Arbitrary ADT

arbitrary-adt.hs

-- Data
data Shape
  = Rectangle Number Number -- length, height
  | Triangle Number Number -- base, height
  | Circle Number -- radius
  | Square Number -- length

-- Behaviour
area :: Shape -> Number
area (Rectangle l h) = l * h
area (Triangle b h)  = b * h / 2
area (Circle r)      = 3.14 * r * r
area (Square l)      = l * l

-- Example
area (Rectangle 5 4) == 20
area (Circle 7) == 154
...