Re: [Ocaml-lib-devel] Array interface

[Brian H. <bri...@ql...>]

On Thu, 19 Jun 2003, John Max Skaller wrote:

> Brian Hurt wrote:
> 
> > On Tue, 17 Jun 2003, John Skaller wrote:
> 
> >>Can't you initialise the empty slots with Obj.magic(0)?
> >>
> > No.  What if it's an array of floats, which is unboxed?
> 
> 
> Ah.
> 
> 
> > If you don't have a convient default, use a 'a option Dynarray.t.  
> 
> 
> No way. That's a hack to support a faulty implementation.
> I want a 'a Dynarray.t, the *filled* slots always have that type.
> I don't see why I should change the type to suit an implementation
> that can't handle the idea of a variable length array properly.

As I can't use Obj.magic() if it's an array of floats, the only other
option I have is to make *everything* a 'a option Dynarray.t implicitly-
make the internal array a 'a option array instead of just a 'a array.  
Which means if it's a dynarray of ints, the ints are not unboxed.  
Futhermore, if I understand things correctly, the ints aren't unboxed in
the option variable either- meaning that I've now added two dereferences
and two extra words for every integer in the array- when I can instead
just use an integer array in the current implementation.

> 
> C++ vector does this correctly.

At the cost of emitting a whole new wad og object code for every single 
type you put into a vector.  Vector<int> requires the compiler to produce 
a whole vector class to deal with ints, seperate from Vector<short>, 
Vector<char>, Vector<void *>, etc.  

> 
> > Note: in the current API there is no way to get access to an element with 
> > a null value.  So you don't ever have to worry about distinguishing null 
> > values from sensible values.  So it's still safe if the null value would 
> > otherwise have meaning.
> 
> 
> I have examples where I CANNOT construct a dummy value.
> The fact that the value is not accessible with
> an array access cannot remove a requirement
> an object may have to connect to other objects.

In this case, you're going to be using a 'a option Dynarray.t one way or 
another.  The question is wether you can ever get rid of the option part.

> It could be there is NO good solution in Ocaml.
> In that case, use a C solution. I want this
> data structure to work exactly right, and I need
> it precisely *because* I can't define it myself.

Feel free to submit code.  

Brian