Propagate an optional through a function (or Init) in Swift

Question

does anyone have a (better) way to do this?

Lets say I have a optional Float

let f: Float? = 2

Now I want to cast it to a Double

Answer 1

You can simply use Optional's map(_:) method, which will return the wrapped value with a given transform applied if it's non-nil, else it will return nil.

let f : Float? = 2

// If f is non-nil, return the result from the wrapped value passed to Double(_:),
// else return nil.
let d = f.map { Double($0) }

Which, as you point out in the comments below, can also be said as:

let d = f.map(Double.init)

This is because map(_:) expects a transformation function of type (Float) -> Double in this case, and Double's float initialiser is such a function.

If the transform also returns an optional (such as when converting an String to a Int), you can use flatMap(_:), which simply propagates a nil transform result back to the caller:

let s : String? = "3"

// If s is non-nil, return the result from the wrapped value being passed to the Int(_:)
// initialiser. If s is nil, or Int($0) returns nil, return nil.
let i = s.flatMap { Int($0) }