How to combine two Arrays that are optional

Question

Having some trouble combining two arrays that are optional. Not arrays that contain optional items.

let a : [String]? = ["foo"]
let b : [String]? = nil

or

let a : [String]? = nil
let b : [String]? = nil

or

let a : [String]? = ["foo"]
let b : [String]? = ["bar"]

This obviously doesn't work because the arrays are optional

let combinedArrays : [String]? = a + b

Is there a more concise way than the traditional if let approach using functional or some other cleaner method to combine arrays a and b?

Update: The above example is contrived but below is a more real world example of what I was attempting to do:

func pinToAllSidesOfSuperView() -> [NSLayoutConstraint]?
{
    let horizontalConstraints : [NSLayoutConstraint]? = pinViewToLefAndRight()
    let verticalConstraints : [NSLayoutConstraint]? = pinViewToTopAndBottom()
    return horizontalConstraints + verticalConstraints
}

It would be nice to return an optional return value vs an empty array so the caller of the method can still use the optional features (i.e. if let , guard etc) vs the simple check if Array.isEmpty.

Answer 1

I would do it like this:

func +<T>(lhs: Array<T>?, rhs: Array<T>?) -> Array<T>? {
    switch (lhs, rhs) {

    case (nil, nil):
        return nil

    case (nil, _):
        return rhs

    case (_, nil):
        return lhs

    default:
        return lhs! + rhs!

    }
}

let foo: [Int]? = nil
let bar: [Int]? = [1]

foo + foo   // -> nil
foo + bar   // -> [1]
bar + foo   // -> [1]
bar + bar   // -> [1, 1]