Re: Structured exception information

Maciej Sobczak <no.spam@xxxxxxxxxxx> writes:

Stephen Leake wrote:

How can I pass some error information from the constructor function
out, so that it's used when the exception is handled?
Why do you want to?

So that the handler has more information about the error?

Generally, all you can do is report the error to
the user.

That's what I want to do.

Then what's wrong with a string?

Can you give a detailed example of passing more complex information
that is actually used?

Imagine a constructor function that calls a database or a script
engine with the query/script provided as a parameter. Some error can
result and I want to raise an exception. The handler might benefit
(even if for the purpose of presenting a nice error pop-up message)
from:

- error code
- error message
- line number
- character position
- hint from the engine
- timestamp from within the engine
- ...

All of this can be put into the string by the routine raising the
exception:

raise Database_Error with
"Error " & Error_Code_Type'image (Error_Code) &
": " & Time_Type'image (Timestamp) &
": " & file_name & ":" & Integer'image (Line_Number) &
":" & Integer'image (Character_Position) &
": " & Message &
": " & Hint;

If you want a GUI pop-up, and it would be wrong to have the routine
raising the exception do the pop-up (which is reasonable), the
pop-up can just display this string. That's what Windex did.

I suppose if you want to put the different pieces of information into
different widgets in the "nice error pop-up", then you now have to
parse the string. With the structure I've shown, that's not hard; it's
colon-delimited. But I'll grant that is a case where more structure
than a string is desired.

As a general matter of user-interface design, I would find that
annoying. As a user of a system, the only thing I want from an error
message is either "how do I fix this myself" or "how do I report this
to the maintenance team".

In the first case, if the error is in a source file, displaying the
source file and line number, in a format that my IDE can understand,
is the best solution. That's a string on standard_output, not a GUI
pop-up.

In the second case, writing the string to a log file, and
displaying a pop-up with the full path name of the log file is a
_much_ better user interface. Make sure you can copy the file name, so
it can be pasted into an email message.

As a maintainer, I want the same thing. I hate conversations like
"what does the pop-up say in the third widget"? "no, the third widget
right to left, not top to bottom". Much better to say "send me the log
file".

Another example - imagine that your compiler just prints
"COMPILER_ERROR" instead of everything that it actually prints.
That wouldn't be very funny!

The compiler prints a string! I have a script interpreter that does
exactly that. The string is formatted at the lowest level, where the
details of the error are known. Sometimes a higher level exception
handler prepends another string with source file information:

exception
when E : Input_Error =>
raise Script_Error with File_Line_Column (current_script) &
Ada.Exceptions.Exception_Message (E);
end;

In building this system, I have never wished for more complex info
from an exception. Sometimes the string length limit is a problem, but
that's an implementation detail, not a fundamental design issue.

--
-- Stephe
.

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