CODEWITHSUNDEEP
perl
GoldenNewby
GoldenNewby

Reputation: 4452

Safely test scalar for Storable compatibility

I have a program which accepts a perl data structure which is intended to be a Storable scalar. Is there a way to test if the scalar is a valid Storable object, without dying if it isn't?

For instance, if I do:

use Storable qw(freeze thaw);
my $ref = thaw("lol_not_storable")

I get back "Storable binary image v54.111 more recent than I am (v2.8) at /usr/local/lib/perl/5.12.4/Storable.pm line 420, at test.pl line 5"

I would like to figure out if it is possible to cleanly handle these exceptions without eval. Is it possible without rewriting the Storable Perl Module?

Upvotes: 1

Views: 441

Answers (2)

Rebell P.K.
Rebell P.K.

Reputation: 1

Do it with magic :)

use Data::Dumper;
use Storable qw(freeze thaw read_magic);

my $storable_str = freeze( [ 1 .. 42 ] );
print Dumper( read_magic($storable_str) );
# prints:
# $VAR1 = {
#     'netorder'   => 0,
#     'hdrsize'    => 15,
#     'version'    => '2.7',
#     'minor'      => 7,
#     'longsize'   => 8,
#     'ptrsize'    => 8,
#     'version_nv' => '2.007',
#     'byteorder'  => '12345678',
#     'major'      => 2,
#     'intsize'    => 4,
#     'nvsize'     => 8
# };

my $ordinary_str = join( ',', (1 .. 42) );
print Dumper( read_magic($ordinary_str) );
# prints:
# $VAR1 = undef;

# So:
if(read_magic($something_to_check)){
    my $ref = thaw($something_to_check);
}else{
    # foo
}

Upvotes: 0

Axeman
Axeman

Reputation: 29854

eval { thaw("lol_not_storable"); };

is not the same thing as 

eval qq/thaw("lol_not_storable");/;

in that Perl has ample chance to parse the first, but waits to parse the second. Observe, the below is a compile error: 

use 5.014;
use strict;
use warnings;

say 'Would print without compile error';
eval { $i++; };
^D

Global symbol "$i" requires explicit package name at - line 8.
Execution of - aborted due to compilation errors.

Whereas eval '$i++' wouldn't be. I think most of the discouragement you have heard about eval is more of the latter type and not of the former. The latter evaluates a string as code, the former mainly tells Perl "don't die."

Here's the string version:

use 5.014;
use strict;
use warnings;

say 'Would print without compile error';
eval ' $i++;';

Outputs:

Would print without compile error

The code still fails to compile, but only when it's eval-ed, and only has effect when I check $@, which reads:

$@= 'Global symbol "$i" requires explicit package name at (eval 24) line 1.
'

Upvotes: 3

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