Hello, here is a reproduction of the classic Shape example adapted for haskell (The equivalent of “hello world” in the OOP world).

With haskell classes and three different data types (think Java interfaces)

-- define the shape data types
data Triangle = Triangle
data Square = Square
data Octagon = Octagon

class Shape s where
  sides :: s -> Integer

instance Shape Triangle where
  sides _ = 3

instance Shape Square where
  sides _ = 4

instance Shape Octagon where
  sides _ = 8

GHCi output

*Main> sides Triangle
3
*Main> sides Square
4
*Main> sides Octagon
8

With one haskell datatype and runtime pattern matching (think erlang or “parametric polymorphism”)

-- define the instances of shape
data Shape =
  Triangle |
  Square |
  Octagon

sides Triangle = 3
sides Square = 4
sides Octagon = 8

GHCi output

*Main> sides Triangle
3
*Main> sides Square
4
*Main> sides Octagon
8

Which do you like better?

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