My first attempt to use swift generics:
extension RealmSwift.List where Element == Object {
// #deprecated use RealmSwift.List<>
func arrayo<T: Object>() -> [T] {
var res: [T] = []
for card in self {
res.append(card) <- here I got
No exact matches in call to instance method 'append'
}
return res
}
convenience init<T: Object>(objects: [T]) {
self.init()
for card in objects {
append(card)
}
}
}
what's a good way to write this adapter once and for all?
Notice the where Element. You can refer to the type of the list items using Element, so you do not need to set up another type parameter T. card is of type Element not T, so you cannot add it to the Array<T>. There is no guarantee that T and Element are equivalent so the compiler doesn't allow it. The same applies for your convenience init.
extension RealmSwift.List where Element == Object {
// #deprecated use RealmSwift.List<>
func arrayo() -> [Element] {
var res: [Element] = []
for card in self {
res.append(card) // Now you are adding an `Element` to the array of `Element` so it will work.
}
return res
}
convenience init(objects: [Element]) {
self.init()
for card in objects {
append(card)
}
}
}
But generics are not really useful here because you are constraining Element to Object already. So there is only one potential type - You could make arrayo() and the init use Object directly.
To make this useful do
extension RealmSwift.List where Elemtn: RealmCollectionValue
Related
I know that it's possible to check if one value is an instance of a potential supertype:
func isInstance<T, U>(_ instance: T, of: U.Type) -> Bool {
return instance is U
}
However, what if you want to check against an entire array? Since you can't have an array of generics, the above approach doesn't really work. What I want to do is something like:
func isInstance<T>(_ instance: T, of types: [Any.Type]) -> Bool {
return types.allSatisfy { instance is $0 }
}
However, a variable (such as $0) isn't allowed as the RHS of an is expression. Is this kind of type check possible?
I saw one type check that used Mirror(reflecting:) and superclassMirror to search the inheritance hierarchy, but that only works for an array of classes, and I need this check to work when the array contains protocols as well.
It is possible, but only for classes and #objc protocols, and only if the object conforms to NSObjectProtocol. It is not possible in the general case for Swift protocols or value types.
import ObjectiveC
func isInstance(_ instance: some NSObjectProtocol, of types: [Any.Type]) -> Bool {
return types.allSatisfy { type in
if let `class` = type as? AnyClass {
return instance.isKind(of: `class`)
} else if let `protocol` = type as AnyObject as? Protocol {
return instance.conforms(to: `protocol`)
} else {
print("Cannot type-check instance against \(type)")
return false
}
}
}
I have an extension for an Array:
extension Array where Element == [String:Double] {
func values (keyOrder : [String]) -> [[Double]] {
return self.map { element in
return (0..<keyOrder.count).compactMap {element[keyOrder[$0]]}
}
}
}
It works pretty well, but only if Dictionary Key is String and Value is Double. I can imagine this function could work exactly same way for Dictionary of any types, like [AnyHashable:Any] but I have no clue how to define header, is it possible?
One useful trick you can use in situations like this is to move the where clause from the extension declaration to the method declaration. This allows you to introduce new generic placeholders for the nested dictionary's Key and Value placeholder types:
extension Array {
func nestedValues<Key, Value>(orderedBy keys: [Key]) -> [[Value]] where Element == [Key: Value] {
return map { element in
return keys.compactMap { element[$0] }
}
}
}
Use can use value of dictionary double as an Any Type. You can try below code.
extension Array where Element == [String: Any] {
func values (keyOrder : [String]) -> [[Any]] {
return self.map { element in
return (0..<keyOrder.count).compactMap {element[keyOrder[$0]]}
}
}
}
Is something like
protocol A {
var intCollection: CollectionType<Int> { get }
}
or
protocol A {
typealias T: CollectionType where T.Generator.Element == Int
var intCollection: T
}
possible in Swift 2.1?
Update for Swift 4
Swift 4 now support this feature! read more in here
Not as a nested protocol, but it's fairly straightforward using the type erasers (the "Any" structs).
protocol A {
var intCollection: AnyRandomAccessCollection<Int> { get }
}
This is actually often quite convenient for return values because the caller usually doesn't care so much about the actual type. You just have to throw a return AnyRandomAccessCollection(resultArray) at the end of your function and it all just works. Lots of stdlib now returns Any erasers. For the return value problem, it's almost always the way I recommend. It has the nice side effect of making A concrete, so it's much easier to work with.
If you want to keep the CollectionType, then you need to restrict it at the point that you create a function that needs it. For example:
protocol A {
typealias IntCollection: CollectionType
var intCollection: IntCollection { get }
}
extension A where IntCollection.Generator.Element == Int {
func sum() -> Int {
return intCollection.reduce(0, combine: +)
}
}
This isn't ideal, since it means you can have A with the wrong kind of collection type. They just won't have a sum method. You also will find yourself repeating that "where IntCollection.Generator.Element == Int" in a surprising number of places.
