Review of DIP49

[Andrei Alexandrescu]

On 2/3/14, 6:07 AM, Kenji Hara wrote:
> 2014-02-03 Andrei Alexandrescu <SeeWebsiteForEmail at erdani.org
> <mailto:SeeWebsiteForEmail at erdani.org>>:
>
>     Here are a few questions and comments:
>
> First of all, thanks for your reviewing!

And thank you for your work and this reply.

This part:

> Note that, currently D has 9 qualifiers:
> - (mutable)
> - const
> - inout
> - inout const (added from 2.065. See issue 6930)
> - shared
> - shared const
> - shared inout
> - shared inout const (added from 2.065. See issue 6930)
> - immutable
>
> So, if we allow defining arbitrary postblit from A to B copy, you can
> define 9 * 9 = 81 postblits!
> I don't want to see such horrible D code.

convinced me something has definitely gotten out of hand. We must devise 
a radically simpler solution to object copying, one that aims straight 
at solving the fundamental problems we're facing. Far as I can tell 
these are it:

1. Reference counting for all objects, including const/immutable/shared. 
If this(this) can't be reasonably made to solve that, we will build it 
into the language and raise the question whether this(this) should exist 
at all. In particular, calling .dup inside this(this) is an antipattern. 
Objects must be cheap to copy. So any example using somrArray.dup inside 
this(this) is an instant indication we're solving the wrong problem (and 
with a wrong solution as well).

2. Removing head const. You have added the nice conversion 
qualified(T[]) -> qualified(T)[] upon calling a function. We must look 
at ways to allow the user a similar generalization for type constructors.

If we figure this(this) doesn't help these two problems, its entire 
existence must be put in question.


Andrei