[Haskell-cafe] operating on nested monads

[ajb at spamcop.net]

G'day Marco.

Quoting Marco Righele <marco_righele at yahoo.it>:

> How can I achieve the same effect (if it is ever possible)?
> I feel like it should be something almost trivial, but I really can't get it.

One approach is to use a monad which works like Maybe, but as a monad
transformer over IO.

You might want to try NegateT, which does pretty much precisely this:

        http://cvs.sourceforge.net/viewcvs.py/hfl/hfl/mtl/Negate.hs?rev=1.2

Cheers,
Andrew Bromage