Purity, @safety, etc., in generic code

[Zach the Mystic]

On Monday, 18 February 2013 at 07:46:24 UTC, kenji hara wrote:
>> Now, as for const, I don't think that the problem has been 
>> solved yet. We
>> really need some sort of inferrence there, which is why a 
>> number of months
>> ago
>> I created this enhancement request for inferring inout:
>>
>> http://d.puremagic.com/issues/show_bug.cgi?id=8407
>>
>> AFAIK though, no one's even looked at it, so I don't think 
>> that it's had
>> any
>> effect as of yet. One of the compiler devs will need to take 
>> it up for it
>> to
>> make it in.
>
> I think const must not be inferred for member function, because 
> it affects
> to the function overloading. For example, the const-ness for 
> class method
> will be inferred from base class overridden method with current 
> compiler,
> but it causes a serious problem.
>
> http://d.puremagic.com/issues/show_bug.cgi?id=8366
>
> Instead, you should declare two methods in generic code - one 
> is mutable,
> and another is const/inout.
>
> Kenji Hara

I'll say upfront that I don't understand this problem completely, 
but I was trying to solve it anyway!

What bothers me is that the spec for 'inout' functions suggests 
that they generate the same runtime code, so it seems weird that 
there should have to be different vtable entries for the same 
code.

Following that thought, perhaps 'inout' class member functions 
could be treated specially in the case of classes. If the base 
class defines a method using inout,

class A
{
     bool foo(inout Object o) inout { return true; }
}

... it defines a single function in the vtable, which applies to 
mutable, const, and immutable. If a subclass then defines a 
mutable method, that function is added to the vtable for that 
class and all descendent classes. It must be defined with 
override, despite that it actually creates a new function 
underneath:

class B : A
{
     override bool foo(Object o) { return false; }
}

I am in the position of someone who knows he doesn't know 
everything he needs to know, but who doesn't want a good idea to 
go unmentioned either. Do you think this idea has merit?