Common Equatable class on Swift

Question

I need container for Any Equatable items in NOT Generic class (for example UI classes initial from storyboard). I need like this

var items: [Equatable]?

but it don't work, Equatable need Generic. the problem that there is no exist common Equatable class.

Ok - Go to generic! But if I do this

class Item<Value: Equatable>: Equatable {
   var value: Value
   init(_ value: Value) {
       self.value = value
   }
   //Equatable
   public static func ==(lhs: Item, rhs: Item) -> Bool {
       return (lhs.value == rhs.value)
   }
}

then I will be forced to specify the type in my nonGeneric-UI class. Like this

var items: [Item<WhatShouldBeHere?>]?

but again we come to the problem that there is no exist common Equatable class

Any solutions for container for All Equatable?

Related: [Operation on an array of structs implementing Equatable](https://stackoverflow.com/questions/41298464/operation-on-an-array-of-structs-implementing-equatable) — Hamish, Sep 18 '17 at 23:02

Alexander · Accepted Answer · 2017-09-19T15:14:13.130

6

In lieu of existential types, you need to use a type eraser:

public struct AnyEquatable: Equatable {
    public let value: Any
    private let equals: (Any) -> Bool

    public init<E: Equatable>(_ value: E) {
        self.value = value
        self.equals = { ($0 as? E) == value }
    }

    public static func == (lhs: AnyEquatable, rhs: AnyEquatable) -> Bool {
        return lhs.equals(rhs.value) || rhs.equals(lhs.value)
    }
}

example usage:

let items = [
    AnyEquatable(1),
    AnyEquatable(1.23),
    AnyEquatable(true),
    AnyEquatable("some string")
]

let desired = "some string"
let desiredAsAnyEquatable = AnyEquatable(desired)
let desiredDescription = String(reflecting: desired)

for item in items {
    let itemDescription = String(reflecting: item.value)
    let isEqualToDesired = item == desiredAsAnyEquatable
    print("\(itemDescription) is \(isEqualToDesired ? "": "not ")equal to \(desiredDescription)")
}

Example output:

1 is not equal to "some string"

1.23 is not equal to "some string"

true is not equal to "some string"

"some string" is equal to "some string"

edited Sep 19 '17 at 15:14

answered Sep 18 '17 at 21:41

Alexander

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Note you should call `equals(_:)` for both `lhs` and `rhs` in order to preserve symmetry for when comparing a subclass with a superclass instance. – Hamish Sep 18 '17 at 23:04
@Hamish Good one, I hadn't considered that. so `lhs.equals(rhs.value) || rhs.equals(lhs.value)`? – Alexander Sep 19 '17 at 00:40
@Alexander Yup :) – Hamish Sep 19 '17 at 14:56
1

@Hamish Looking back on this, is that really necessary? A proper conformance to `Equatable` establishes an equivalence relationship, which implies symmetry. `a == b ≡ b == a` – Alexander Oct 07 '18 at 18:38
@Alexander It does, but casting isn't symmetrical – i.e for `class C {}; class D : C {}`, casting `C() as? D` fails, but `D() as? C` succeeds. See https://gist.github.com/hamishknight/80f2ef2dd328892439eaa20eb3478deb for an example of how that breaks `Equatable`'s requirement of symmetry. – Hamish Oct 07 '18 at 19:13
@Hamish Ahhh, I see what you mean – Alexander Oct 07 '18 at 19:49

Common Equatable class on Swift

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