在OCAML中，这种相互递归类型的定义的用途是什么？

Question

贝洛是OCAML中的有效类型定义。

type t = U of u and u = T of t

有使用吗？

如何创建类型

t

的值？或键入

u

？

您所展示的示例没有多大意义。递归无法结束。但是这样，相互递归类型可以是 非常有用。考虑从标准库中的

Answer 1

定义。

type 'a t = unit -> 'a node
(** A sequence [xs] of type ['a t] is a delayed list of elements of
    type ['a]. Such a sequence is queried by performing a function
    application [xs()]. This function application returns a node,
    allowing the caller to determine whether the sequence is empty
    or nonempty, and in the latter case, to obtain its head and tail. *)

and +'a node =
  | Nil
  | Cons of 'a * 'a t (**)
(** A node is either [Nil], which means that the sequence is empty,
    or [Cons (x, xs)], which means that [x] is the first element
    of the sequence and that [xs] is the remainder of the sequence. *)

Source