Re: Rational for not making cursor tagged in Containers

[Anh Vo <anhvofrcaus@gmail.com>]

On Apr 19, 7:53 pm, "Randy Brukardt" <r...@rrsoftware.com> wrote:
> "Anh Vo" <anhvofrc...@gmail.com> wrote in message
>
> > It seems that it is not balanced between Container types and its
> > cursors. I just want to know what rationale behind it for not making
> > cursor a tagged type.
>
> I think there are two reasons:
>
> (1) The original designer of the containers library intended for cursors to
> be implemented as a bare access value; adding a tag makes that messier and
> doubles the size of the cursors. I personally don't find this too important.
>
> (2) Ada does not allow an operation to be primitive for two tagged types.
> (I'm sure we'll hear at length from Dmitry on this. ;-) That would mean that
> operations like:
>      procedure Delete (Container : in out Vector; Index : in Cursor);
> would be illegal. That could be worked around by making the cursor
> classwide:

Thanks for your explanation. Reason in (2) is more legitimate.

>      procedure Delete (Container : in out Vector; Index : in Cursor'Class);
> but now you don't have much taggedness left. On most operations, you're not
> even losing prefix notation (as the cursor is the second parameter).

agreed.

> Indeed, the real reason for your problem is that container is implicit in
> Cursor operations that read from the container:
>      function Element (Position : Cursor) return Element_Type;
>
> A prefix call like
>     My_Cursor.Element
> doesn't make much sense, because a cursor is just an accessor, you're not
> extracting the element from it! You really ought to be able to say:
>    My_Container.Element (Position => My_Cursor)
> which makes it clear where the element is coming from.

The latter is already given. I am not quite agreed with the former
because I do not see any thing wrong to extract an element pointed by
an accessor if and only if Cursor is tagged.

> I admit we didn't think much about these issues. When it was suggested that
> perhaps the container type should be visibly tagged, I recall someone noted
> that would allow the prefix notation. And that seemed good; but I don't
> think anyone noted that it doesn't work sometimes.

It is a good lesson learned.

AV