Function that lifts a function with non-optional input to one with optional input.

LiftToOptional.swift

// Thanks to @jckarter for pointing that this is just Optional's map implementation... 
{ $0.map(f) }

/// Takes a function with non-optional input and non-optional return and lifts it to a function
/// with optional input and optional return. 
/// The lifted function will return `nil` iff the input is `nil`, otherwise the input will be 
/// applied to the original function which will return a non-`nil` value.
public func liftToOptional<T,U>(_ f: @escaping (T) -> U) -> (T?) -> U? {
    return { arg in
        guard let arg = arg else { return nil }
        return f(arg)
    }
}

// Example: Allows to lift (String) -> UIView, to (String?) -> UIView?
// When working directly with optionals, the above can be resolved with by using `flatMap`. 
// But if optionals are in another container (e.g. an Observable), this function seems necessary.