NULL indicator in Variant

[Steven Schveighoffer]

On Thu, 27 Oct 2011 14:06:47 -0400, Steve Teale  
<steve.teale at britseyeview.com> wrote:

> On Thu, 27 Oct 2011 13:58:54 -0400, Steven Schveighoffer wrote:
>
>> On Thu, 27 Oct 2011 13:53:02 -0400, Steve Teale
>> <steve.teale at britseyeview.com> wrote:
>>
>>> On Thu, 27 Oct 2011 13:09:43 -0400, Steven Schveighoffer wrote:
>>>
>>>> On Thu, 27 Oct 2011 12:58:57 -0400, Steve Teale
>>>> <steve.teale at britseyeview.com> wrote:
>>>>
>>>>> On Thu, 27 Oct 2011 16:16:26 +0200, Alex Rønne Petersen wrote:
>>>>>
>>>>>> On 27-10-2011 15:50, Steve Teale wrote:
>>>>>>>>
>>>>>>>> Surely Variant.init should do the trick?
>>>>>>>
>>>>>>> Dmitry,
>>>>>>>
>>>>>>> Are you talking about the new std.variant - I don't see 'init'
>>>>>>> currently.
>>>>>>>
>>>>>>> Steve
>>>>>>
>>>>>> He is probably referring to the 'init' property on the *type*, i.e.
>>>>>> int.init and so on.
>>>>>>
>>>>>> - Alex
>>>>>
>>>>> Same catch 22. In order to have an init property, the Variant would
>>>>> have to have been set to some type, at which point hasValue() would
>>>>> say yes.
>>>>>
>>>>> Steve
>>>>
>>>> import std.variant;
>>>>
>>>> void main()
>>>> {
>>>>      Variant v = 1;
>>>>      assert(v.hasValue());
>>>>      v = v.init;
>>>>      assert(!v.hasValue());
>>>> }
>>>>
>>>> -Steve
>>>
>>> Exactly, and v has reverted to being typeless:
>>
>> Sorry to jump in mid-stream, but you seemed to be suggesting Variant
>> didn't have an init without an assigned type.
>
> No, I was just saying it didn't help.
>
>> Why wouldn't you just wrap variant if you want to introduce a nullable
>> variant type?  Why does Variant have to be muddied with requirements for
>> database API?  Database null is an entirely different animal from D's
>> null.
>
> Well, yes, that was my first reaction, but I thought I'd ask - if there
> was a spare bit somewhere in Variant, would it do much harm, and Variant
> is getting a makeover. Maybe there are circumstances other than database
> interactions where it could be useful.

I don't like Variant having this behavior when it's only of specific use.   
Variant is not a database-only type.

Here's another idea:

struct DBNull(T) { }

Where T is the type for the column.  i.e.:

row.column = DBNull!int;

Now, you have your flag indicating it's null (you should be able to write  
a function that returns whether the column is null or not), it doesn't  
consume any more space (value is already part of a union that's bigger  
than DBNull!int), and the type of field is still valid.

-Steve