measure.hs

class Unit a where 
  standard :: a

class RatioUnit a where
  standardFactor :: a -> Float

toStandard :: (RatioUnit a) => a -> Float -> Float
toStandard a x = (standardFactor a) * x  

data Measure b = Measure Float b deriving (Show)

data LengthUnit = Meter | Kilometer deriving (Show,Eq)
data AreaUnit = SqMeter | SqKilometer deriving (Show,Eq)
data TemperatureUnit = Celsius | Fahrenheit | Kelvin deriving (Show,Eq)

instance Unit LengthUnit where
  standard = Meter :: LengthUnit

instance Unit AreaUnit where
  standard = SqMeter :: AreaUnit

instance Unit TemperatureUnit where
  standard = Kelvin :: TemperatureUnit

instance (RatioUnit a) => Eq (Measure a) where
  (Measure x u1) == (Measure y u2) = (toStandard u1 x) == (toStandard u2 y)

add :: (RatioUnit a, Unit a) => Measure a -> Measure a -> Measure a
add (Measure q1 u1) (Measure q2 u2) = 
  let standardQ1 = toStandard u1 q1
      standardQ2 = toStandard u2 q2
  in Measure (standardQ1 + standardQ2) (standard)
 
instance RatioUnit LengthUnit where
  standardFactor Meter = 1.0
  standardFactor Kilometer = 1000.0

instance RatioUnit AreaUnit where
  standardFactor SqMeter = 1.0
  standardFactor SqKilometer = 1000000.0

units.hs

class Unit a where
  baseFactor :: a -> Float
  coefficient :: a -> a -> Float 
  coefficient a b = (baseFactor a)/(baseFactor b)

data Measure b = Measure Float b deriving (Show,Eq)

convert :: (Unit b) => Measure b -> b -> Measure b
convert (Measure qty unit) newUnit = Measure (qty*(coefficient unit newUnit)) newUnit

data LengthUnit = Meter | Kilometer deriving (Show,Eq)
data AreaUnit = SqMeter | SqKilometer deriving (Show,Eq)

instance Unit LengthUnit where 
  baseFactor Meter = 1
  baseFactor Kilometer = 1000

instance Unit AreaUnit where 
  baseFactor SqMeter = 1
  baseFactor SqKilometer = 1000000