VB "On Error" Equivalent in Perl

[Gorby]

Hello Wise Monks,

In VB, it's possible to instruct the program to continue even if an unexpected error occurs. Is there an equivalent way to do that in Perl? My problem is that I'm using Net::FTP, and that module times out after a few seconds, and my Perl program halts and exists without giving me a choice to reconnect and try again. How can I handle errors like this and keep my Perl program running?

Thanks in advance.

Gorby

[jrsimmon]

Have you tried eval? It's similar to catch in C/C++. Details can be found in perldoc: "perldoc -f eval" on your cmd line.

[diotalevi]

It's possible to resume, sort of. See chromatic's Resumable Exceptions (sort of).

[wind]

Check out Conway's Perl Best Practices. From Chapter 13: Error Handling, Section Recoverable Failure, we find the following technique:

use Carp qw(croak);
use English qw(-no_match_vars);
use Net::FTP;
use Time::HiRes qw(sleep);

use strict;

my $MAX_TRIES = 15;

my $ftp;

TRY:
for my $try (1..$MAX_TRIES) {
    # If successful, we're done.
    eval {
        $ftp = Net::FTP->new("some.host.name", Debug => 0)
            or die "Cannot connect to some.host.name: $@";
        last TRY;
    };

    # Report non-recoverable failure if no more tries
    croak( $EVAL_ERROR ) if $try == $MAX_TRIES;
    
    # Try again after a nap (approximate fibonacci)
    sleep( rand (1.618 ** $try) );
}

- Miller

Update: Missing Semi-colon.

[erroneousBollock]

    eval {
        $ftp = Net::FTP->new("some.host.name", Debug => 0)
            or die "Cannot connect to some.host.name: $@";
        last TRY;
    }

I'm curious about your use of $@ om the second line of the eval. How would $@ be set without causing an exception that would throw control outside the eval?

If the die "Cannot ..." ever occurs, wouldn't $@ be empty?

Surely $@ is only useful outside the eval?

Also, eval {} is a statement and should be followed by a semicolon if further statements are to follow in the current block. Your code doesn't compile.

-David

[wind]

That is explained in the documentation for Net::FTP.

If the constructor fails undef will be returned and an error message will be in $@.

All other Net::FTP methods save their error message in $ftp->message.

I've also sometimes wondered about the logic of this. Nevertheless, that's the way it is.

- Miller

[bdimych]

I may be not exactly right, but in perl, quite the contrary, majority of errors do not make script exit. Most functions return false or undef on error and it can be checked just with "if" operator. Imho, there is rather opposite problem in perl - how to implement exceptions and try/catch

I agree too, that 'eval'+module documentation is the guaranteed right way

PS could you provide example