Learn & Master ⚔️ Type Erasure in 6 Minutes

22. Jul 2019

Example constant of type Any

Some of you may already be familiar with Type Erasure and know how to use it. But I’m certain there are still people out there which don’t know about it or don’t know how to use it.

Honestly, I didn’t take a look at it before writing my article about Advanced Lists in SwiftUI. It was a little mystery for me since I listened to a not so good talk at a developer conference a while ago.

Before lifting the secret Type Erasure let’s take a look at the problem it solved for me.

The Problem 💣

I wanted to store objects conforming to two different protocols in an array. When dealing with simple protocols the implementation is simple as well.

protocol ProtocolA {}
protocol ProtocolB {}
let objects: [ProtocolA & ProtocolB] = []

In the real world protocols are often more complex (like in my case) and have *associated type* and/or *Self requirements*. Let’s take the Equatable protocol for example.

let objects: [Equatable] = []

This code will not compile because the Equatable protocol has Self requirements. Therefore the protocol can only be used as a generic constraint.

One more thing left before the solution: a definition of Type Erasure.

Type Erasure

Here is my definition of Type Erasure:

It’s a technique used to solve the problems which occur when dealing with protocols having associated type and/or Self requirements.

Type Erasure erases the type information in the first place while still constraining to the specific type. Its achieved with the implementation of a box/wrapper type.

I know it’s weird to erase the type information because we all love the statically typed language Swift, right? 🙃

Nevertheless let’s implement a solution to the aforementioned problem.

The Solution 💡

I implemented a type erased wrapper called AnyEquatable. It has a generic initializer which expects an object conforming to the Equatable protocol. The object is stored internally in an Any property, so the type information is lost.

The key thing here is the *isEqualTo* block. This block gets another AnyEquatable object and tries to cast the equatable property of it to the type of the Equatable object passed to the initializer.

AnyEquatable itself conforms to the Equatable protocol and uses its isEqualTo block to do the equation. That way the conformance to the Equatable protocol is preserved regardless of the type erasure.

struct AnyEquatable {
    private let isEqualTo: (AnyEquatable) -> Bool
    let equatable: Any
    
    init<T: Equatable>(_ equatable: T) {
        self.equatable = equatable
        self.isEqualTo = { anotherEquatable in
            guard let anotherEquatable = anotherEquatable.equatable as? T else {
                return false
            }
            
            return anotherEquatable == equatable
        }
    }
}
extension AnyEquatable: Equatable {
    static func == (lhs: AnyEquatable, rhs: AnyEquatable) -> Bool {
        return lhs.isEqualTo(rhs)
    }
}

Now, we are able to store different Equatable objects in the same array:

struct Foo {
    let propertyA: String
}
extension Foo: Equatable {
    static func == (lhs: Foo, rhs: Foo) -> Bool {
        return lhs.propertyA == rhs.propertyA
    }
}
struct Bar {
    let propertyB: String
}
extension Bar: Equatable {
    static func == (lhs: Bar, rhs: Bar) -> Bool {
        return lhs.propertyB == rhs.propertyB
    }
}
var equatableArray: [AnyEquatable] = []
let equatable1 = AnyEquatable(Foo(propertyA: "foo"))
equatableArray.append(equatable1)
let equatable2 = AnyEquatable(Foo(propertyA: "foo"))
equatableArray.append(equatable2)
let equatable3 = AnyEquatable(Bar(propertyB: "bar"))
equatableArray.append(equatable3)
print(equatableArray[0] == equatableArray[1]) // true
print(equatableArray[1] == equatableArray[2]) // false

There is another common problem where Type Erasure comes to the rescue:

Think about a delegate protocol where you want to use a type which has associated type and/or Self requirements. Again the solution is a type erased wrapper. The only downside is that the delegate has to cast the Any value inside the type erased wrapper back to its original type.

Okay, let’s head over to other type erased wrappers.

More Type erased wrappers 🌟

In this section I’ll give you a short overview of some other type erased wrappers you can use to solve the aforementioned problems.

AnyHashable

It’s part of the Swift Standard Library.

Usage examples: Store Hashable objects in an array, require Hashable conformance in another protocol

AnyView

The SwiftUI framework provides AnyView which erases the type information of views.

Usage examples: Store objects conforming to the *View* protocol in an array, require conformance to the View protocol in another protocol

AnyIdentifiable

The Identifiable protocol is part of SwiftUI as well and needs to be implemented by the items used in a List view or in a ForEach, so they can be uniquely identified by the framework.

