Re: API design problem - buffer scatter I/O

[Robert A Duff <bobduff@shell01.TheWorld.com>]

christoph.grein@eurocopter.com writes:

> Thank you, Bob, for these illustrative comments. Problems with wiki is
> that people with no good knowledge about the innards of a language
> implementation may contribute (and I include myself into this class).
>
> I fear you do not feel like correcting the wiki. I know, it's much
> work, the whole chapter has to be rearranged.

You're right -- I don't have time to do a good job of that,
so I'm not even going to try.

I do appreciate the efforts of others who choose to help out
with the wiki, even if it's not always perfect.

>> >...This is not an implementation
>> > detail.
>>
>> Sure it is. �How a compiler represents data types (including access
>> types) is an implementation detail.
>
> What I meant is the incompatibility between pointers to constrained
> and unconstrained:
>
> type Unc     is array (Integer range <>) of Character;
> type Acc_Unc is access Unc;
>
> CO: aliased Unc (-1 .. +1) := (-1 .. +1 => ' ');
> UO: aliased Unc            := (-1 .. +1 => ' ');
>
> A: Acc_Unc            := CO'Access;  -- illegal
> B: Acc_Unc            := UO'Access;  -- OK
> C: Acc_Unc (CO'Range) := CO'Access;  -- also illegal
>
> which is not an implementation detail.

Ah, sorry, I misunderstood what you were referring to.  I agree that
this restriction (A is illegal) is not an implementation detail.  The
declaration of A makes perfect sense, but it is forbidden for efficiency
reasons.  Well, not efficiency, really -- distributed overhead.  Some
say that was a bad language design choice, and I'm at least half
convinced.  ;-)

I'd suggest removing C from the wiki.  Constrained subtypes of access
types are a weird and nearly-useless feature.  I wouldn't teach
about them to beginners -- they're confusing.

- Bob