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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Vector of Vectors.
Date: Sun, 18 Jan 2009 16:29:39 -0500
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Date: 2009-01-18T16:29:39-05:00
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Georg Bauhaus <see.reply.to@maps.futureapps.de> writes:

> Robert A Duff wrote:
>
>> The loop body is usually self explanatory.  E.g:
>
> I wish it were, yes. A loop body _should_ be
> elf explanatory.  FWIW, I have not seen a loop
> with just single statement in quite a while.
> Hope things are in different shape elsewhere.

Not sure what you mean.  I'm saying that Ada's current support for
iterators requires you to wrap the loop body in a named procedure.
The fact that people don't normally wrap loop bodies in procedures
(when using 'for' and 'while' loops) proves that doing so is a burden.

I want to create procedures for exactly the code that represents
a coherent abstraction -- and that's not necessarily the body of a loop.

>> for Index in S'Range loop
>>     S(Index) := To_Upper_Case(S(Index));
>> end loop;
>> 
>> Do you want me to call it "Convert_Current_Character_To_Upper_Case",
>> and wrap it in a procedure by that name?
>
> No, but I think I'd prefer something like
>
>   Iterate(S, Process => To_Upper_Case);

Without the 'Access.  ;-)

Normally, the loop body needs access to some local variable (or
parameter).  Here's another example: Suppose we have a procedure
to send a character to an output stream:

    procedure Put(Stream: in out ...; C: Character);

And we want to write a procedure to send a String.  So we write:

    procedure Put(Stream: in out ...; S : String) is
    begin
        for I in S'Range loop
            Put(Stream, S(I));
        end loop;
    end Put;

It would be silly to write:

    procedure Put(Stream: in out ...; S : String) is
        procedure Put(C: Character) is
        begin
            Put(Stream, C);
        end Put;
    begin
        for I in S'Range loop
            Put(S(I));
        end loop;
    end Put;

No matter what you call that inner Put, it doesn't deserve to be a
named procedure at all.  Note that it has to be nested, because
it needs to refer to Stream -- we can't create a reusable one
and put it in a library.

But if we have a Vector of Characters, or Linked_List of Characters,
instead of an array of Characters, we would be forced to write that
silly nested procedure.

Not sure whether we are agreeing or disagreeing, here...  ;-)

> That is, To_Upper_Case is a good name, already.
> The loop above is a "for-all" loop, Index is used
> for nothing but a concrete, clumsy, and---if you
> will---redundant handle of the abstraction "for-all".
> So why a detailed loop?

I definitely agree with this part.  The Index is conceptually
unnecessary.  Furthermore, the fact that arrays and Vectors use
totally different syntax for iteration and element selection
and so forth is a Bad Thing.

Ideally, we want to say something like:

    for C: Character in S loop -- Whether S is an array, or some other sequence data structure
        <do something with C> -- not a named procedure
    end loop;

- Bob