LinkedList.hs

data List a = Nothing | Cons a (List a)
mylist = (Cons 1 (Cons 2 (Cons 3 Nothing)))
print mylist
-- This will show errors

-- You cannot run a `show` on your mylist 
-- because you are not deriving them while defining your list in line 1
-- So your new definition becomes

data List a = Nothing | Cons a (List a) deriving (Show)
mylist = (Cons 1 (Cons 2 (Cons 3 Nothing)))
print mylist
-- Cons 1 (Cons 2 (Cons 3 Nothing))


-- Maybe read the arrows tutorial> https://wiki.haskell.org/Arrow_tutorial
showHead :: List a -> a   
showHead l = case l of Cons a _ -> a

-- showHead mylist  => 10 (Not Cons 10)

showTail :: List a -> List a
showTail Nothing = Nothing
showTail l = case l of Cons _ a -> a

-- showTail mylist => Cons 99 (Cons 11 Nothing)

showLength :: List a -> Int
showLength Nothing = 0
showLength xs = 1 + (showLength (showTail xs)) -- I hope this is right :/

-- showLength mylist => 3

-- The evaluations are lazy. If you define the list of infinite length, it will give you head, tail. 
-- But if you compute length, it will go into stackoverflow. This is concept called - Holding on to the head. 
-- The program is holding on to the first value and mutating it - infinitely. 
-- One way to handle it to make your evaluations strict instead of lazy.