関数型言語ML (SML, OCaml, etc.), Part 6

[564 デフォルトの名無しさん 2012/10/12(金) 11:22:15.18]

# type t = int * t;;
Error: The type abbreviation u is cyclic
# type t = { x: int; y: t };;
type t = { x : int; y : t; }

# type u = u option;;
Error: The type abbreviation u is cyclic
# type u = Some of u | None;;
type u = Some of u | None

だれか解説してくれ。