I have to establish whether the current value is equal to the given comparing value.
public static func Is<TBaseValue, TComparingValue>(baseValue: TBaseValue, comparingValue: TComparingValue) -> Bool
{
return baseValue == comparingValue
}
I almost try this
public static func Is<TBaseValue: Comparable, TComparingValue: Comparable>(baseValue: TBaseValue, comparingValue: TComparingValue) -> Bool
{
return baseValue==comparingValue
}
and this
public static func Is<TBaseValue: Equatable, TComparingValue: Equatable>(baseValue: TBaseValue, comparingValue: TComparingValue) -> Bool
{
return baseValue == comparingValue
}
but always I have the same result Binary operator cannot be applied....
Equatable doesn't exactly mean that. Think about things that are equatable - Int, for instance. 2 == 2 makes sense. 2 == 3 will return false - but it still makes sense. Now, think of something else that's Equatable - String, for instance.
"abc" == "abc" // true
"acb" == "abc" // false
That's fine - but what about this:
"abc" == 4
An Equatable thing is Equatable to itself - not to everything else. In your example, you're comparing two different types - TBaseValue, TComparingValue.
If you want to compare any values you can use overloading:
static func Is<T: Equatable>(first: T, second: T) -> Bool {
return first == second
}
static func Is<T1, T2>(first: T1, second: T2) -> Bool {
return false
}
So the most appropriate function gets called automatically. If the first function cannot be called with the passed parameters than the second one gets called anyhow.
This works in Swift 2.0.
func Is<Any : Equatable>(l: Any, _ r: Any) -> Bool {
return l == r
}
Is("a", "a") // true
Is("a", "b") // false
Is(1, 1) // true
It also would work with AnyObject.
For comparing two generics, you can declare the generics such that, types, that are capable to be in the place of your generic type, should conform to Comparable protocol.
struct Heap<T: Comparable>{
var heap = [T]()
mutating func insert(element: T){
heap.append(element)
var index = heap.count - 1
heapify(childIndex: index)
}}
Now we will be able to do:-
if heap[parentIndex] < heap[childIndex] {
//Your code
}
How this works?
As we know, conforming to a protocol means implementing all the required methods in that protocol. Comparable protocol has got all the comparison methods as required parameters, and any type that is implementing Comparable will be able to do a comparison.
Happy coding using Generics
Related
I'm writing a component for caching instances of classes. The classes are not per se Comparable, Hashable or Equatable. If they were, the semantics of the respective operations would not necessarily serve our purposes, so let's pretent we can not use those protocols.
Objects can be cached w.r.t. multiple keys. So when asking the cache for a list of all cached objects, I need to remove duplicates from the value set of the underlying dictionary -- with respect to object identity.
Obviously, this does the job:
var result: [C] = []
for c in dict.values {
if !result.contains(where: { (rc: C) in rc === c }) {
result.append(c)
}
}
return result
However, this has quadratic runtime behaviour. Compared to linearithmic or expected linear behaviour that are easy to get when using abovementioned protocols (using set implementations), this is bad.
So how can we efficiently remove duplicates w.r.t. object identity from a Swift collection?
We can wrap our objects into something that is Hashable and Comparable:
struct ClassWrap<T: AnyObject>: Hashable, Comparable {
var value: T
var hashValue: Int {
return ObjectIdentifier(self.value).hashValue
}
static func ==(lhs: ClassWrap, rhs: ClassWrap) -> Bool {
return lhs.value === rhs.value
}
static func <(lhs: ClassWrap<T>, rhs: ClassWrap<T>) -> Bool {
return ObjectIdentifier(lhs.value) < ObjectIdentifier(rhs.value)
}
}
Now, any regular Set implementation or otherwise unique-fying operation should do the job.
I'm experimenting with using Composition instead of Inheritance and I wanted to use diff on an array of objects that comply with a given protocol.
To do so, I implemented a protocol and made it comply with Equatable:
// Playground - noun: a place where people can play
import XCPlayground
import Foundation
protocol Field:Equatable {
var content: String { get }
}
func ==<T: Field>(lhs: T, rhs: T) -> Bool {
return lhs.content == rhs.content
}
func ==<T: Field, U: Field>(lhs: T, rhs: U) -> Bool {
return lhs.content == rhs.content
}
struct First:Field {
let content:String
}
struct Second:Field {
let content:String
}
let items:[Field] = [First(content: "abc"), Second(content: "cxz")] // đź’Ą boom
But I've soon discovered that:
error: protocol 'Field' can only be used as a generic constraint because it has Self or associated type requirements
I understand why since Swift is a type-safe language that needs to be able to know the concrete type of these objects at anytime.
