> ocaml -rectypes
Objective Caml version 3.09.3

# let rec len v = List.fold_left (fun s v -> s + (len v)) 0 v;;
val len : ('a list as 'a) -> int = <fun>
# len (let rec v = [v; v; v] in v);;
Stack overflow during evaluation (looping recursion?).

はっ、初学者を混乱させるだけか...。