[This really has *nothing* to do with the original tips and tricks thread]

> What I meant is that when you load/unload a package that extend
> a class by adding  an instance variable you may not want to be
> forced to recompile a hierarchy of classes. May be this is the
> price to pay.

No, no, no. A "variable" is a name in Smalltalk not an object. Because of
that there is absolutely no reason for "recompiling an entire class
hierarchy" unless you bind the name to a "slot" which is represented as part
of the object. For example, you might *easily* stick that slot into a
dictionary (such as Morph's extensions) and simply tell the compiler that
what it should do when it sees the name "foo" is to send messages like #foo
and #foo:. At which point, when you load the package you merely tell the
compiler about that "variable" so the compiler knows what to do when it
comes across the places where foo is used, and when you unload it you simply
remove the methods (if you want to be pedantic you can remove the values
too).

But most definitely there's no reason whatsoever to recompile an entire
class hierarchy just because you told the compiler that it's okay to use
"foo" as a name in some portion of code. The behavior we see today is simply
the result of having only one, and heavily optimized, way of representing
variables.

As for George's note about adding a variable to Object - precisely the same
could be done here. So if we wanted to add a "variable" to Object all we
need to do is to tell the compiler that (for example) it should use the name
"dependents" to refer to

    DependentsFields at: self

and be done with it (there are some subtleties with the above which I won't
discuss in detail here - see the above as a conceptual example). So there's
a significant difference between a "variable" (representing a name we can
use in some program) and its associated "slot". If you want to add a "slot"
to Object, yeah, you'll be screwed in Squeak. But adding a *name* under
which something is accessed is no problem whatsoever (except that the
current compiler doesn't support it ;-)

Cheers,
  - Andreas