Subtyping can be closed, too (see Scala's sealed classes, for example) and algebraic data types can be open (see OCaml's extensible variants [1] or polymorphic

Subtyping can be closed, too (see Scala's sealed classes, for example) and algebraic data types can be open (see OCaml's extensible variants [1] or polymorphic variants [2]). This is simply a distinction between closed and open sum types. [1] https://caml.inria.fr/pub/docs/manual-ocaml/extn.html#s%3Aextensible-variants https://caml.inria.fr/pub/docs/manual-ocaml/extn.html#s%3Aex... [2] http://caml.inria.fr/pub/docs/manual-ocaml-400/manual006.html#toc36 http://caml.inria.fr/pub/docs/manual-ocaml-400/manual006.htm...