Sorry, oops again: The problem is that it complains that it cannot find an instance for Datasource' z [MyKeyVal] [MyKeyVal] Float Int Whilst I have an instance Datasource [MyKeyVal] Float Int Thus, it seems that it needs an explicit type for the z here. Both datasources stored in the joined datasource are of type [MyKeyVal], so I would expect this to compile correctly, and then take the left datasource. Sorry that my explanation is all so confusing.... Robert