swift3 - Unable to use indices.contains() in a Collection extension in Swift 3

Question

I wrote following extension in Swift 2.3:

extension CollectionType {
    /// Returns the element at the specified index iff it is within bounds, otherwise nil.
    subscript (safe index: Index) -> Generator.Element? {
        return indices.contains(index) ? self[index] : nil
    }
}

However, it turns out that Swift 3.0 does not have contains() function. Instead, it offers me following syntax for this method:

indices.contains(where: { (<#Self.Indices.Iterator.Element#>) -> Bool in
    <# code ??? what should it do??? #>
})

The problem is that I don't know what should it contain inside the block. Any help with migrating it, please?

Answer 1

Swift 4 Update

In Swift 4, thanks to the ability to have where clauses on associated types, Collection now enforces that Indices's Element type is the same type as the Collection's Index.

This therefore means that we can just say:

extension Collection {

    /// Returns the element at the specified index iff it is within bounds, otherwise nil.
    subscript (safe index: Index) -> Element? {
        return indices.contains(index) ? self[index] : nil
    }
}

---

Swift 3

The Sequence protocol in Swift 3 still has a contains(_:) method, which accepts an element of the sequence if the sequence is of Equatable elements:

extension Sequence where Iterator.Element : Equatable {
    // ...
    public func contains(_ element: Self.Iterator.Element) -> Bool
    // ...
}

The problem you're encountering is due to the change in the type of Collection's indices property requirement. In Swift 2, it was of type Range<Self.Index> – however in Swift 3, it is of type Indices (an associated type of the Collection protocol):

/// A type that can represent the indices that are valid for subscripting the
/// collection, in ascending order.
associatedtype Indices : IndexableBase, Sequence = DefaultIndices<Self>

As there's currently no way in Swift for the Collection protocol itself to express that Indices's Iterator.Element is of type Index (this will however be possible in a future version of Swift), there's no way for the compiler to know that you can pass something of type Index into contains(_:). This is because it's currently fully possible for a type to conform to Collection and implement Indices with whatever element type it wants.

Therefore the solution is to simply constrain your extension to ensure that Indices does have elements of type Index, allowing you to pass index into contains(_:):

extension Collection where Indices.Iterator.Element == Index {

    /// Returns the element at the specified index iff it is within bounds, otherwise nil.
    subscript (safe index: Index) -> Iterator.Element? {
        return indices.contains(index) ? self[index] : nil
    }
}