No, it wants me to define an instance for
(StateT s) which is supposed to be defined
be the authors of the library.

Actually I discovered that I have two libraries
called transformers.

transformers-0.2.2.0
transformers-0.3.0.0

So when I'm doing

import Control.Monad.Trans (0.2.2.0)

I get the error

And when I'm doing

import Control.Monad.Trans.Class (0.3.0.0)

It compiles.

Anton