Extended Type Design.

[Daniel Keep]

Andrei Alexandrescu (See Website For Email) wrote:
> Bruno Medeiros wrote:
>> What is the status of the experimental designs for the "storage
>> classes" manipulation that Andrei and others where thinking of for D.
>> The last I heard from it was Andrei's max suggestion from his max
>> design challenge, however, I think that suggestion may suffer from
>> some problems in regards to the "maxtype" requirement, plus it is
>> wholly incomplete in regards to how storage classes interact between
>> each other. Like Andrei said, what is a "const inout lazy const
>> char[]", if valid at all? Is there any news here? Is there a
>> working(aka complete) design?
> 
> We have talked about a design. In short, the intent is to define three
> flavors of immutability:
> 
> a) final - a simple storage class controlling the immutability of the
> bits allocated for the symbol per se;
> 
> b) const - type qualifier meaning an immutable view of an otherwise
> modifiable data. const does not control the bits of the object, only the
> storage addressed indirectly by it (transitively);
> 
> c) "superconst" - denoted as "const!" or "super const": type qualifier
> meaning that the data is genuinely unmodifiable.
> 
> There is talk about deprecating lazy if it's best implemented via other
> mechanisms. There is also talk about deprecating "inout" in favor of
> "ref" on grounds that the often-useful "inout const" is likely to become
> #1 reason for bashing D.
> 
> To read a declaration like "const inout lazy const char[]", you can
> first parenthesize it appropriately:
> 
> const(inout(lazy(const(char[]))))
> 
> The lazy thing is really a delegate that returns a const char[]. The
> inout around it passes that delegate by reference, and the const at the
> top makes the delegate immutable.
> 
> 
> Andrei

This is fantastic news.  I'm really happy to know you guys are making
progress on this.

So if I understand this right:

* "final int*" is an immutable pointer that points to mutable data,

* "const int*" is a mutable pointer to immutable data, but that data
*may* be mutable in another context (ie: const((new int[10]).ptr)) and

* "const! int*" means "seriously, this *will not* change".

In any case, great to know things are moving along :)

	-- Daniel

-- 
Unlike Knuth, I have neither proven or tried the above; it may not even
make sense.

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