Combining distinct-thread state monads?

[Dr Mark H Phillips]

Hi Wolfgang,

Thanks for your informative reply.  At first I didn't
understand it, but a search on "StateT" lead me to the
paper "Monad Transformers and Modular Interpreters" by
Liang, Hudak and Jones, which clarified some of the
ideas for me.

The state transformer approach seems to have
advantageous in that it provides a framework for
building new monads from old, and accessing the
components.  One disadvantage is that it lacks 
symmetry in that one monad is arbitrarily chosen
to sit inside the other.  I found another approach
mentioned called "stratification", developed by
David Espinosa in his PhD thesis "Semantic Lego".  I
am finding it a little hard to read because he codes
in Scheme, not Haskell, but it sounds promising and
seems to preserve symmetry better.

Are you (or others) aware of this kind of approach being
used in Haskell?

Cheers,

Mark.