On 29/01/2011 20:56, Henning Thielemann wrote: > Is there a reason why you use an individual type for every unit? > The existing implementations of typed physical units only encode the > physical dimension in types and leave the unit factors to the value > level. I found this to be the most natural way.
I am studying type classes using examples from the literature [1]. There is no particular intension to implement anything. I am confused about the unit function in the code below. My understanding is: The signature of the unit function is defined in the MetricDescription class. Any valid instantce of MetricDescription will respect the functional dependency (FD): The FD | description -> unit is exactly the signature of the unit function. My confusions I do not understand the definitions of unit in the instances. I do not know how the constant 1 can be equated with a *type*, Where did 1 come from? I do not know how the constant 1 can be equated with *two distinct* definitions of the function uint and still produce the following correct results Ok, modules loaded: A. *A> unit (LengthInMetres 7) Metre *A> unit (LengthInCentimetres 7) Centimetre *A> [1] http://ifgi.uni-muenster.de/~lutzm/odbase04_schade_et_al.pdf ====================================================================== module A where -- 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 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 Dog = Dog deriving Show data Metre = Metre deriving Show data Centimetre = Centimetre deriving Show -- An instance for metres, where the convert factor is 1.0 instance Unit Metre where convertFactorToBaseUnit Metre = 1.0 -- An instance for centimetres, 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 instance MetricDescription LengthInMetres Metre where valueInUnit (LengthInMetres d) = d unit l = Metre instance MetricDescription LengthInCentimetres Centimetre where valueInUnit (LengthInCentimetres d) = d unit l = 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 _______________________________________________ Haskell-Cafe mailing list [email protected] http://www.haskell.org/mailman/listinfo/haskell-cafe