A type erased wrapper AnyIdentifiable is easily implemented with the use of the existing type erased wrapper AnyHashable.

struct AnyIdentifiable: Identifiable {
    let id: AnyHashable
    
    init<T: Identifiable>(_ identifiable: T) {
        self.id = identifiable.id
    }
}

AnyComparable

Similar to the AnyEquatable wrapper is the implementation of the following type erased wrapper for Comparable objects.

struct AnyComparable {
    private let isEqualTo: (AnyComparable) -> Bool
    private let compareTo: (AnyComparable) -> Bool
    let comparable: Any
    
    init<T: Comparable>(_ comparable: T) {
        self.comparable = comparable
        self.isEqualTo = { anotherEquatable in
            guard let anotherEquatable = anotherEquatable.comparable as? T else {
                return false
            }
            
            return anotherEquatable == comparable
        }
        self.compareTo = { anotherComparable in
            guard let anotherComparable = anotherComparable.comparable as? T else {
                return false
            }
            
            return comparable < anotherComparable
        }
    }
}
extension AnyComparable: Comparable {
    static func < (lhs: AnyComparable, rhs: AnyComparable) -> Bool {
        return lhs.compareTo(rhs)
    }
}
extension AnyComparable: Equatable {
    static func == (lhs: AnyComparable, rhs: AnyComparable) -> Bool {
        return lhs.isEqualTo(rhs)
    }
}

The following code shows a basic usage example of the type erased wrapper:

let comparables = [AnyComparable(5), AnyComparable(10)]
print(comparables[0] < comparables[1]) // true
print(comparables[0] > comparables[0]) // false
print(comparables[0] == comparables[0]) // true

AnyNumber

Last but not least I proudly present a type erased wrapper for storing objects conforming to the Numeric protocol in the same array (Int, Float and Double numbers).

Keep in mind that you can only use add, substract or multiply on AnyNumbers with the same underlying type (Int, Float or Double).

struct AnyNumber: Numeric {
    private let add: (AnyNumber) -> AnyNumber
    private let substract: (AnyNumber) -> AnyNumber
    private let multiply: (AnyNumber) -> AnyNumber
    private let isEqualTo: (AnyNumber) -> Bool
    
    var magnitude: Double {
        let stringValue = String(describing: value)
        return Double(stringValue)?.magnitude ?? -1
    }
    
    let value: Any
    
    init?<T>(exactly source: T) where T : BinaryInteger {
        self.init(source)
    }
    
    init<T: Numeric>(_ number: T) {
        self.value = number
        self.add = { anotherNumber in
            guard let anotherNumber = anotherNumber.value as? T else {
                return AnyNumber(number)
            }
            
            let numbers = number + anotherNumber
            return AnyNumber(numbers)
        }
        self.substract = { anotherNumber in
            guard let anotherNumber = anotherNumber.value as? T else {
                return AnyNumber(number)
            }
            
            let numbers = number - anotherNumber
            return AnyNumber(numbers)
        }
        self.multiply = { anotherNumber in
            guard let anotherNumber = anotherNumber.value as? T else {
                return AnyNumber(number)
            }
            
            let numbers = number * anotherNumber
            return AnyNumber(numbers)
        }
        self.isEqualTo = { anotherNumber in
            guard let anotherNumber = anotherNumber.value as? T else {
                return false
            }
            
            return anotherNumber == number
        }
    }
}
extension AnyNumber: Equatable {
    static func == (lhs: AnyNumber, rhs: AnyNumber) -> Bool {
        return lhs.isEqualTo(rhs)
    }
}
extension AnyNumber: AdditiveArithmetic {
    static func -= (lhs: inout AnyNumber, rhs: AnyNumber) {
        lhs = lhs - rhs
    }
    
    static func - (lhs: AnyNumber, rhs: AnyNumber) -> AnyNumber {
        return lhs.substract(rhs)
    }
    
    static func += (lhs: inout AnyNumber, rhs: AnyNumber) {
        lhs = lhs + rhs
    }
    
    static func + (lhs: AnyNumber, rhs: AnyNumber) -> AnyNumber {
        return lhs.add(rhs)
    }
    
    static func * (lhs: AnyNumber, rhs: AnyNumber) -> AnyNumber {
        lhs.multiply(rhs)
    }
    
    static func *= (lhs: inout AnyNumber, rhs: AnyNumber) {
        lhs = lhs * rhs
    }
}
extension AnyNumber: ExpressibleByIntegerLiteral {
    init(integerLiteral value: Int) {
        self.init(value)
    }
}

The following code shows a simple usage example:

let floatNumber: Float = 5.5323498539485
let doubleNumber = 6.549123234234234
let intNumber = 3
let numbers = [AnyNumber(floatNumber),
               AnyNumber(doubleNumber),
               AnyNumber(intNumber)]
print(type(of: numbers[0].value)) // Float
print(type(of: numbers[1].value)) // Double
print(type(of: numbers[2].value)) // Int

You did it 🎉. Head over to your todo list:

✅ Understand and use Type Erasure

I hope that I could unlock the secret Type Erasure for you. If yes then you are ready to use it in the near future 🚀🚀🚀. Otherwise don’t hesitate to ask questions ✌️.

Thanks again for reading one of my articles. Check out my other articles if you like.

Stay tuned 📺.

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