Re: TR : Helton article Throw/Catch

[Scott Klement <rpg400-l@xxxxxxxxxxxxxxxx>]

Hi Paul,

> I was just wondering what you meant by
>  "on-error catches all message IDs.  In fact, that's the problem
>  with it!  it doesn't give you a change to specify that some of them are
>  "unhandled",
As you, yourself, pointed out, on-error will catch everything.  It not 
only will catch the messages that you send with the throw() subprocedure, 
but it'll also catch an MCH1202 or some other error that you may not have 
expected.
For example, let's say I'm writing a subprocedure to delete a file.  The 
caller might expect that the file doesn't exist, so it might want to 
monitor for CPF1205, and if that's the only error that gets thrown, 
continue normally.
On the other hand, if something totally unexpected occurs, for example an 
MCH3601, I might want NOT want to catch it. OR I might want to let another 
program that's higher up on teh call stack decide whether or not it should 
handle the error.
You can do that easily in CL with the MONMSG command, but it's not so easy 
in RPG, because you can't monitor for specific msgid's, only for RPG 
status codes.
When you find an exception like this that you DON'T want to handle (but 
would prefer the OS's error handler to kick in) then what you have to do 
in RPG (because there isn't a better alternative) is send a new *ESCAPE 
message.
Because the procedure that you called can return ANY escape message, the 
easiest way to handle these "unexpected exceptions" is to use the QMHRSNEM 
API. That API will look up the last exception that occurred, and it'll 
re-send it to your caller.
That way, when the MCH3601 occurs in a subprocedure that I called, I call 
QMHRSNEM and the exact same MCH3601 message gets sent to my caller.  It's 
very much like the way ILE would've propagated the error message up the 
call stack if I had not caught the error.
I posted an example of the QMHRSNEM API to this mailing list in the past. 
Here's the link to where I posted that example:
http://archive.midrange.com/rpg400-l/200510/msg00610.html

I hope this explanation was better.