I see now at least three MTL libraries at hackage, namely mtl mmtl mtl-tf They all define both the example data types and according type classes in modules with the same name. This way you can only use one of these packages in all packages you import. However the data types are defined the same way in the packages, only the classes differ. I wished, there would be one package defining only the datatypes, say "mtl-data", and three ones with the names "mtl", "mmtl", "mtl-tf" that define their classes with corresponding instances. These classes should be in different modules, so you can use different class frameworks in the same project. The data type package would be useful on its own and could be run on the simplest Haskell compilers, since no functional dependencies are required. However there is the compatibility problem: Currently Control.Monad.State exports also the MonadState class, which would no longer work. Maybe we can reserve Control.Monad.State for mtl, which exports Control.Monad.State.Data.Lazy from mtl-data and Control.Monad.State.Class from mtl. I assume that mmtl and mtl-tf are not as much used as mtl, such that it would not too bad to break compatibility for them. They could no longer export Control.Monad.State, but instead the user of mmtl has to import Control.Monad.State.Data from mtl-data. The class file of mmtl could be named Control.Monad.Modular.State.Class. (Replace "State" by "Writer", "Reader" and so on, and "mmtl" by "mtl-tf" in order to get all names that need to be adapted.)