Re: Labeled exit within generalized labeled loop not allowed?

[Niklas Holsti <niklas.holsti@tidorum.invalid>]

On 12-07-16 12:33 , Thomas L�cke wrote:
> On 07/13/2012 06:19 PM, Marc C wrote:
>> I find it hard to believe that this would be a GNAT GPL 2012 compiler
>> bug, but I don't see anything in the LRM that indicates this wouldn't
>> be legal.
>>
>> Code containing a labeled generalized loop (the "for Elem of
>> Some_Container" type), with a conditional exit referencing the loop
>> label, fails to compile with an "Invalid loop name in exit statement"
>> error.
>>
>> What's the rationale for this? Or a consequence of what? Or is it a
>> compiler bug?
>
>
> Hmm, it appears we're the only people who experience this, or perhaps
> the only people who care?  :D

My guess is a compiler bug, not discovered earlier because the labeled 
exit statements are rare, and users of Ada 2012 also (so far).

> I would've thought that labeled exits were something that were widely
> used.

A data point on this:

I scanned an Ada 2005 project of mine. It has 893 loops but only 35 
labeled exit statements. These are always on large, complex loops, 
sometimes nested loops where the exit leaves the outermost loop from an 
innner loop. When these loops are traversing a data structure, the 
structure is usually dynamic (e.g. a work-list) that can shrink and grow 
during the execution of the loop.

This project has many loop exits from small traversal loops of fixed 
data structures. These loops could use the generalized loop syntax, but 
the loops are small and usually not nested, so they are not named.

To me, the generalized loop syntax suggests that the loop should be 
executed for all the elements in the container, that the amount of data 
in the container does not change during the loop, and that the traversal 
order is not important. I know that the order is defined; I am talking 
about my impression while reading the program. I'm not sure if data can 
be added to or removed from a container during a generalized iteration 
over the container; would that be "tampering"?

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
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