You can't parametrize type classes over one another in the same way. In practice, this often makes ML style modules much more expressive: for example, I can hav

You can't parametrize type classes over one another in the same way. In practice, this often makes ML style modules much more expressive: for example, I can have a Hashable type class with one implementation per type that is used by a HashSet data type or module, but with MLs there could be a number of modules for the same type implementing the hash function, which can be used to construct the set. This also means things like Monoids or Semigroups greatly benefit, since now you can easily have more than just (+) for ints, for example. You can even use this to derive operations like folds from your base module where you'd never have to apply the arguments to a folder; instead you could do Sum.fold or Product.fold based on the different base modules.