use 5.006;
use strict;
use Exporter ();
use Carp ();

#######################################################
                   package Const;
# allows export of function to faciliate constant usage
#######################################################
our @EXPORT_OK = qw/Const ConstHash ConstArray/;
our @ISA = qw/Exporter/;

sub Const ($$) { tie $_[0], 'ConstScalar', $_[1] }
sub ConstHash (\%$) { tie %{ $_[0] }, 'ConstHash', $_[1]; }
sub ConstArray (\@$) { tie @{ $_[0] }, 'ConstArray', $_[1]; }

#############################################
            package ConstScalar;
# scalars set on tying, altering causes croak
#############################################

use Tie::Scalar ();
our @ISA = qw/Tie::StdScalar/;

sub STORE { Carp::croak "fatal: Const cannot be altered" }


###########################################
           package ConstHash;
# hashes which are fully set on tying,
#  altering, or accessing unset keys croaks
###########################################

use Tie::Hash ();
our @ISA = qw/Tie::StdHash/;

sub TIEHASH { bless { %{$_[1]} }, $_[0] }       # shallow clone
sub STORE { Carp::croak "fatal: ConstHash cannot be altered" }
sub DELETE { Carp::croak "fatal: ConstHash cannot be altered" }
sub CLEAR { Carp::croak "fatal: ConstHash cannot be altered" }
sub FETCH {
        if (exists $_[0]->{$_[1]}) { return $_[0]->{$_[1]} }
        else { Carp::croak "fatal: key '$_[1]' doesn't exist in this ConstHash"
}
}


#############################################
           package ConstArray;
# Arrays which are fully set on tying,
#  altering or accessing unset indexes croaks
#############################################

use Tie::Array ();
our @ISA = qw/Tie::StdArray/;

sub TIEARRAY { bless [ @{$_[1]} ], $_[0] }      # shallow clone

BEGIN{ #lazy shortcut
        for (qw/STORESIZE STORE CLEAR POP PUSH SHIFT UNSHIFT DELETE SPLICE/) {
                eval "sub $_ { Carp::croak 'fatal: ConstArray cannot be altered'
 }";
        }
}

sub FETCH {
        if ($_[1] > $#{$_[0]}) { Carp::croak "fatal: array element $_[1] does no
t exist in this ConstArray" }
        $_[0]->[$_[1]];
}

#############################################
42;