Well, my point was that you can do something akin to Perl's taint - force operations to be done only within a particular type context.

So you could do something similar to the ST monad but instead of accepting functions which generate any type output, it operates on, say, a parse tree/ADT representing a Lisp function which is evaluated with the rest of the homomorphic data.

But it's not really important; any such strategy would probably be done in a new language (for efficiency, if nothing else) and the into/escape invariant enforced by manual code inspection or something.