(*
Copyright (c) 2013, Simon Cruanes
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(** {1 Generators}

Values of type ['a Gen.t] represent a possibly infinite sequence of values
of type 'a. One can only iterate once on the sequence, as it is consumed
by iteration/deconstruction/access. [None] is returned when the generator
is exhausted. Most functions consume elements.

The submodule {!Restart} provides utilities to work with
{b restartable generators}, that is, functions [unit -> 'a Gen.t] that
allow to build as many generators from the same source as needed.
*)

(** {2 Global type declarations} *)

type 'a t = unit -> 'a option
  (** A generator may be called several times, yielding the next value
      each time. It returns [None] when no elements remain *)

type 'a gen = 'a t

(** {b NOTE}: version informations ("@since" annotations) in CCGen_intf
    will not be reliable, for they will represent versions of Gen
    rather than containers. *)
module type S = Gen_intf.S

(** {2 Transient generators} *)

val get : 'a t -> 'a option
  (** Get the next value *)

val next : 'a t -> 'a option
  (** Synonym for {!get} *)

val get_exn : 'a t -> 'a
  (** Get the next value, or fails
      @raise Invalid_argument if no element remains *)

val junk : 'a t -> unit
  (** Drop the next value, discarding it. *)

val repeatedly : (unit -> 'a) -> 'a t
  (** Call the same function an infinite number of times (useful for instance
      if the function is a random generator). *)

include S with type 'a t := 'a gen
  (** Operations on {b transient} generators *)

(** {2 Restartable generators} *)

module Restart : sig
  type 'a t = unit -> 'a gen

  type 'a restartable = 'a t

  include S with type 'a t := 'a restartable

  val cycle : 'a t -> 'a t
    (** Cycle through the enum, endlessly. The enum must not be empty. *)

  val lift : ('a gen -> 'b) -> 'a t -> 'b

  val lift2 : ('a gen -> 'b gen -> 'c) -> 'a t -> 'b t -> 'c
end

(** {2 Utils} *)

val persistent : 'a t -> 'a Restart.t
  (** Store content of the transient generator in memory, to be able to iterate
      on it several times later. If possible, consider using combinators
      from {!Restart} directly instead. *)

val persistent_lazy : 'a t -> 'a Restart.t
  (** Same as {!persistent}, but consumes the generator on demand (by chunks).
      This allows to make a restartable generator out of an ephemeral one,
      without paying a big cost upfront (nor even consuming it fully). 
      @since 0.6.1 *)

val start : 'a Restart.t -> 'a t
  (** Create a new transient generator.
      [start gen] is the same as [gen ()] but is included for readability. *)