Haskell: Functional dependency example -- for any instance of the Pet type class, the type of animal uniquely determines the sound it can make.

Functional_Dependencies_Animals.hs

-- | Sandbox Haskell package
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
module Main where

class Pet animal sound | animal -> sound where
  speak :: animal -> sound

data Sound = Bark | Meow deriving (Show)

type Name = String

newtype Dog = Dog Name

newtype Cat = Cat Name

instance Pet Dog Sound where
  speak (Dog _) = Bark

instance Pet Cat Sound where
  speak (Cat _) = Meow

oscar :: Dog
oscar = Dog "Oscar"

felix :: Cat
felix = Cat "Felix"

main :: IO ()
main = do 
  putStrLn "Dog says: "
  print $ speak oscar
  
  putStrLn "Cat says: "
  print $ speak felix