package Foo;
sub new { ... };
sub xyzzy { ... };
1;
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require Foo;
my $foo = new Foo ( );
print $foo->xyzzy ( );
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# The new Foo.
package Foo;
require System::Foo;
@ISA = qw ( System::Foo );
...
1;
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And then set @INC to be:

• First, the parent directory of Custom and System.
• Second, the directory Custom.
• Third, the directory System.

With this, "require System::Foo" (needed in the subclass) will work as will "require Foo" which will find the custom Foo module if it exists and will find the system Foo module if it doesn't.

So far so good. I can get the system to at least find my custom modules before the system modules. This example doesn't work though unless I change the namespace of the system Foo by changing the package call. That is, I have to change the definition of the system module as follows:

package System::Foo;
...
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So I'm willing to do this to the system objects but there's still a problem with those modules that do not have a custom subclass. That is, now all the places where I used the system module Bar as "require Bar" or "new Bar" have to be changed to "require System::Bar" and "new System::Bar." Well, that doesn't help at all as when I want to subclass Bar I would have to change these all back again.

I've tried various things like using a variable for the package name but perl doesn't like that. (I take it "package" is a compile-time command and not a run-time one.) I also tried moving the %Foo:: namespace to %System::Foo:: after loading the module but this failed because of a circular require path though I might try this route again unless I hear a better solution.

The only other solution I can think of is to write stub modules for all the system modules I might like to subclass someday. These stubs would subclass (in an identity relationship) the system modules but would reserve the namespace Foo for future changes. That is, the system modules would be named System::Foo but would only ever be accessed through the stub modules and so appear to be named straight-up Foo. Then I would change my installer to overwrite the stub modules as needed. This seems really ugly.

But then, is what I'm trying to do really ugly? Perhaps. I'm pretty sure the same problem exists in C++ and Java - maybe I'm just spoiled by the openness and flexibility of perl in so many other situations.

Thanks for any help or ideas,
Mike

Edit kudra, 2002-06-06 s/pre/code/, added readmore