Nullable or Optional? Or something else?

[Andrei Alexandrescu]

Daniel Keep wrote:
> Andrei Alexandrescu wrote:
>> I plan to add a Nullable struct to Phobos (akin to C#'s Nullable,
>> Boost's Optional).
>>
>> Apparently a good design is to define Optional!T with a minimum of
>> member functions (ideally none) and have it use the "alias this" feature
>> to masquerade as a T. That way Optional!T looks and feels much like a T,
>> except that it supports a function
>>
>> bool isNull(T)(Optional!T value);
>>
>> Am I on the right track? If so, what is the name you'd prefer for this
>> artifact?
>>
>>
>> Andrei
> 
> Given the apocalyptic battle raging in this thread at the moment, maybe
> it would be pertinent to have both.  Sometimes, you do want a generic
> mechanism for representing "this may or may not have a value".  Other
> times, you just want something with a "no value set" sentinel.
> 
> Nullable!T would be a type for which 'null' is a valid value.  By this
> definition, Nullable!(Nullable!T) is obviously equivalent to Nullable!T;
> it's equivalent to T if T is either a pointer, array or object.
> 
> Optional!T would be a type where an instance may or may not have a value
> of type T; even if T is itself Optional.
> 
> I'm tempted to put an alias this into Optional and just define an
> external contentsOf function that pulls out the value contained in the
> Optional.  Not sure if that's the right choice or not.
> 
> So that gives us:
[snip]

Thanks. I think what I'll do for now is define Optional and see where 
things go.

Andrei