[Ocaml-lib-devel] Re: VList

[Bardur A. <sp...@sc...>]

Alain Frisch wrote:
> Bardur Arantsson wrote:
> 
>> I think there *might* be a problem with doing this. I don't remember 
>> the details right now, but I do remember being quite sure it was 
>> actually necessary to fill up using NULLs instead of copies. 
>> (Obviously it had something to do with the extra references causing 
>> objects to hang around for longer than necessary).
> 
> 
> I've thought about that, and I don't see any problem. Since the "used 
> part" of the array is read-only, I don't see any problem. If the value 
> used as an initializer is referenced through any cell of the array, then 
> the first cell - which still contain the same value - is also referenced 
> as well.
> 

Yup, you're certainly right about that. The only potential "problem" is 
in situations like with VList.map where you have references *after* the 
last_used:

      |---| \             |----|
      | . | |-- not x     | .. |
      | . | /             | .. |
  L-> | x |    ---map---> | x' | <- L'
      | x |               | ?? |
      | x |               | ?? |
      |---|               |----|

Since the reference to L may be dropped later (and there are none before 
position L), you certainly don't want to set '??' to x; you want to set 
them to x'.

So the "problem" is just that you have to be a bit more careful about 
initializers when copying the array. But since Array.create will 
actually "force" you to think about the initializer should be, it 
shouldn't be a problem in practise. (And besides, I only need to get the 
code right *once* :))

Thanks for the help, I'll remove all the NULL stuff some time today and 
make a new snapshot.

-- 
Bardur Arantsson
<ba...@im...>
<ba...@sc...>

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