Hi all, I'm definitely stuck with the migration of my project from mtl to transformers. The use of transformer is mandated by a new dependency that I need to add. Overlapping instances for Monad (Either String) arising from a use of `tstampToForecasts' at ThreeLab/Gfs.hs:94:32-55 Matching instances: instance (Control.Monad.Trans.Error.Error e) => Monad (Either e) -- Defined in Control.Monad.Trans.Error instance Monad (Either e) -- Defined in mtl-1.1.1.0:Control.Monad.Error I really have no clue of who and why is bringing mtl instance in the scope. If I compile the file directly with: ghc -package monads-tf -package transformers -hide-package mtl ThreeLab/Gfs.hs -c I can successfully compile the module, so it seems the problem is not specific to that module. But the compilation fails if I do ghc -package monads-tf -package transformers -hide-package mtl --make Main.hs What's the best practice to debug this problem? What are the constraints of indirectly depending on both mtl and transformers? I'm really stuck :-/ Thanks Paolo