Re: VB "On Error" Equivalent in Perl

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) );
}
[download]

- Miller

Update: Missing Semi-colon.

Comment on Re: VB "On Error" Equivalent in Perl Download Code

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Re^2: VB "On Error" Equivalent in Perl by erroneousBollock (Curate) on Nov 29, 2007 at 01:28 UTC
`eval { $ftp = Net::FTP->new("some.host.name", Debug => 0) or die "Cannot connect to some.host.name: $@"; last TRY; }` [download] I'm curious about your use of `$@` 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	[reply] [d/l] [select]
Re^3: VB "On Error" Equivalent in Perl by wind (Priest) on Nov 29, 2007 at 01:41 UTC
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	[reply]