lazy.mli

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(*                                                                        *)
(*                                 OCaml                                  *)
(*                                                                        *)
(*             Damien Doligez, projet Para, INRIA Rocquencourt            *)
(*                                                                        *)
(*   Copyright 1997 Institut National de Recherche en Informatique et     *)
(*     en Automatique.                                                    *)
(*                                                                        *)
(*   All rights reserved.  This file is distributed under the terms of    *)
(*   the GNU Lesser General Public License version 2.1, with the          *)
(*   special exception on linking described in the file LICENSE.          *)
(*                                                                        *)
(**************************************************************************)

(** Deferred computations. *)

type 'a t = 'a CamlinternalLazy.t
(** A value of type ['a Lazy.t] is a deferred computation, called
   a suspension, that has a result of type ['a].  The special
   expression syntax [lazy (expr)] makes a suspension of the
   computation of [expr], without computing [expr] itself yet.
   "Forcing" the suspension will then compute [expr] and return its
   result. Matching a suspension with the special pattern syntax
   [lazy(pattern)] also computes the underlying expression and
   tries to bind it to [pattern]:

  {[
    let lazy_option_map f x =
    match x with
    | lazy (Some x) -> Some (Lazy.force f x)
    | _ -> None
  ]}

   Note: If lazy patterns appear in multiple cases in a pattern-matching,
   lazy expressions may be forced even outside of the case ultimately selected
   by the pattern matching. In the example above, the suspension [x] is always
   computed.


   Note: [lazy_t] is the built-in type constructor used by the compiler
   for the [lazy] keyword.  You should not use it directly.  Always use
   [Lazy.t] instead.

   Note: [Lazy.force] is not thread-safe.  If you use this module in
   a multi-threaded program, you will need to add some locks.

   Note: if the program is compiled with the [-rectypes] option,
   ill-founded recursive definitions of the form [let rec x = lazy x]
   or [let rec x = lazy(lazy(...(lazy x)))] are accepted by the type-checker
   and lead, when forced, to ill-formed values that trigger infinite
   loops in the garbage collector and other parts of the run-time system.
   Without the [-rectypes] option, such ill-founded recursive definitions
   are rejected by the type-checker.
*)


exception Undefined

(* val force : 'a t -> 'a  *)
external force : 'a t -> 'a = "%lazy_force"
(** [force x] forces the suspension [x] and returns its result.
   If [x] has already been forced, [Lazy.force x] returns the
   same value again without recomputing it.  If it raised an exception,
   the same exception is raised again.
   @raise Undefined if the forcing of [x] tries to force [x] itself
   recursively.
*)

val force_val : 'a t -> 'a
(** [force_val x] forces the suspension [x] and returns its
    result.  If [x] has already been forced, [force_val x]
    returns the same value again without recomputing it.

    If the computation of [x] raises an exception, it is unspecified
    whether [force_val x] raises the same exception or {!Undefined}.
    @raise Undefined if the forcing of [x] tries to force [x] itself
    recursively.
*)

val from_fun : (unit -> 'a) -> 'a t
(** [from_fun f] is the same as [lazy (f ())] but slightly more efficient.

    [from_fun] should only be used if the function [f] is already defined.
    In particular it is always less efficient to write
    [from_fun (fun () -> expr)] than [lazy expr].

    @since 4.00.0 *)

val from_val : 'a -> 'a t
(** [from_val v] returns an already-forced suspension of [v].
    This is for special purposes only and should not be confused with
    [lazy (v)].
    @since 4.00.0 *)

val is_val : 'a t -> bool
(** [is_val x] returns [true] if [x] has already been forced and
    did not raise an exception.
    @since 4.00.0 *)

val lazy_from_fun : (unit -> 'a) -> 'a t
  [@@ocaml.deprecated "Use Lazy.from_fun instead."]
(** @deprecated synonym for [from_fun]. *)

val lazy_from_val : 'a -> 'a t
  [@@ocaml.deprecated "Use Lazy.from_val instead."]
(** @deprecated synonym for [from_val]. *)

val lazy_is_val : 'a t -> bool
  [@@ocaml.deprecated "Use Lazy.is_val instead."]
(** @deprecated synonym for [is_val]. *)