Counting with natural numbers

{-
To execute this program, paste this source code into:

https://replit.com/languages/haskell

Counts the items in any finite list (which may be empty).

The return value of the 'count' function is a natural number (which, as this codes demonstrates, may be Zero, otherwise we cannot count the items of *any* finite list).
-}

data Natural = Zero | SuccessorOf Natural
   deriving Show

plus :: Natural -> Natural -> Natural
plus Zero n = n
plus (SuccessorOf m) n = SuccessorOf (m `plus` n)

data CharList = EmptyList | InhabitedListMadeOf Char CharList
   deriving Show

count :: CharList -> Natural
count EmptyList = Zero
count (InhabitedListMadeOf head tail) = (SuccessorOf Zero) `plus` (count tail)

listOf1Item = InhabitedListMadeOf 'a' EmptyList

listOf2Items = InhabitedListMadeOf 'b' listOf1Item

listOf3Items = InhabitedListMadeOf 'c' listOf2Items

main = putStrLn $
   (show EmptyList) ++ " => " ++ (show $ count EmptyList) ++ "."
   ++ "\n\n" ++
   (show listOf1Item) ++ " => " ++ (show $ count listOf1Item) ++ "."
   ++ "\n\n" ++
   (show listOf2Items) ++ " => " ++ (show $ count listOf2Items) ++ "."
   ++ "\n\n" ++
   (show listOf3Items) ++ " => " ++ (show $ count listOf3Items) ++ "."

{-
Output:

EmptyList => Zero.

InhabitedListMadeOf 'a' EmptyList => SuccessorOf Zero.

InhabitedListMadeOf 'b' (InhabitedListMadeOf 'a' EmptyList) => SuccessorOf (SuccessorOf Zero).

InhabitedListMadeOf 'c' (InhabitedListMadeOf 'b' (InhabitedListMadeOf 'a' EmptyList)) => SuccessorOf (SuccessorOf (SuccessorOf Zero)).
-}