[plt-scheme] Typed Scheme 2.0

I've just checked Typed Scheme version 2.0 into SVN.

One persistent limitation of Typed Scheme has been that while this expression
works as expected:

(if (number? x) (add1 x) 7)

The simple transformation of making `x' a part of a structure
breaks Typed Scheme's ability to reason about the code. So this
expression doesn't typecheck:

(if (number? (car x)) (add1 (car x)) 7)

With the newest version of Typed Scheme, now available in SVN, both of
these will now work.  In general, Typed Scheme can now follow paths
into arbitrary immutable structures, including pairs.

This is part of a more general reworking of underlying mechanisms of
the Typed Scheme typechecker, which makes it both simpler and more
flexible.  I hope that it will be possible, using this new foundation,
to add additional features that make more programs easy to express in
Typed Scheme.

Of course, these changes mean that Typed Scheme may be more unstable,
so if you notice any new bugs, please let us know.

Unfortunately, this won't be available in the upcoming 4.2 release, but it will
be in the release after that.

If you have any questions or comments or feature requests for Typed
Scheme, please let me know, either privately or on the list.

Thanks!
-- 
sam th
samth at ccs.neu.edu