2_3_2_kelvin_celsius.hs

{-
GHCi> k2c 0
Temperature (-273.15)
GHCi> k2c 0 == Temperature (-273.15)
True
GHCi> k2c 273.15
Temperature 0.0
-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
newtype Temperature a = Temperature Double deriving (Num, Show, Eq, Fractional)

data Celsius
data Fahrenheit
data Kelvin

comfortTemperature :: Temperature Celsius
comfortTemperature = Temperature 23

c2f :: Temperature Celsius -> Temperature Fahrenheit
c2f (Temperature c) = Temperature (1.8 * c + 32)

k2c :: Temperature Kelvin -> Temperature Celsius
k2c (Temperature k) = Temperature (k - 273.15)

ex1 = k2c 0
ex2 = k2c 0 == Temperature (-273.15)
ex3 = k2c 273.15