type 'a status =
  | Delayed of (unit -> 'a)
  | Value of 'a
  | Exception of exn
;;
type 'a t = 'a status ref;;

A value of type 'a Lazy.t is a deferred computation (also called a suspension) that computes a result of type 'a. The expression lazy (expr) returns a suspension that computes expr.

exception Undefined;;
val force: 'a t -> 'a;;

Lazy.force x computes the suspension x and returns its result. If the suspension was already computed, Lazy.force x returns the same value again. If it raised an exception, the same exception is raised again. Raise Undefined if the evaluation of the suspension requires its own result.