Creating an extension to filter nils from an Array in Swift

Question

I'm trying to write an extension to Array which will allow an array of optional T's to be transformed into an array of non-optional T's.

e.g. this could be written as a free function like this:

func removeAllNils(array: [T?]) -> [T] {
    return array
        .filter({ $0 != nil })   // remove nils, still a [T?]
        .map({ $0! })            // convert each element from a T? to a T
}

But, I can't get this to work as an extension. I'm trying to tell the compiler that the extension only applies to Arrays of optional values. This is what I have so far:

extension Array {
    func filterNils<U, T: Optional<U>>() -> [U] {
        return filter({ $0 != nil }).map({ $0! })
    }
}

(it doesn't compile!)

Answer 1

As of Swift 2.0, you don't need to write your own extension to filter nil values from an Array, you can use flatMap, which flattens the Array and filters nils:

let optionals : [String?] = ["a", "b", nil, "d"]
let nonOptionals = optionals.flatMap{$0}
print(nonOptionals)

Prints:

[a, b, d]

Note:

There are 2 flatMap functions:

• One flatMap is used to remove non-nil values which is shown above. Refer - https://developer.apple.com/documentation/swift/sequence/2907182-flatmap

• The other flatMap is used to concatenate results. Refer - https://developer.apple.com/documentation/swift/sequence/2905332-flatmap