Type Inheritance - Swift

Recently I’ve been going a little bit crazy doing a project in Javascript/Node and the lack of types (safety) is disturbing to me. So today I was playing around with some types in Swift and noticed something interesting. So first lets set up the test playground:

public protocol CellInfo { }

public struct WelcomeCellInfo: CellInfo { }
public struct SummaryCellInfo: CellInfo { }
public struct AmountCellInfo: CellInfo {
    let amount: Int64
}

var cells = [CellInfo]()

let amounts: [Int64] = [10, -10, 15, 5, 10]
let amountCellInfo = amounts.flatMap{ AmountCellInfo(amount: $0) }

This is for a data source for a table/collection view and we’re passing back a struct with some info in it to the view (for context). We have an array of type CellInfo and we want to add structs to it that conform to that protocol.

As expected the following works just fine:

cells.append(WelcomeCellInfo())
cells.append(SummaryCellInfo())
cells.append(AmountCellInfo(amount: 100))

But we run into an interesting problem when we try to append the entire array of `transfer

// This works
amountCellInfo.forEach{
    cells.append($0)
}

// This does not
cells.appendContentsOf(amountCellInfo)

The problem of course is type: amountCellInfo has the type AmountCellInfo while cells has the type CellInfo. Its interesting how type inheritance works fine with the append function but does not for appendContentsOf. Making the following changes fixes everything.

let amountCellInfo: [CellInfo] = amounts.flatMap{ AmountCellInfo(amount: $0) }
// This works now
cells.appendContentsOf(amountCellInfo)