1. RE: Exception Handling
-------Phoenix-Boundary-07081998-
Hi Chris Bensler, you wrote on 3/24/02 4:34:58 PM:
>I'm hoping to get some feedback on this library. Positive and negative
>criticism is appreciated.
>
>I've written an error library and implemented what I 'think' is a
>exception handling system.
>
>Sorry for the lack of documentation, but it's still in prototype.
>
>Basically, I know nothing of exception handling other than my own
>theories and the other implementations that I have seen. I'd like to get
>some more insight as to the how and why.
>Is this system sufficient? What are the main properties of an exception
>handling system?
>
As I understand it:
You throw an exception with something like
if file_open (nm, mode) = ERROR then
throw (file_open_error)
While throw() looks like a call, it is actually a return which returns
to the nearest catch statement. This catch statement
may be in the throwing function, or in any of it's callers.
Throw is therfore a sort of multilevel return, that 'unwinds' the
call stack looking for a catcher.
Example:
procedure open_file (sequence name)
if open(name, "r") == -1 then
throw (file_open_err)
end if
... --perform other housekeeping
end procedure
procedure init_logfile (sequence path)
open_file (path & "log")
...
end procedure
procedure main ()
again:
init_logfile (path)
do_process()
report_results ()
:catch:
if error = file_open_err and retrys = 0 then
retrys = 1
path = getenv_path(); --try a different path this time
goto again
end if
rethrow () --let a higher catch (if any) handle it
end procedure
The important thing is that we have decoupled the place where
the error is handled from the place where it was generated.
I can't imagine how this can be done except by modifying the
interpreter code (which is quite easy).
Karl Bochert
-------Phoenix-Boundary-07081998---
