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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: 'for cursor in container.first loop' bug?
Date: Thu, 28 Aug 2014 14:56:18 -0500
Organization: Jacob Sparre Andersen Research & Innovation
Message-ID: <lto1h3$cja$1@loke.gir.dk>
References: <85y4u8xztj.fsf@stephe-leake.org>
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Xref: news.eternal-september.org comp.lang.ada:21962
Date: 2014-08-28T14:56:18-05:00
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"Stephen Leake" <stephen_leake@stephe-leake.org> wrote in message 
news:85y4u8xztj.fsf@stephe-leake.org...
> I'm trying to use the 'for cursor in container.first loop' syntax in Ada
> 2012 with my own container, and having lots of problems.
>
> So first I tried a simple example using Ada.Containers.Vector, with GNAT
> 2014:
>
> with Ada.Text_Io; use Ada.Text_Io;
> with Ada.Containers.Vectors;
> procedure Try_Containers
> is
>   package Integer_Vectors is new Ada.Containers.Vectors (Natural, 
> Integer);
>   use Integer_Vectors;
>
>   A : Vector := To_Vector (1, 10);
> begin
>   Loop_1 :
>   for Element of A loop
>      Put_Line ("A (i) => " & Integer'Image (Element));
>      -- can't do Element := 2;
>   end loop Loop_1;
>
>   Loop_2 :
>   for Cursor in First (A) loop
>      Put_Line ("A (I) => " & Integer'Image (Element (Cursor)));
>      Replace_Element (A, Cursor, 2);
>      Reference (A, Cursor) := 2;
>   end loop Loop_2;
>
> end Try_Containers;
>
> This gives compilation errors:
>
> gnatmake -k ../try_containers.adb
> gcc -c -I../ -I- ../try_containers.adb
> try_containers.adb:17:28: invalid prefix in call
> try_containers.adb:17:28: invalid prefix in selected component "I572b"
> gnatmake: "../try_containers.adb" compilation error
>
> Same error (with a different generated name) with GnatPro 7.2.
>
> Commenting out loop_2 gives the expected results.
>
> Am I missing something simple, or is this a compiler bug? At the very
> least, it's not a good error message.

You're missing something simple: "First" is not an iterator (it returns a 
cursor). You need to use one of the functions named "Iterate" that return a 
value that has an iterator type:

Loop_2 :
for Cursor in Iterate (A) loop

I'd still call it a bug, though, because the error message is inpenatrable 
for something simple (a violation of 5.5.2(3/3)). Indeed, this shouldn't 
even resolve.

I checked the two more recent GNAT compilers that I have (Gnatpro 7.2.2 and 
a wavefront), but they both have this message. I'd report it as a bug, 
because you probably wasted at lot of time (and even some of mine) on this 
rather simple mistake.

I was a bit surprised by this, because I thought the new ACATS tests covered 
this. (After some checking...) Interestingly, there is a test that checks 
for this sort of problem, and it produces sensible error messages for cases 
like:
    95.    for Item in Sparse_Cursor loop                            --  
ERROR:
                       |
        >>> cannot iterate over "Cursor"
        >>> to iterate directly over the elements of a container, write "of 
Sparse_Cursor"

   116.    for Item of Sparse_Cursor loop                            --  
ERROR:
               |
        >>> cannot iterate over "Cursor"

But your test (which looks the same to me) produces gibberish. This is 
obviously more subtle than I understand. Time to get some use out of your 
AdaCore support contract. :-)

                                        Randy.