After lengthy discussion on the libraries list, the mtl package has been updated to depend on the transformers package, from which mtl re-exports the monad transformers and the MonadTrans and MonadIO classes. This makes the two packages compatible. The monads-fd package is now a deprecated stub re-exporting the mtl modules. Ths transformers/mtl-2 combination is under consideration for inclusion in the Haskell Platform: http://trac.haskell.org/haskell-platform/wiki/Proposals/transformers Discussion is on the libraries list. So far there have been no objections. The new version has a few incompatibilities with the old, which will break a few packages, though the great majority build unchanged. The above web page lists the incompatibilities and suggested fixes. You might also consider whether the portable transformers package suffices for your package. As packages move to the new version, there will be increasing dependency clashes with packages that currently specify a bound on their mtl dependency excluding the new version. In some cases the dependency can be simply relaxed with no other change, or with small changes obtain compatibility with both versions. In other cases one must choose a version, or use cpp hackery to support both.