which introduces the associatedtype constraints needed for this)
Declare your original protocol that things will conform to:
protocol HasSequenceOfInts {
associatedType IntSequence : Sequence where IntSequence.Element == Int
var seq : IntSequence { get }
}
Now, you can just write
class ArrayOfInts : HasSequenceOfInts {
var seq : [Int] = [0,1,2]
}
like you always wanted.
However, if you try to make an array of type [HasSequenceOfInts], or assign it to a variable (or basically do anything with it), you'll get an error that says
Protocol 'HasSequenceOfInts' can only be used as a generic constraint because it has Self or associated type requirements
Now comes the fun part.
We will create another protocol HasSequenceOfInts_ (feel free to choose a more descriptive name) which will not have associated type requirements, and will automatically be conformed to by HasSequenceOfInts:
protocol HasSequenceOfInts: HasSequenceOfInts_ {
associatedType IntSequence : Sequence where IntSequence.Element == Int
var seq : IntSequence { get }
}
protocol HasSequenceOfInts_ {
var seq : AnySequence<Int> { get }
}
extension HasSequenceOfInts_ where Self : HasSequenceOfInts {
var seq_ : AnySequence<Int> {
return AnySequence(seq)
}
}
Note that you never need to need to explicitly conform to HasSequenceOfInts_ , because HasSequenceOfInts already conforms to it, and you get a full implementation for free from the extension.
Now, if you need to make an array or assign an instance of something conforming to this protocol to a variable, use HasSequenceOfInts_ as the type instead of HasSequenceOfInts, and access the seq_ property (note: since function overloading is allowed, if you made a function seq() instead of an instance variable, you could give it the same name and it would work):
let a: HasSequenceOfInts_ = ArrayOfInts()
a.seq_
This needs a bit more setup than the accepted answer, but means you don't have to remember to wrap your return value in AnySequence(...) in every type where you implement the protocol.
I believe you need to drop the requirement for it to only be Int's and work around it with generics:
protocol HasSequence {
typealias S : SequenceType
var seq : S { get }
}
struct A : HasSequence {
var seq = [1, 2, 3]
}
struct B : HasSequence {
var seq : Set<String> = ["a", "b", "c"]
}
func printSum<T : HasSequence where T.S.Generator.Element == Int>(t : T) {
print(t.seq.reduce(0, combine: +))
}
printSum(A())
printSum(B()) // Error: B.S.Generator.Element != Int
In Swift's current state, you can't do exactly what you want, maybe in the future though.
it is very specific example on request of Daniel Howard
1) type conforming to SequenceType protocol could be almost any sequence, even though Array or Set are both conforming to SequenceType protocol, most of their functionality comes from inheritance on CollectionType (which conforms to SequenceType)
Daniel, try this simple example in your Playground
import Foundation
public struct RandomIntGenerator: GeneratorType, SequenceType {
public func next() -> Int? {
return random()
}
public func nextValue() -> Int {
return next()!
}
public func generate() -> RandomIntGenerator {
return self
}
}
let rs = RandomIntGenerator()
for r in rs {
print(r)
}
As you can see, it conforms to SequenceType protocol and produce infinite stream of Int numbers. Before you will try to implement something, you have to answer yourself few questions
can i reuse some functionality, which is available 'for free' in standard Swift library?
am i trying to mimic some functionality which is not supported by Swift? Swift is not C++, Swift is not ObjectiveC ... and lot of constructions we used to use before Swift has no equivalent in Swift.
Define your question in such way that we can understand you requirements
are you looking for something like this?
protocol P {
typealias Type: SequenceType
var value: Type { get set }
}
extension P {
func foo() {
for v in value {
dump(v)
}
}
}
struct S<T: CollectionType>: P {
typealias Type = T
var value: Type
}
var s = S(value: [Int]())
s.value.append(1)
s.value.append(2)
s.foo()
/*
- 1
- 2
*/
let set: Set<String> = ["alfa", "beta", "gama"]
let s2 = S(value: set)
s2.foo()
/*
- beta
- alfa
- gama
*/
// !!!! WARNING !!!
// this is NOT possible
s = s2
// error: cannot assign value of type 'S<Set<String>>' to type 'S<[Int]>' (aka 'S<Array<Int>>')
I'm trying to use Protocol-Oriented Pgrogramming for model layer in my application.
I've started with defining two protocols:
protocol ParseConvertible {
func toParseObject() -> PFObject?
}
protocol HealthKitInitializable {
init?(sample: HKSample)
}
And after implementing first model which conforms to both I've noticed that another model will be basically similar so I wanted to create protocol inheritance with new one:
protocol BasicModel: HealthKitInitializable, ParseConvertible {
var value: AnyObject { get set }
}
A you can see this protocol has one additional thing which is value but I want this value to be type independent... Right now I have models which use Double but who knows what may show up in future. If I leave this with AnyObject I'm sentenced to casting everything I want to use it and if I declare it as Double there's no sense in calling this BasicModel but rather BasicDoubleModel or similar.
Do you have some hints how to achieve this? Or maybe I'm trying to solve this the wrong way?
You probably want to define a protocol with an "associated type",
this is roughly similar to generic types.
From "Associated Types" in the Swift book:
When defining a protocol, it is sometimes useful to declare one or
more associated types as part of the protocol’s definition. An
associated type gives a placeholder name (or alias) to a type that is
used as part of the protocol. The actual type to use for that
associated type is not specified until the protocol is adopted.
Associated types are specified with the typealias keyword.
In your case:
protocol BasicModel: HealthKitInitializable, ParseConvertible {
typealias ValueType
var value: ValueType { get set }
}
Then classes with different types for the value property can
conform to the protocol:
class A : BasicModel {
var value : Int
func toParseObject() -> PFObject? { ... }
required init?(sample: HKSample) { ... }
}
class B : BasicModel {
var value : Double
func toParseObject() -> PFObject? { ... }
required init?(sample: HKSample) { ... }
}
For Swift 2.2/Xcode 7.3 and later, replace typealias in the
protocol definition by associatedtype.