[Haskell-cafe] Instantiation problem

[Patrick Browne]

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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