## κατα-morphism: Finally length

All needed information for making length.

    instance Functor (StrF a) =
        fmap f (Cons c x) = Cons c (f x)
        fmap _ Nil = Nil

    length' :: Str -> Int
    length' = cata phi where
        phi :: Algebra StrF Int -- StrF Int -> Int
        phi (Cons a b) = 1 + b
        phi Nil = 0

    main = do
        l <- length' $ stringToStr "Toto"
        ...