How can I extend append method for Array in optional in Swift?

Question

I am working to make this code works for optional array, but Xcode complain about:

Cannot assign value of type '[Wrapped.Element]' to type 'Wrapped'

Not sure why should be an error?

extension Optional where Wrapped: RangeReplaceableCollection, Wrapped.Element: Equatable {

    mutating func customAppend(_ value: Wrapped.Element) {
        if (self != nil) {
            self?.append(value)
        }
        else {
            self = [value] // Cannot assign value of type '[Wrapped.Element]' to type 'Wrapped'
        }
    }
    
}

The solution is pretty similar to that answer to your question stackoverflow.com/a/73179138/2303865 — Leo Dabus
– Leo Dabus, Commented Aug 3, 2022 at 2:03
Note that the Wrapped.Element's constraint to Equatable there is pointless. — Leo Dabus
– Leo Dabus, Commented Aug 3, 2022 at 3:07

asyncawait · Accepted Answer · 2022-08-03 01:42:10Z

1

This will compile:

extension Optional where Wrapped: RangeReplaceableCollection, Wrapped.Element: Equatable {
    mutating func customAppend(_ value: Wrapped.Element) {
        if (self != nil) {
            self?.append(value)
        }
        else {
            self = .init([value]) // <--- RangeReplaceableCollection.init(_ elements:)
        }
    }
    
}

This works because RangeReplaceableCollection declares this init:

init<S>(_ elements: S) where S : Sequence, Self.Element == S.Element

So that init is available to any conforming type, and gives the appropriate concrete type.

On the other hand, trying to do self = [value] doesn't work, because that's trying to set self to an Array. But since the extension is on RangeReplaceableCollection, you can't set self to an Array, because that's not its type.

answered Aug 3, 2022 at 1:42

asyncawait

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5 Comments

Leo Dabus Over a year ago

Why not simply init if nil and then just append? if self == nil { self = .init() } self?.append(value)

asyncawait Over a year ago

@LeoDabus Sure! That would be equivalent and maybe clearer.

swiftPunk Over a year ago

Your answer is not enough as a correct answer, there is no initialization for RangeReplaceableCollection as you commented like "RangeReplaceableCollection.init(_ elements:)" or in explaining, Also there is no need to use Equatable protocol as Leo said! Your answer does some issues, also you just used .init() which that is not enough, you could say Wrapped.init([value]) or Wrapped([value])

asyncawait Over a year ago

RangeReplaceableCollection does declare that init, here's documentation: developer.apple.com/documentation/swift/…

asyncawait Over a year ago

Additionally, .init([value]) and Wrapped([value]) are equivalent here. This is because of type inference: the compiler knows that self is of type Wrapped, so it infers that .init([value]) actually means Wrapped.init([value])

swiftPunk · Accepted Answer · 2022-08-03 18:39:22Z

0

Here is the correct and simplified answer:

extension Optional where Wrapped: RangeReplaceableCollection {

    mutating func customAppend(_ value: Wrapped.Element) {
        if (self != nil) {
            self?.append(value)
        }
        else {
            self = Wrapped([value])
        }
    }
    
}

answered Aug 3, 2022 at 18:39

swiftPunk

1

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