Part 2 of the linked-to article, for example, shows that the list data type
data List a = Nil | Cons a (List a)
L(a) = 1 + a * L(a),
L(a) = 1 / (1 - a).
F(a) = 1 / (1 - f(a))
The topic is incredible. Basically all the math you learn in Undergraduate mathematics gets pulled in to solve counting problems and perform algorithm analysis. It's inspiring. :)