Hash parsing considered ugly

[Jonathan Stowe]

On Sat, 2006-02-25 at 22:27, drkjam wrote:

> When I say :-
> 
>     use strict;
>     use warnings;
>     use Data::Dumper;
> 
>     my %h = (
>         foo => 1,
>         foo => 2,
>     );
> 
>     print Dumper(\%h);
> 
> Perl provides :-
> 
>     $VAR1 = {
>               'foo' => 2
>             };
> 
> Assuming there is an awful lot more items in the hash than shown here
> this could lead to unexpected results. Is there any way at all to get
> Perl to flag this with at least some form of warning? It can lead to
> some rather insidious bugs if you aren't paying attention.


If you want to have a hash that has this kind of behaviour then you
might want to consider a tied hash, you can do something likeL

package MyHash;
 
use base Tie::Hash;
 
use Carp;
 
sub STORE
{
   my ( $self, $key, $value ) = @_;
   if ( exists $self->{$key} )
   {
      carp "Key $key already exists";
   }
 
   $self->{$key} = $value;
}
 
sub TIEHASH
{
   my ( $class ) = @_;
 
   my $hash = bless {}, $class;
 
   return $hash;
}
 
sub FIRSTKEY {};
 
package main;
 
my %foo;
 
tie %foo, 'MyHash';
 
 
%foo = ( bar => 1, foo => 2 );
 
$foo{bar} = 2;


Of course you will want to add some extra stuff to the tied class to
make the hash behave properly.

/J\
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