Patrick Browne schrieb:
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 see a constant 1 that is equated with a type.
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
Where is the constant 1 equated with two distinct definitions of 'unit'?
Ok, modules loaded: A. *A> unit (LengthInMetres 7) Metre *A> unit (LengthInCentimetres 7) Centimetre *A>
'unit' is a method of the class MetricDescription. The particular implementation of 'unit' is chosen by the compiler according to the type of the actual parameter and result of 'unit'.
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
Since valueInUnit and valueInBaseUnit do not need the 'unit' type, I would put them in a separate class.
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
I assumed that 0.01m = 1cm
data LengthInMetres = LengthInMetres Double deriving Show data LengthInCentimetres = LengthInCentimetres Double deriving Show
instance MetricDescription LengthInMetres Metre where valueInUnit (LengthInMetres d) = d unit l = Metre
If you enable compiler warnings, then the compiler will warn you, that 'l' is not used. You can also write unit _ = Metre
instance MetricDescription LengthInCentimetres Centimetre where valueInUnit (LengthInCentimetres d) = d unit l = Centimetre