a = [1, 23, 4]

len :: Num a => [b] -> a
len [] = 0  
len (x : xs) = 1 + len xs


-- data Empty = Empty



lst :: [a] -> a
lst (x : []) = x
lst (x : xs) = lst xs
-- lst [] = Empty

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

data EltOrEmpty a = Real a | Empty 

instance Show a => Show (EltOrEmpty a) where
    show (Real a) = show a
    show Empty    = "<>"            
 
hd :: List a -> EltOrEmpty a
hd (Cons x _)    = Real x
hd (Cons x Nil)  = Real x
hd Nil           = Empty

tl (Cons _ xs)  = xs
tl (Cons _ Nil) = Nil

ln (Cons x xs) = 1 + ln xs
ln Nil           = 0

(#) a b = Cons a b