Folding a list is like replacing the cons and empty constructors

Folding.elm

-- GIVEN

type MyList a
  = Cons a (MyList a)
  | Empty
  

add : Int -> Int -> Int
add a b =
  a + b
  
  
-- WHEN FOLDING

foldl add 0

-- NOTE THE SYMMETRY

Cons 3 (Cons 2 (Cons 1 Empty))
add  3 (add  2 (add  1 0    ))