[Haskell-beginners] Instance but no instance

[Jeroen van Maanen]

I am trying to upgrade some update facility in my program to allow for IO:

  class Updater modelType updateType where
    update :: updateType -> modelType -> Maybe modelType

  class UpdaterIO modelType updateType where
    updateIO :: updateType -> modelType -> IO (Maybe modelType)

  pureUpdateToIO :: (updateType -> modelType -> (Maybe modelType)) -> updateType -> modelType -> IO (Maybe modelType)
  pureUpdateToIO updateFunction update model = return $ updateFunction update model

  instance (Updater modelType updateType) => UpdaterIO modelType updateType where
    updateIO = pureUpdateToIO update

My code still works for all my old Updater instances even when I use them in the IO monad:

  do maybeUpdatedModel <- updateIO newUpdate oldModel

So far so good. However, when I try to upgrade one of the Updater instances to an UpdateIO instance I get into a kind of a catch 22 situation where not only

  maybeUpdatedModel = update newUpdate oldModel

results in a "No instance for Updater [...] [...]" compiler error as expected, but also

  do maybeUpdatedModel <- updateIO newUpdate oldModel

unexpectedly results in an "Overlapping instances for UpdaterIO [...] [...]" compiler error. Where one of the overlapping instance declarations is:

  instance (Updater modelType updateType) => UpdaterIO modelType updateType

How is that possible?

Cheers,  Jeroen