make perl shout when trying to access undefined hash key

Question

Hi,

I think the title is self-explanatory. Many times I have small typos and I get unexpected results when trying to access undefined hash keys. I know I can add some defined check before each time I access a hash key, but I wonder if there's any cleaner way to warn against such cases....

Best, Dave

Answer 1

Use Hash::Util:

use Hash::Util "lock_keys";
my %hash = (foo => 42, bar => 23);
lock_keys(%hash);
print $hash{foo};
print $hash{baz};
print $hash{bar};

output:

42
Attempt to access disallowed key 'baz' in a restricted hash at foo line 5.

There are other functions that allow specifying which keys to allow, not just defaulting to what's already there.

Answer 2

You can write a simple function for this:

sub get {
    my ($hash, $key) = @_;
    die "No such key: $key" unless exists $hash->{$key};
    return $hash->{$key};
}

my %hash = (...);    
my $val = get(\%hash, "mykey");

Answer 3

This is probably best done with a tied hash. Tied variables allow you to define the implementation of the low level operations of the variable. In this case, we want a special fetch method that dies when accessing non-existant keys:

use warnings;
use strict;

{package Safe::Hash;
        require Tie::Hash;
        our @ISA = 'Tie::StdHash';
        use Carp;

        sub FETCH { 
                exists $_[0]{$_[1]} or croak "no key $_[1]";
                $_[0]{$_[1]}
        }
}

tie my %safe => 'Safe::Hash';

$safe{a} = 5;  # ok

print $safe{a}, "\n";  # ok

$safe{b} = 10; # ok 

print $safe{bb}, "\n";  # dies

In the implementation of Safe::Hash above, I first load Tie::Hash which provides Tie::StdHash. Setting @ISA to Tie::StdHash provides our new package with tie methods that behave the same way as normal hashes. Each of the tie methods are outlined on http://perldoc.perl.org/perltie.html

In this case the only method to override is FETCH which is passed a reference to the hidden tied object (a hashref in this case), and the key to use. It checks if the slot exists, and either returns it or throws an error