base - Establish an ISA relationship with base classes at compile time

    package Baz;
    use base qw(Foo Bar);

    package Baz;
    BEGIN {
        require Foo;
        require Bar;
        push @ISA, qw(Foo Bar);
    }

        package Foo;
        sub exclaim { "I can have such a thing?!" }

        package Bar;
        use base "Foo";

package base;

use strict 'vars';
use vars qw($VERSION);
$VERSION = '2.15';
$VERSION = eval $VERSION;

# constant.pm is slow
sub SUCCESS () { 1 }

sub PUBLIC     () { 2**0  }
sub PRIVATE    () { 2**1  }
sub INHERITED  () { 2**2  }
sub PROTECTED  () { 2**3  }


my $Fattr = \%fields::attr;

sub has_fields {
    my($base) = shift;
    my $fglob = ${"$base\::"}{FIELDS};
    return( ($fglob && 'GLOB' eq ref($fglob) && *$fglob{HASH}) ? 1 : 0 );
}

sub has_version {
    my($base) = shift;
    my $vglob = ${$base.'::'}{VERSION};
    return( ($vglob && *$vglob{SCALAR}) ? 1 : 0 );
}

sub has_attr {
    my($proto) = shift;
    my($class) = ref $proto || $proto;
    return exists $Fattr->{$class};
}

sub get_attr {
    $Fattr->{$_[0]} = [1] unless $Fattr->{$_[0]};
    return $Fattr->{$_[0]};
}

if ($] < 5.009) {
    *get_fields = sub {
        # Shut up a possible typo warning.
        () = \%{$_[0].'::FIELDS'};
        my $f = \%{$_[0].'::FIELDS'};

        # should be centralized in fields? perhaps
        # fields::mk_FIELDS_be_OK. Peh. As long as %{ $package . '::FIELDS' }
        # is used here anyway, it doesn't matter.
        bless $f, 'pseudohash' if (ref($f) ne 'pseudohash');

        return $f;
    }
}
else {
    *get_fields = sub {
        # Shut up a possible typo warning.
        () = \%{$_[0].'::FIELDS'};
        return \%{$_[0].'::FIELDS'};
    }
}

sub import {
    my $class = shift;

    return SUCCESS unless @_;

    # List of base classes from which we will inherit %FIELDS.
    my $fields_base;

    my $inheritor = caller(0);
    my @isa_classes;

    my @bases;
    foreach my $base (@_) {
        if ( $inheritor eq $base ) {
            warn "Class '$inheritor' tried to inherit from itself\n";
        }

        next if grep $_->isa($base), ($inheritor, @bases);

        if (has_version($base)) {
            ${$base.'::VERSION'} = '-1, set by base.pm' 
              unless defined ${$base.'::VERSION'};
        }
        else {
            my $sigdie;
            {
                local $SIG{__DIE__};
                eval "require $base";
                # Only ignore "Can't locate" errors from our eval require.
                # Other fatal errors (syntax etc) must be reported.
                die if $@ && $@ !~ /^Can't locate .*? at \(eval /;
                unless (%{"$base\::"}) {
                    require Carp;
                    local $" = " ";
                    Carp::croak(<<ERROR);
Base class package "$base" is empty.
    (Perhaps you need to 'use' the module which defines that package first,
    or make that module available in \@INC (\@INC contains: @INC).
ERROR
                }
                $sigdie = $SIG{__DIE__} || undef;
            }
            # Make sure a global $SIG{__DIE__} makes it out of the localization.
            $SIG{__DIE__} = $sigdie if defined $sigdie;
            ${$base.'::VERSION'} = "-1, set by base.pm"
              unless defined ${$base.'::VERSION'};
        }
        push @bases, $base;

        if ( has_fields($base) || has_attr($base) ) {
            # No multiple fields inheritance *suck*
            if ($fields_base) {
                require Carp;
                Carp::croak("Can't multiply inherit fields");
            } else {
                $fields_base = $base;
            }
        }
    }
    # Save this until the end so it's all or nothing if the above loop croaks.
    push @{"$inheritor\::ISA"}, @isa_classes;

    push @{"$inheritor\::ISA"}, @bases;

    if( defined $fields_base ) {
        inherit_fields($inheritor, $fields_base);
    }
}


sub inherit_fields {
    my($derived, $base) = @_;

    return SUCCESS unless $base;

    my $battr = get_attr($base);
    my $dattr = get_attr($derived);
    my $dfields = get_fields($derived);
    my $bfields = get_fields($base);

    $dattr->[0] = @$battr;

    if( keys %$dfields ) {
        warn <<"END";
$derived is inheriting from $base but already has its own fields!
This will cause problems.  Be sure you use base BEFORE declaring fields.
END

    }

    # Iterate through the base's fields adding all the non-private
    # ones to the derived class.  Hang on to the original attribute
    # (Public, Private, etc...) and add Inherited.
    # This is all too complicated to do efficiently with add_fields().
    while (my($k,$v) = each %$bfields) {
        my $fno;
        if ($fno = $dfields->{$k} and $fno != $v) {
            require Carp;
            Carp::croak ("Inherited fields can't override existing fields");
        }

        if( $battr->[$v] & PRIVATE ) {
            $dattr->[$v] = PRIVATE | INHERITED;
        }
        else {
            $dattr->[$v] = INHERITED | $battr->[$v];
            $dfields->{$k} = $v;
        }
    }

    foreach my $idx (1..$#{$battr}) {
        next if defined $dattr->[$idx];
        $dattr->[$idx] = $battr->[$idx] & INHERITED;
    }
}


1;

__END__