LinkedList in Haskell created by essic - https://repl.it/GuFF/49

LinkedList in Haskell.hs

data LinkedList a =
  Cons a (LinkedList a) | Nil
    deriving (Show)

push :: a -> LinkedList a -> LinkedList a
push x y =
  Cons x y
  
push_back :: a -> LinkedList a -> LinkedList a
push_back a Nil =
  Cons a Nil

push_back a (Cons x y) =
  Cons x $ push_back a y
  
length_list :: LinkedList a -> Integer
length_list Nil =
  0

length_list (Cons x y) =
  (+) 1 $ length_list y

last_element :: LinkedList a -> Maybe a
last_element x =
  case x of
    Nil -> Nothing
    Cons x Nil -> Just x
    Cons _ y -> last_element y

find_first :: (a -> Bool) -> LinkedList a -> Maybe a
find_first f Nil =
  Nothing

find_first f (Cons x y) 
  | f x = Just x
  | otherwise = find_first f y

remove :: (a -> Bool) -> LinkedList a -> Maybe (LinkedList a)
remove _ Nil =
  Nothing

remove predicate (Cons x xs) 
  | predicate x = Just xs
  | otherwise = fmap (\a -> Cons x a) $ remove predicate xs


filter :: (a -> Bool) -> LinkedList a -> Maybe (LinkedList a)
filter _ Nil = 
  Nothing

filter predicate (Cons x xs) =
  case predicate x of
    True -> case xs of
              (Cons y Nil) -> Just Nil
              (Cons y ys) -> fmap (\a -> Cons y a) $ filter predicate ys
    False -> fmap (\a -> Cons x a) $ filter predicate xs

filter' :: (a -> Bool) -> LinkedList a -> LinkedList a
filter' _ Nil = Nil
filter' predicate (Cons x xs) =
  go $ predicate x
    where 
          go True = filter' predicate xs
          go _ = Cons x $ filter' predicate xs
        
main :: IO ()
main = do
  print $ "list members at push_back 3 on :: Cons 2 Nil -> " ++ show result
  print $ "list members after push 1 on :: Cons 2 (Cons 3 Nil) -> " ++ show result2
  print $ "list length of :: Cons 1 (Cons 2 (Cons 3 Nil) ) -> " ++ (show . length_list) result2
  print $ "last element of :: Cons 1 (Cons 2 (Cons 3 Nil) ) -> " ++ (show . last_element) result2
  print $ "last element of empty list Nil -> " ++ (show . last_element)  Nil
  print $ "find 50 on (Cons 2 (Cons 3 (Cons 50 Nil))) -> " ++ ( show . find_first  (\a -> a == 50) ) (Cons 2 (Cons 3 (Cons 50 Nil)))
  print $ "find 20 on (Cons 2 (Cons 3 (Cons 50 Nil))) -> " ++ ( show . find_first  (\a -> a == 20) ) (Cons 2 (Cons 3 (Cons 50 Nil)))
  print $ "remove member 2 of :: Cons 1 (Cons 2 (Cons 3 Nil) ) ->  : " ++ (show . remove (\a -> a == 2)) result2
  print $ "remove member 45 of :: Cons 1 (Cons 2 (Cons 3 Nil) ) " ++ (show . remove (\a -> a == 45)) result2
  print $ "filter all member 3 of :: (Cons 3 (Cons 3 Nil)) : " ++ (show . filter (\a -> a == 3)) (Cons 3 (Cons 3 Nil))
  print $ "filter all member 2 of :: (Cons 3 (Cons 3 Nil)) -> " ++ (show . filter (\a -> a == 2)) (Cons 3 (Cons 3 Nil))
  print $ "filter all member 2 of :: (Cons 3 (Cons 3 Nil)) with filter'-> " ++ (show . filter' (\a -> a == 2)) (Cons 3 (Cons 3 Nil))
  print $ "filter all member 3 with filter' of :: (Cons 3 (Cons 3 (Cons 3 Nil))) -> " ++ (show . filter' (\a -> a == 3)) (Cons 3 (Cons 3 (Cons 3 Nil)))
  print $ "filter all member 2 on empty list of Nil -> " ++ (show . filter' (\a -> a == 2)) Nil
    where result = push_back 3 $ Cons 2 Nil
          result2 = push 1 result