{- typeclasses.hs

This file contains some of the examples on typeclasses to be shown in
the second class of the course "Haskell for Life", by Sergiu Ivanov
(sivanov@lacl.fr):

  http://lacl.fr/~sivanov/doku.php?id=en:haskell_for_life

This file is distributed under the Creative Commons Attribution Alone
licence.-}

-- | A simple redefinition of the 'Eq' typeclass.
class BasicEq a where
  isEqual :: a -> a -> Bool

isElement :: BasicEq a => a -> [a] -> Bool
isElement _ [] = False
isElement y (x:xs) | isEqual y x = True
                   | otherwise = isElement y xs

instance BasicEq Bool where
  isEqual True True = True
  isEqual False False = True
  isEqual _ _ = False

instance BasicEq a => BasicEq [a] where
  isEqual [] [] = True
  isEqual _ [] = False
  isEqual [] _ = False
  isEqual (x:xs) (y:ys) | isEqual x y = isEqual xs ys
                        | otherwise = False


-- | This typeclasses are simpler than 'Show' and 'Read', but
-- essentially capture their principal function.
class SimplifiedShow a where
  show :: a -> String

class SimplifiedRead a where
  read :: String -> a