In my experience, it is seldom worth this effort, and you quickly come back to Arrays (which are the dominant CollectionType anyway). But when you need it, these are the two major approaches. That's the best we have today.
You can't do this upright as in your question, and there exists several thread here on SO on the subject of using protocols as type definitions, with content that itself contains Self or associated type requirements (result: this is not allowed). See e.g. the link provided by Christik, or thread Error using associated types and generics.
Now, for you example above, you could do the following workaround, however, perhaps mimicing the behaviour you're looking for
protocol A {
typealias MyCollectionType
typealias MyElementType
func getMyCollection() -> MyCollectionType
func printMyCollectionType()
func largestValue() -> MyElementType?
}
struct B<U: Comparable, T: CollectionType where T.Generator.Element == U>: A {
typealias MyCollectionType = T
typealias MyElementType = U
var myCollection : MyCollectionType
init(coll: MyCollectionType) {
myCollection = coll
}
func getMyCollection() -> MyCollectionType {
return myCollection
}
func printMyCollectionType() {
print(myCollection.dynamicType)
}
func largestValue() -> MyElementType? {
guard var largestSoFar = myCollection.first else {
return nil
}
for item in myCollection {
if item > largestSoFar {
largestSoFar = item
}
}
return largestSoFar
}
}
So you can implement blueprints for your generic collection types in you protocol A, and implement these blueprints in the "interface type" B, which also contain the actual collection as a member property. I have taken the largestValue() method above from here.
Example usage:
/* Examples */
var myArr = B<Int, Array<Int>>(coll: [1, 2, 3])
var mySet = B<Int, Set<Int>>(coll: [10, 20, 30])
var myRange = B<Int, Range<Int>>(coll: 5...10)
var myStrArr = B<String, Array<String>>(coll: ["a", "c", "b"])
myArr.printMyCollectionType() // Array<Int>
mySet.printMyCollectionType() // Set<Int>
myRange.printMyCollectionType() // Range<Int>
myStrArr.printMyCollectionType() // Array<String>
/* generic T type constrained to protocol 'A' */
func printLargestValue<T: A>(coll: T) {
print(coll.largestValue() ?? "Empty collection")
}
printLargestValue(myArr) // 3
printLargestValue(mySet) // 30
printLargestValue(myRange) // 10
printLargestValue(myStrArr) // c
Given a struct-based generic CollectionType …
struct MyCollection<Element>: CollectionType, MyProtocol {
typealias Index = MyIndex<MyCollection>
subscript(i: Index) -> Element { … }
func generate() -> IndexingGenerator<MyCollection> {
return IndexingGenerator(self)
}
}
… how would one define an Index for it …
struct MyIndex<Collection: MyProtocol>: BidirectionalIndexType {
func predecessor() -> MyIndex { … }
func successor() -> MyIndex { … }
}
… without introducing a dependency cycle of death?
The generic nature of MyIndex is necessary because:
It should work with any type of MyProtocol.
MyProtocol references Self and thus can only be used as a type constraint.
If there were forward declarations (à la Objective-C) I would just[sic!] add one for MyIndex<MyCollection> to my MyCollection<…>. Alas, there is no such thing.
A possible concrete use case would be binary trees, such as:
indirect enum BinaryTree<Element>: CollectionType, BinaryTreeType {
typealias Index = BinaryTreeIndex<BinaryTree>
case Nil
case Node(BinaryTree, Element, BinaryTree)
subscript(i: Index) -> Element { … }
}
Which would require a stack-based Index:
struct BinaryTreeIndex<BinaryTree: BinaryTreeType>: BidirectionalIndexType {
let stack: [BinaryTree]
func predecessor() -> BinaryTreeIndex { … }
func successor() -> BinaryTreeIndex { … }
}
One cannot (yet?) nest structs inside generic structs in Swift.
Otherwise I'd just move BinaryTreeIndex<…> inside BinaryTree<…>.
Also I'd prefer to have one generic BinaryTreeIndex,
which'd then work with any type of BinaryTreeType.
You cannot nest structs inside structs because they are value types. They aren’t pointers to an object, instead they hold their properties right there in the variable. Think about if a struct contained itself, what would its memory layout look like?
Forward declarations work in Objective-C because they are then used as pointers. This is why the indirect keyword was added to enums - it tells the compiler to add a level of indirection via a pointer.
In theory the same keyword could be added to structs, but it wouldn’t make much sense. You could do what indirect does by hand instead though, with a class box:
// turns any type T into a reference type
final class Box<T> {
let unbox: T
init(_ x: T) { unbox = x }
}
You could the use this to box up a struct to create, e.g., a linked list:
struct ListNode<T> {
var box: Box<(element: T, next: ListNode<T>)>?
func cons(x: T) -> ListNode<T> {
return ListNode(node: Box(element: x, next: self))
}
init() { box = nil }
init(node: Box<(element: T, next: ListNode<T>)>?)
