Nullable with reference types

[Jonathan M Davis via Digitalmars-d-learn]

On Tuesday, 30 June 2015 at 00:02:38 UTC, Meta wrote:
> On Monday, 29 June 2015 at 19:29:37 UTC, sigod wrote:
>> Hi, everyone.
>>
>> ```
>> import std.typecons : Nullable;
>>
>> class Test {}
>>
>> Nullable!Test test;
>> assert(test.isNull);
>> ```
>>
>> Why does `Nullable` allowed to be used with reference types 
>> (e.g. classes)?
>>
>> P.S. I have experience with C#, where `Nullable<T>` cannot be 
>> used with reference types. And it sounds logical to me.
>
> It's a design mistake in Nullable. I would suggest that either 
> never use Nullable with a type that already has a null value, 
> or use the "overload" of Nullable that takes a null value, and 
> set it to null. Example:
>
> Class Test {}
> alias NullableTest = Nullable!(Test, null);

I tend to think that it's incredibly stupid to use something like 
Nullable for a type that's already Nullable. It's just silly. If 
a type is already nullable, then just use that and stop being 
adding extra overhead for no good reason. However, it _is_ true 
that if you need to have a nullable variable in generic code 
where the type that you need to be nullable could be any type, 
then having Nullable work with all types - and work with them all 
in the same way - is useful. Without that, you'd have to special 
case your code for types which were naturally nullable (and thus 
used null) and those which required Nullable. So, I can see why 
it could be useful to have Nullable work with classes, but I also 
question how common such a use case is.

- Jonathan M Davis