Below is some code that is produces information about the *types* used for measuring (e.g. metres). The following evaluation returns 1.00 which the convert factor for metres. convertFactorToBaseUnit (unit (LengthInMetres 7)) . The next evaluation returns the type, Metre, of data being measured unit (LengthInMetres 7) Using the particular definitions below is it possible to make an instance of MetricDescription for centimetres? I have an error on the defintion of the unit function in the definition of the MetricDescription instance. As far as possible I would like to retain the data types and class structures. Thanks, Pat class (Unit unit) => MetricDescription description unit | description -> unit where unit :: description -> unit valueInUnit :: description -> Double valueInBaseUnit :: description -> Double valueInBaseUnit d = (convertFactorToBaseUnit(unit d)) * (valueInUnit d) data Metre = Metre deriving Show data Centimetre = Centimetre deriving Show -- Each member of the Unit class has one operator convertFactorToBaseUnit -- that takes a measurement unit (say metre) and returns a conversion factor for that unit of measurement class Unit unit where convertFactorToBaseUnit :: unit -> Double -- An instance for metres, where the convert factor is 1.0 instance Unit Metre where convertFactorToBaseUnit Metre = 1.0 -- An instance for metres, where the convert factor is 0.1 instance Unit Centimetre where convertFactorToBaseUnit Centimetre = 0.1 data LengthInMetres = LengthInMetres Double deriving Show data LengthInCentimetres = LengthInCentimetres Double deriving Show -- This seems fine instance MetricDescription LengthInMetres Metre where valueInUnit (LengthInMetres d) = d unit l = Metre -- This is the instance that I cannot get to work -- The unit 2 function seems to be the problem. -- instance MetricDescription LengthInCentimetres Centimetre where -- valueInUnit (LengthInCentimetres d) = d -- unit 2 = Centimetre This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie