poly.ml

utop # class intlist (l: int list) =
object
  method empty = (l = [])
  method fold f acc = List.fold_left f acc l
end;;
Error: Some type variables are unbound in this type:                                                                                 class intlist :                                                                                                               int list ->
           object
             method empty : bool
             method fold : ('a -> int -> 'a) -> 'a -> 'a
           end
       The method fold has type ('a -> int -> 'a) -> 'a -> 'a where 'a
       is unbound