Hi, Your definition of 'unit' in the instance MetricDescription LengthInCentimetres Centimetre is not well-typed. Maybe you want to write either unit (LengthInCentimitres 2.0) = Centimetre -- (pattern match fail for all (LengthInCentimetres l), l /= 2.0) or unit l = Centimetre -- i.e. unit = const Centimetre as in the instance for Metre Steffen On 01/28/2011 12:42 PM, Patrick Browne wrote:
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
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