!standard 11.4.1      (3)                        00-10-04  AI95-00241/01
!class amendment 00-10-04
!status work item 00-10-04
!status received 00-10-04
!priority Medium
!difficulty Easy
!subject Testing for Null_Occurrence

!summary

Function Is_Null_Occurrence is added to Ada.Exceptions.

!problem

Ada.Exceptions exports the constant Null_Occurrence, but doesn't
provide any way to test whether a given exception occurrence is
Null_Occurrence (type Exception_Occurrence is limited).

!proposal

We propose that a function should be added to Ada.Exceptions that
would allow testing whether an exception occurrence is Null_Occurrence.

!wording

Add the following to Ada.Exceptions (11.4.1(3)):

  function Is_Null_Occurrence (X : Exception_Occurrence) return Boolean;

Add the following after 11.4.1(7):

  Is_Null_Occurrence yields True if the argument is Null_Occurrence, and False
  otherwise.

!example


!discussion

When logging information, an application probably will use Null_Occurrence to
represent the absence of an exception.

When displaying the log, it is necessary to determine whether a particular
occurrence is Null_Occurrence, or an occurrence containing information about an
exception.

A Constraint_Error handler can be used for this purpose, but it is awkward,
does not reflect the fact that this is a normal case, and potentially can mask
other errors.

!appendix

!topic Testing for Null_Occurrence
!reference RM95-11.4.1(3)
!from Gary Dismukes (for Ada Core Technologies)
!keywords Ada.Exceptions, Null_Occurrence
!discussion

Ada.Exceptions exports the constant Null_Occurrence, but doesn't
provide any way to test whether a given exception occurrence is
Null_Occurrence (type Exception_Occurrence is limited).

We propose that a function should be added to Ada.Exceptions that
would allow testing whether an exception occurrence is Null_Occurrence.
In GNAT, we have added a child function Ada.Exceptions.Is_Null_Occurrence
for this purpose:

--  This is a GNAT-specific child function of Ada.Exceptions. It provides
--  clearly missing functionality for its parent package, and most reasonably
--  would simply be an added function to that package, but this change cannot
--  be made in a conforming manner.

function Ada.Exceptions.Is_Null_Occurrence
  (X    : Exception_Occurrence)
   return Boolean;
   --  This function yields True if X is Null_Occurrence, and False otherwise

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From: Tucker Taft
Sent: Friday, September 22, 2000 9:19 PM

I believe the original intent was that:

   Exception_Identity(Null_Occurrence) = Null_Id

providing a mechanism for testing for Null_Occurrence,
but I see that 11.4.1(14) contradicts that by indicating that
Exception_Identity raises Constraint_Error in this case.  I think
that is a mistake, though others might remember otherwise.

> function Ada.Exceptions.Is_Null_Occurrence
>   (X    : Exception_Occurrence)
>    return Boolean;
> --  This function yields True if X is Null_Occurrence, and False otherwise

This seems like a reasonable proposal, in any case, although it
is obviously an amendment, not an interpretation.

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From: Robert Dewar
Sent: Friday, September 22, 2000 10:03 PM

<<I believe the original intent was that:

   Exception_Identity(Null_Occurrence) = Null_Id

providing a mechanism for testing for Null_Occurrence,
but I see that 11.4.1(14) contradicts that by indicating that
Exception_Identity raises Constraint_Error in this case.  I think
that is a mistake, though others might remember otherwise.
>>

Well I would be happy to allow this identity, though this would be a change
(in GNAT we could add a child, but not change clear RM semantics), but
if one is in the amendment business, perhaps allowing the above identity
is the cleanest approach.

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