Re: S-expression I/O in Ada

[Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org>]

On 08/19/2010 01:09 AM, Natasha Kerensikova wrote:
>
> And now I have just had a look at GNAT FSF (v4.4) implementation, and
> there is a normal array assignment, which I guess triggers an assignment
> of each item, which in our case (record containing a Vector) triggers a
> deep copy. Or am I wrong here?
>
> So it seems a simple Vector.Append call might trigger a copy of the
> entire S-expression, which makes a strong point for the linked-list,
> doesn't it?

Yes, an Append to a Vector might trigger several recursive calls to Vector 
assignment. If using a Vector is clearer than using a list, that may offset any 
advantage of using a list. Since this is a general, reusable package, 
"efficiency" is not a real concern: projects with special requirements usually 
shouldn't use general packages and need special versions of such things to meet 
their requirements.

> And indeed, your implementation was the first I understood almost
> completely, but I can't tell how much is due to your clarity intend and
> how much is due to having already tried to write one.

Having tried to write one probably helped.

> I'm still wondering which is better between vector and array. I thought
> a vector could be easily be turned into an array, hence my initial
> choice of arrays, but it seems there is no such vector primitive in
> A.18.2. Another tough choice...

Yes, that was a mistake, IMO. An unbounded-array abstraction should have 
operations to convert to and from the fixed equivalent, like To_String and 
To_Unbounded_String in Ada.Strings.Unbounded (which is a special case of 
Vectors; Ada.Characters.Vectors, anyone?).

> Is there any other (perhaps more difficult for a beginner) way to
> address that issue?

That's hardly simple for a beginner. Another way is to use explicit access 
values (and types, though some would prefer the more C-like use of anonymous 
access types). I prefer to avoid explicit access types and values whenever 
possible, and it's clearly possible here.

-- 
Jeff Carter
"That was the most fun I've ever had without laughing."
Annie Hall
43

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