Optional<T> equivalent in D?

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On Saturday, 16 November 2013 at 05:04:42 UTC, Jonathan M Davis 
wrote:
> I really don't understand this. Optional<T> is one of the most 
> useless ideas
> that I've ever seen in Java. Just use null. It's built into the 
> language. It
> works just fine. And wrapping it in a class isn't going to make 
> it go away.
> Just learn to deal with null properly. I've known many 
> programmers who have no
> problems with null whatsoever, and I'd be worried about any 
> programmer who is
> so scared of it that they feel the need to wrap nullable 
> objects in another
> type which has its own concept of null.

The value of an Option<T> type is that it moves checking for null 
into the type system. It forces you to check for null before you 
perform any potentially NullPointerException-throwing operations, 
whereas using naked class references in Java, it's easy to 
forget, or miss a null check, or just ignore it and hope 
everything is fine. With Option<T>, you have no choice but to 
check for null before you perform an operation on the wrapped 
class reference.

> The only types which aren't nullable in Java are the primitive 
> types, and if
> you use them in generics (like Optional<T> does), then you get 
> a class that
> boxes the primitive type rather than using the primitive type 
> directly, and
> that object is of course nullable. So, you might as well just 
> use the class
> that boxes the primitive type directly and set its reference to 
> null when you
> need it to be null. And Optional<T> doesn't even protect 
> against null, since
> it's perfectly possible to make its contents null. So, as far 
> as I can see,
> Optional<T> is utterly pointless. IMHO, it's outright bad 
> software design.

I would have to respectfully disagree with that. Proper use of an 
Option<T> type can dramatically reduce the chance of calling a 
method on a null object, or even eliminate it entirely.