{ box = node }
}
let nodes = ListNode().cons(1).cons(2).cons(3)
nodes.box?.unbox.element // first element
nodes.box?.unbox.next.box?.unbox.element // second element
You could turn this node directly into a collection, by conforming it to both ForwardIndexType and CollectionType, but this isn’t a good idea.
For example, they need very different implementations of ==:
the index needs to know if two indices from the same list are at the same position. It does not need the elements to conform to Equatable.
The collection needs to compare two different collections to see if they hold the same elements. It does need the elements to conform to Equatable i.e.:
func == <T where T: Equatable>(lhs: List<T>, rhs: List<T>) -> Bool {
// once the List conforms to at least SequenceType:
return lhs.elementsEqual(rhs)
}
Better to wrap it in two specific types. This is “free” – the wrappers have no overhead, just help you build the right behaviours more easily:
struct ListIndex<T>: ForwardIndexType {
let node: ListNode<T>
func successor() -> ListIndex<T> {
guard let next = node.box?.unbox.next
else { fatalError("attempt to advance past end") }
return ListIndex(node: next)
}
}
func == <T>(lhs: ListIndex<T>, rhs: ListIndex<T>) -> Bool {
switch (lhs.node.box, rhs.node.box) {
case (nil,nil): return true
case (_?,nil),(nil,_?): return false
case let (x,y): return x === y
}
}
struct List<T>: CollectionType {
typealias Index = ListIndex<T>
var startIndex: Index
var endIndex: Index { return ListIndex(node: ListNode()) }
subscript(idx: Index) -> T {
guard let element = idx.node.box?.unbox.element
else { fatalError("index out of bounds") }
return element
}
}
(no need to implement generate() – you get an indexing generator “for free” in 2.0 by implementing CollectionType)
You now have a fully functioning collection:
// in practice you would add methods to List such as
// conforming to ArrayLiteralConvertible or init from
// another sequence
let list = List(startIndex: ListIndex(node: nodes))
list.first // 3
for x in list { print(x) } // prints 3 2 1
Now all of this code looks pretty disgusting for two reasons.
One is because box gets in the way, and indirect is much better as the compiler sorts it all out for you under the hood. But it’s doing something similar.
The other is that structs are not a good solution to this. Enums are much better. In fact the code is really using an enum – that’s what Optional is. Only instead of nil (i.e. Optional.None), it would be better to have a End case for the end of the linked list. This is what we are using it for.
For more of this kind of stuff you could check out these posts.
While Airspeed Velocity's answer applies to the most common cases, my question was asking specifically about the special case of generalizing CollectionType indexing in order to be able to share a single Index implementation for all thinkable kinds of binary trees (whose recursive nature makes it necessary to make use of a stack for index-based traversals (at least for trees without a parent pointer)), which requires the Index to be specialized on the actual BinaryTree, not the Element.
The way I solved this problem was to rename MyCollection to MyCollectionStorage, revoke its CollectionType conformity and wrap it with a struct that now takes its place as MyCollection and deals with conforming to CollectionType.
To make things a bit more "real" I will refer to:
MyCollection<E> as SortedSet<E>
MyCollectionStorage<E> as BinaryTree<E>
MyIndex<T> as BinaryTreeIndex<T>
So without further ado:
struct SortedSet<Element>: CollectionType {
typealias Tree = BinaryTree<Element>
typealias Index = BinaryTreeIndex<Tree>
subscript(i: Index) -> Element { … }
func generate() -> IndexingGenerator<SortedSet> {
return IndexingGenerator(self)
}
}
struct BinaryTree<Element>: BinaryTreeType {
}
struct BinaryTreeIndex<BinaryTree: BinaryTreeType>: BidirectionalIndexType {
func predecessor() -> BinaryTreeIndex { … }
func successor() -> BinaryTreeIndex { … }
}
This way the dependency graph turns from a directed cyclic graph into a directed acyclic graph.
Sounds ridiculous, but I'm unable to fix this piece of code:
self.runningScripts.filter({ $0 != scriptRunner })
No matter how I write the closure I always get this error:
Cannot invoke 'filter' with an argument list of type '((_) -> _)'
runningScripts is defined like this:
var runningScripts = [ScriptRunner]()
and ScriptRunner is a Swift class (does not inherit from NSObject)
I'm using nearly the same in many other places without problems. Any suggestions?
You can get that error if you didn't make ScriptRunner conform to Equatable:
class ScriptRunner : Equatable {
// the rest of your implementation here
}
func ==(lhs: ScriptRunner, rhs: ScriptRunner) -> Bool {
return ... // change this to whatever test that satisfies that lhs and rhs are equal
}
I needed an explicit cast like this:
#NSManaged private var storage: [String]
private var objects: Set<String>?
func remove(element:String) {
initSetIfNeeded()
if(objects!.contains(element)) {
objects!.remove(element)
storage = storage.filter({($0 as NSObject) !== (element as NSObject)}) // Explicit cast here!!
}
}