After tinkering around, I ended up removing Equatable from the protocol and overloading the == operator:
// Playground - noun: a place where people can play
import XCPlayground
import Foundation
protocol Field {
var content: String { get }
}
func ==(lhs: Field, rhs: Field) -> Bool {
return lhs.content == rhs.content
}
func ==(lhs: [Field], rhs: [Field]) -> Bool {
return (lhs.count == rhs.count) && (zip(lhs, rhs).map(==).reduce(true, { $0 && $1 })) // naive, but let's go with it for the sake of the argument
}
struct First:Field {
let content:String
}
struct Second:Field {
let content:String
}
// Requirement #1: direct object comparison
print(First(content: "abc") == First(content: "abc")) // true
print(First(content: "abc") == Second(content: "abc")) // false
// Requirement #2: being able to diff an array of objects complying with the Field protocol
let array1:[Field] = [First(content: "abc"), Second(content: "abc")]
let array2:[Field] = [Second(content: "abc")]
print(array1 == array2) // false
let outcome = array1.diff(array2) // đź’Ą boom
error: value of type '[Field]' has no member 'diff'
From here on, I'm a bit lost to be honest. I read some great posts about type erasure but even the provided examples suffered from the same issue (which I assume is the lack of conformance to Equatable).
Am I right? And if so, how can this be done?
UPDATE:
I had to stop this experiment for a while and totally forgot about a dependency, sorry! Diff is a method provided by SwiftLCS, an implementation of the longest common subsequence (LCS) algorithm.
TL;DR:
The Field protocol needs to comply with Equatable but so far I have not been able to do this. I need to be able to create an array of objects that comply to this protocol (see the error in the first code block).
Thanks again
The problem comes from a combination of the meaning of the Equatable protocol and Swift’s support for type overloaded functions.
Let’s take a look at the Equatable protocol:
protocol Equatable
{
static func ==(Self, Self) -> Bool
}
What does this mean? Well it’s important to understand what “equatable” actually means in the context of Swift. “Equatable” is a trait of a structure or class that make it so that any instance of that structure or class can be compared for equality with any other instance of that structure or class. It says nothing about comparing it for equality with an instance of a different class or structure.
Think about it. Int and String are both types that are Equatable. 13 == 13 and "meredith" == "meredith". But does 13 == "meredith"?
The Equatable protocol only cares about when both things to be compared are of the same type. It says nothing about what happens when the two things are of different types. That’s why both arguments in the definition of ==(::) are of type Self.
Let’s look at what happened in your example.
protocol Field:Equatable
{
var content:String { get }
}
func ==<T:Field>(lhs:T, rhs:T) -> Bool
{
return lhs.content == rhs.content
}
func ==<T:Field, U:Field>(lhs:T, rhs:U) -> Bool
{
return lhs.content == rhs.content
}
You provided two overloads for the == operator. But only the first one has to do with Equatable conformance. The second overload is the one that gets applied when you do
First(content: "abc") == Second(content: "abc")
which has nothing to do with the Equatable protocol.
Here’s a point of confusion. Equability across instances of the same type is a lower requirement than equability across instances of different types when we’re talking about individually bound instances of types you want to test for equality. (Since we can assume both things being tested are of the same type.)
However, when we make an array of things that conform to Equatable, this is a higher requirement than making an array of things that can be tested for equality, since what you are saying is that every item in the array can be compared as if they were both of the same type. But since your structs are of different types, you can’t guarantee this, and so the code fails to compile.
Here’s another way to think of it.
Protocols without associated type requirements, and protocols with associated type requirements are really two different animals. Protocols without Self basically look and behave like types. Protocols with Self are traits that types themselves conform to. In essence, they go “up a level”, like a type of type. (Related in concept to metatypes.)
That’s why it makes no sense to write something like this:
let array:[Equatable] = [5, "a", false]
You can write this:
let array:[Int] = [5, 6, 7]
or this:
let array:[String] = ["a", "b", "c"]
or this:
let array:[Bool] = [false, true, false]
Because Int, String, and Bool are types. Equatable isn’t a type, it’s a type of a type.
It would make “sense” to write something like this…
let array:[Equatable] = [Int.self, String.self, Bool.self]
though this is really stretching the bounds of type-safe programming and so Swift doesn’t allow this. You’d need a fully flexible metatyping system like Python’s to express an idea like that.
