why cann't $@ output the eval error in this case?

lisa2moon has asked for the wisdom of the Perl Monks concerning the following question:

use Win32::OLE;

my $book;
my $sheet;
my $sheet_name = "rota";
my $rota_location = 'e:/rota.xls';

if( -e $rota_location ){
    $book = $ex->Workbooks->open($rota_location);    
}else{
    $book = $ex->Workbooks->Add;
}

unless($sheet = eval { $book->Worksheets($sheet_name) }){
    $sheet = $book->Worksheets->Add;
    $sheet->{Name} = $sheet_name;
}
[download]

Using eval { $book->Worksheets($sheet_name) }, I want eval catch the error if the sheet doesn't exist. But the method doesn't work. when the sheet doesn't exist,the perl showes the following error:

Win32::OLE(0.1701) error 0x8002000b: "" in METHOD/PROPERTYGET "Worksheets" at create_rota.pl line 48

I'll appreciate your help.

Comment on why cann't $@ output the eval error in this case? Select or Download Code

Replies are listed 'Best First'.
Re: why cann't $@ output the eval error in this case? by ELISHEVA (Prior) on Jan 05, 2011 at 10:52 UTC
lisa2moon, welcome to Perl Monks! You are correct in believing that `unless (eval {...}) { #error handling }` should catch the exception and add the worksheet if the worksheet you requested can't be found, but that only works if the API is coded/configured to throw exceptions. Perl errors are only stored in `$@` if the author of the API codes it that way. Sometimes packages will generate warnings rather than throw errors. Sometimes they will use the return value for an error. System level routines sometimes also set an error id variable: `$!` - see perlvar for further discussion. As for your specific package, Win32::OLE, you can configure error handling (skim down the documentation page for the discussion of `warn` option (`Win32::OLE->Option(Warn => 3);`). I haven't used this module, but from what I can see in the documentation, if you want your code to die and throw an exception you can catch rather than disappear in the vapour of warnings, you'll need to set the warning level to 4 (croak). Alternatively, you can completely customize error handling by setting Warn to a code reference. Update: added note about using code references to customize error handling.	[reply] [d/l] [select]
Re^2: why cann't $@ output the eval error in this case? by moritz (Cardinal) on Jan 05, 2011 at 11:31 UTC
You are correct in believing that `unless (eval {...}) { #error handling }` should catch the exception If the code `...` returns a false value, the unless block is still executed even if no exception is thrown, in which case $@ doesn't meet your expectations at all. (I don't know the used modules enough to know if that can happen in this particular case). To be on the safe side and only exeucte a block if an exception occors, I often write `unless (eval { ...; 1} ( { # exception handling code here }` [download] Perl 6 - second systems done right	[reply] [d/l] [select]
Re^2: why cann't $@ output the eval error in this case? by lisa2moon (Initiate) on Jan 05, 2011 at 11:40 UTC
Thanks a lot. I added the `Win32::OLE->Option(Warn => 4);` before `unless($sheet = eval { $book->Worksheets($sheet_name) })` . And it works well exactly what I wanted.	[reply] [d/l] [select]
Re: why can't $@ output the eval error in this case? by moritz (Cardinal) on Jan 05, 2011 at 10:42 UTC
Please show the actual code you're running (you don't print $@ anywhere in your code, and there's no line 48 in the short snippet you've shown), what you get and also what you expect. Perl 6 - second systems done right	[reply]
Re: why cann't $@ output the eval error in this case? by Anonymous Monk on Jan 05, 2011 at 10:36 UTC
Which line is line 48?	[reply]