See change above!
Also note type of fundep for Datasource should now be:
class Datasource s k v | s -> k v where ...
I see.... But the cool thing was, that my datasources were generic, in the sence that they could store multiple k's and v's. Now, they would be unique for the actual storage mechanism used, meaning, for example, that I could only read values from 1 table, if I'd instantiate the datasource for a database coupling. Currently, I use the Boilerplate approach to make it possible to store multiple types in one datasource, for example: data MyKeyVal = IntXString Int String | FloatXInt Float Int deriving (Eq, Ord, Show) Furthermore, I generate an instance of KeyHasValue, to tell my framework which keys are valid for a datasource, for example: instance KeyHasValue MyKeyVal Int String where constructor = IntXString instance KeyHasValue MyKeyVal Float Int where constructor = FloatXInt I have an instance instance (..., KeyHasValue a k v) => Datasource [a] k v where ... This way, I can read Ints from a [MyKeyVal], and get a String, and read Floats, and get an Int. If I would have a fundep class Datasource s k v | s -> k v where ... this wouldn't be possible anymore, I guess? Regards, Robert