On Mon, Dec 13, 2010 at 9:30 PM, Brent Yorgey <byorgey@seas.upenn.edu> wrote:

Hi all,

Today I wanted this function

strongLocal :: (MonadReader r1 m1, MonadReader r2 m2) =>
(r2 -> r1) -> m1 a -> m2 a

Of course, after staring at this type for ten seconds I realized that
it cannot be implemented. But I wondered whether anyone has any
pointers to anything related, or if anyone has good ideas for a
reasonable interface that would allow this.

Here's a draft by Bruno Oliveira and me on manipulating the monad stack.
The examples in Section 4.3 on p.12-13 are related:

http://users.ugent.be/~tschrijv/Research/papers/virtual_monad_stack.pdf