To make it a bit nicer, you could use Exception::Class::TryCatch in tandem with Exception::Class -

use Exception::Class::TryCatch;

...

# try
eval { $obj->some_method('foo') };

# catch
if ( catch my $err ) {
  $err->isa('Bio::Phylo::Exceptions::BadNumber') and do { handle_this(
+$err) };
  $err->isa('Bio::Phylo::Exceptions::ObjectMismatch') and do { handle_
+that($err) };
}
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Arunbear's answer is a good one. I'd add that you want to be sure you know why you want to use Exception::Class. If you aren't planning to handle various exceptions, you almost might as well define some standard error messages and just die with them -- but then the major advantage of Exception::Class is that it's easier/saner/maintainable to determine what type of error occured from an object type than from parsing the error message.

You want to consider the right place to use eval. Do you want to:

• handle exceptions yourself
• allow end-users to manage exceptions

You should wrap routines with eval when you either know what the fallback behavior should be or you need to exit more gracefully than just allowing the program to die. (That's all the throw is -- just a die with an Exception::Class object instead of an error message.) And you want to wrap it at the right point for the error to be handled. That means probably not wrapping every little call, but rather wrapping as high up in the calling stack as possible:

use Exception::Class::TryCatch;

sub user_input_loop() {

  USER_INPUT:
  while ( my $line = <> ) {
      # try the command
      eval {  process_command( $line ) };

      # catch any error
      if( catch my $err ) {
          if( $err->isa('MyErr::Recoverable') ) { 
              warn "Input error: $err\n"; 
              next USER_INPUT;
          }
          else {  # not recoverable
              warn "Unrecoverable error: $err\n";
              $err->rethrow;
          }
      }    
  }
 
  print "Exiting normally\n";
}
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You want to leave it to your end-users when the right thing to do in response to an error condition is up to them. Or you can stage it: catch the exception, do some cleanup, then rethrow the exception.

It's not really much of a contortion if you consider what an alternative might be. If an end-user wants to know why some eval died, their only other option is to examine the $@ as a string. That's requires similar contortions and probably breaks the moment you change one of your error messages.

Thank you both for your clarifications - that's very helpful. I think I am just going to have to think hard in each situation whether I want to catch exceptions myself or leave it to the user. I suppose it depends, in some cases I might be able to do something gracefully, but in other cases the user is going to have to be told forcefully that their input isn't sane. I don't suppose there's a hard and fast rule for that, is there?

I agree with salva's rule.

the lowliest monk

You have the basic flow down, but the "catch" part can be simplified. Exception::Class provides a "caught" method that returns the exception object if it is of the right class or false. See "Catching Exceptions" in the Fine Manual for Exception::Class.

package MyExceptions;

use strict;
use warnings;

use Exception::Class (
    'MyExceptions',
    'MyExceptions::RegionNotFound' =>
    { isa => 'MyExceptions' },
    'MyExceptions::CommandNotExecuted' =>
    { isa => 'MyExceptions' }
);

1;
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Then I have another module to report the exceptions handled

package ReportGenerator;

use strict;
use warnings;

sub CheckResult {
   my ( $result, $info ) = @_;
   #Here is want to check of $result and throw an exception of the kin
+d MyExceptions::RegionNotFound->throw(error => 'bad number');
#how do I do it??
}
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The user would script something like this

$Report->CheckResult($ABC->Command(100,100),"Someinfo")
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See Using Perl Exception Class.

It makes little sense to ask the same question in two locations, especially in a thread that is 5 years old.

Yes Corion..was in fix to get this done and... i didn't notice that the thread was 5 years old.. cud finally figure out how to do this ...

use Exception::Class (
        'MyExceptions::Test' => {                  
                   fields => [qw{message}],
   }  
);

use Moose::Util::TypeConstraints;

class_type 'MyExceptions::ExecutionTest';

no Moose::Util::TypeConstraints;
1;
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then to throw the exception

MyExceptions::ExceptionTest->throw( message => $data )
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then the script has something like

 if ( my $ex = $@ )
{
my $e;

  if ( $e = Exception::Class->caught('MyExceptions::ExecutionTest'))
 {       
   print $e->message;
  }
 }
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Thanks for your help