Backwards emulation rather than backwards compatibility?

[Neil Hodgson]

John Roth:

> Also, it should be possible to mix levels in one program,
> the limitation being that any one module is interpreted
> under one set of rules.

   Some of the language changes affect communication between modules. How do
you deal with changes in the language that affect the interface provided by
one module and consumed by another with a different language level? Say I
have some 1.52 code that uses introspection and point it at a 2.2 module
that uses the new property support? Or a 1.52 module asked to deal with
Unicode?

   While most dependencies are of the form of new code using old code, it is
common for this use to include subclassing old code and other forms where
'use' is really bidirectional.

   To me, providing a language level feature looks like a huge time sink,
with a need for exhaustive pairwise (at least!) cross level testing.

   Neil