[racket] Typed Racket vs. Haskell

> Developing statically typed code in TR and making it work later
> on other Scheme implementations should be easy, since the TR type
> system is intended to allow the same style of code that is used in
> Racket, which is also similar to the style in other scheme
> implementations.

Actually, this is what I was asking about.
(Guile people told me to use C or bins, but I don't want to go that deep.)

> The required "translation" step will be mostly
> removing the type declarations.  Doing the same with Haskell etc would
> most likely be much harder.

Is it possible not to remove types?
Vincent told me that one can "mix typed and untyped code" in Racket.
How it's done in Racket (the implementation details)? How these modules
interact? Could you give me some pointers?
Why it's not possible in other Lisp-like languages (e.g. Guile)?