Re: Discriminants or Constructor Function for Limited Types

[Doctor Who <doc@tardis.org>]

On Sun, 8 May 2022 20:00:15 +0200, "Dmitry A. Kazakov"
<mailbox@dmitry-kazakov.de> wrote:

>On 2022-05-08 19:19, Doctor Who wrote:
>> On Sun, 8 May 2022 10:37:48 +0200, "Dmitry A. Kazakov"
>
>>> Note, that your code is already unsafe because nobody knows if
>>> Create_or_Open_Output always opens the file and because there is no
>>> guarantee that the file is closed, while the design
>>>
>>>     declare
>>>        Output_File : Some_File_Type;
>>>     begin
>>>        Write_Output (Output_File, Data);
>>>     end;
>>>
>>> is 100% safe.
>> 
>> Not in the case that your data space is exhausted, in that case
>> Write_Output will fail, because you have no checks of free space
>> before writing.
>
>Upon a write error an exception will propagate and Output_File will go 
>out of the scope. The file will be closed and the exception will 
>continue its way. This is a safe behavior. Randy's code is unsafe and 
>requires catching and re-raising I/O exceptions in the client code.
>
>This is an illustration why exceptions should be allowed to propagate 
>out of Finalize. When Finalize is used to close a handle this may fail 
>and there is no way to retry inside Finalize. The choice in Ada is 
>between aborting the program or sweeping the problem under the rug 
>ignoring the problem. A more sane approach would be to propagate 
>exception out of Finalize indicating the problem.

if you do a check before write there is no need to rise and propagate
an exception.