So how do we solve your problem? Well, first of all realize that the only reason it makes sense to apply SwiftLCS on your array is because, at some level, all of your array elements can be reduced to an array of keys that are all of the same Equatable type. In this case, it’s String, since you can get an array keys:[String] by doing [Field](...).map{ $0.content }. Perhaps if we redesigned SwiftLCS, this would make a better interface for it.
However, since we can only compare our array of Fields directly, we need to make sure they can all be upcast to the same type, and the way to do that is with inheritance.
class Field:Equatable
{
let content:String
static func == (lhs:Field, rhs:Field) -> Bool
{
return lhs.content == rhs.content
}
init(_ content:String)
{
self.content = content
}
}
class First:Field
{
init(content:String)
{
super.init(content)
}
}
class Second:Field
{
init(content:String)
{
super.init(content)
}
}
let items:[Field] = [First(content: "abc"), Second(content: "cxz")]
The array then upcasts them all to type Field which is Equatable.
By the way, ironically, the “protocol-oriented” solution to this problem actually still involves inheritance. The SwiftLCS API would provide a protocol like
protocol LCSElement
{
associatedtype Key:Equatable
var key:Key { get }
}
We would specialize it with a superclass
class Field:LCSElement
{
let key:String // <- this is what specializes Key to a concrete type
static func == (lhs:Field, rhs:Field) -> Bool
{
return lhs.key == rhs.key
}
init(_ key:String)
{
self.key = key
}
}
and the library would use it as
func LCS<T: LCSElement>(array:[T])
{
array[0].key == array[1].key
...
}
Protocols and Inheritance are not opposites or substitutes for one another. They complement each other.
I know this is probably now what you want but the only way I know how to make it work is to introduce additional wrapper class:
struct FieldEquatableWrapper: Equatable {
let wrapped: Field
public static func ==(lhs: FieldEquatableWrapper, rhs: FieldEquatableWrapper) -> Bool {
return lhs.wrapped.content == rhs.wrapped.content
}
public static func diff(_ coll: [Field], _ otherCollection: [Field]) -> Diff<Int> {
let w1 = coll.map({ FieldEquatableWrapper(wrapped: $0) })
let w2 = otherCollection.map({ FieldEquatableWrapper(wrapped: $0) })
return w1.diff(w2)
}
}
and then you can do
let outcome = FieldEquatableWrapper.diff(array1, array2)
I don't think you can make Field to conform to Equatable at all as it is designed to be "type-safe" using Self pseudo-class. And this is one reason for the wrapper class. Unfortunately there seems to be one more issue that I don't know how to fix: I can't put this "wrapped" diff into Collection or Array extension and still make it support heterogenous [Field] array without compilation error:
using 'Field' as a concrete type conforming to protocol 'Field' is not supported
If anyone knows a better solution, I'm interested as well.
P.S.
In the question you mention that
print(First(content: "abc") == Second(content: "abc")) // false
but I expect that to be true given the way you defined your == operator
I'm currently trying out with some basic data structures like LinkedList. I defined a ListNode class of generics values, like this:
class ListNode<T> {
var nodeContent: T
var nextNode: ListNode<T>? = nil
init() {
// details omitted here
}
And then a linked list. I want to implement the contains() method, so I have sth like this:
func contains<T>(_ item: T) -> Bool {
var currNode = self.head
while (currNode != nil) {
if currNode?.nodeContent == item {
return true
}
currNode = currNode?.nextNode
}
return false
}
Then it's giving me error saying that '==' cannot applied to T and T types. I then looked through the language guide and changed ListNode class and LinkedList struct to this:
class ListNode<T: Equatable>{}
struct LinkedList<T: Equatable>{}
But it's not working, so I added 'Equatable' to func itself:
func contains<T: Equatable>(_ item: T) -> Bool
Still fails. I tried pasting the sample function from the language guide inside,
func findIndex<T: Equatable>(of valueToFind: T, in array:[T]) -> Int? {
for (index, value) in array.enumerated() {
if value == valueToFind {
return index
}
}
return nil
}
No error occurs. May I know why it's like this? I tried searching, but all suggested answers like this doesn't clear my doubts. Thanks in advance!
You just don't need to make the contains method generic (twice). It's inside of your already generic class and knows about T type. It's right to require T: Equatable in the type declaration.
findIndex(of:in:) works as is, because it's not a method, but rather a standalone generic function.
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.