NULL indicator in Variant

[Regan Heath]

On Thu, 27 Oct 2011 19:06:47 +0100, Steve Teale  
<steve.teale at britseyeview.com> wrote:
> On Thu, 27 Oct 2011 13:58:54 -0400, Steven Schveighoffer wrote:
>> 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.

Could you wrap std.typecons.Nullable around it?  It's currently disabled,  
but a quick copy/paste into my own src, plus templating the opAssign e.g.

/**
Assigns $(D value) to the internally-held state.
  */
     void opAssign(T value)
     {
         enforce(_value);
         *_value = value;
     }

and this works:

import std.exception;		//nullable
import std.variant;
import std.stdio;

void main()
{
     Nullable!Variant a;
	assert(a.isNull);
	a = 5;
	assert(!a.isNull);
	assert(a == 5);
}

I believe Nullable is going to be re-enabled soon?

One argument in the nullable pull request:
https://github.com/D-Programming-Language/phobos/pull/153

which also applies, is;  why not just use Variant*'s in your database  
cases?

R

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