File: enum.mli

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(* 
 * Enum - enumeration over abstract collection of elements.
 * Copyright (C) 2003 Nicolas Cannasse
 * 
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version,
 * with the special exception on linking described in file LICENSE.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 *)

(** Enumeration over abstract collection of elements.

 Enumerations are entirely functional and most of the operations do not
 actually require the allocation of data structures. Using enumerations
 to manipulate data is therefore efficient and simple. All data structures in
 ExtLib such as lists, arrays, etc. have support to convert from and to
 enumerations.
*)


type 'a t

(** {6 Final functions}

 These functions consume the enumeration until
 it ends or an exception is raised by the first
 argument function.
*)

val iter : ('a -> unit) -> 'a t -> unit
(** [iter f e] calls the function [f] with each elements of [e] in turn. *)

val iter2 : ('a -> 'b -> unit) -> 'a t -> 'b t -> unit
(** [iter2 f e1 e2] calls the function [f] with the next elements of [e] and
 [e2] repeatedly until one of the two enumerations ends. *)

val fold : ('a -> 'b -> 'b) -> 'b -> 'a t -> 'b
(** [fold f v e] returns [v] if [e] is empty,
  otherwise [f aN (... (f a2 (f a1 v)) ...)] where a1..N are
  the elements of [e]. 
*)

val fold2 : ('a -> 'b -> 'c -> 'c) -> 'c -> 'a t -> 'b t -> 'c
(** [fold2] is similar to [fold] but will fold over two enumerations at the
 same time until one of the two enumerations ends. *)

(** Indexed functions : these functions are similar to previous ones
 except that they call the function with one additional argument which
 is an index starting at 0 and incremented after each call to the function. *)

val iteri : (int -> 'a -> unit) -> 'a t -> unit

val iter2i : ( int -> 'a -> 'b -> unit) -> 'a t -> 'b t -> unit

val foldi : (int -> 'a -> 'b -> 'b) -> 'b -> 'a t -> 'b

val fold2i : (int -> 'a -> 'b -> 'c -> 'c) -> 'c -> 'a t -> 'b t -> 'c

(** {6 Useful functions} *)

val find : ('a -> bool) -> 'a t -> 'a
(** [find f e] returns the first element [x] of [e] such that [f x] returns
 [true], consuming the enumeration up to and including the
 found element, or, raises [Not_found] if no such element exists
 in the enumeration, consuming the whole enumeration in the search.

 Since [find] consumes a prefix of the enumeration, it can be used several 
 times on the same enumeration to find the next element. *)

val is_empty : 'a t -> bool
(** [is_empty e] returns true if [e] does not contains any element. *)

val peek : 'a t -> 'a option
(** [peek e] returns [None] if [e] is empty or [Some x] where [x] is
 the next element of [e]. The element is not removed from the enumeration. *)

val get : 'a t -> 'a option
(** [get e] returns [None] if [e] is empty or [Some x] where [x] is
  the next element of [e], in which case the element is removed from the enumeration. *)

val next : 'a t -> 'a
(** [next e] returns the next element of [e] (and removes it from enumeration).
  @raise No_more_elements if enumeration is empty *)

val push : 'a t -> 'a -> unit
(** [push e x] will add [x] at the beginning of [e]. *)

val junk : 'a t -> unit
(** [junk e] removes the first element from the enumeration, if any. *)

val clone : 'a t -> 'a t
(** [clone e] creates a new enumeration that is copy of [e]. If [e]
 is consumed by later operations, the clone will not get affected. *)

val force : 'a t -> unit
(** [force e] forces the application of all lazy functions and the
 enumeration of all elements, exhausting the enumeration. 
 
  An efficient intermediate data structure
  of enumerated elements is constructed and [e] will now enumerate over
  that data structure. *)

(** {6 Lazy constructors}

 These functions are lazy which means that they will create a new modified
 enumeration without actually enumerating any element until they are asked
 to do so by the programmer (using one of the functions above).
 
 When the resulting enumerations of these functions are consumed, the
 underlying enumerations they were created from are also consumed. *)

val map : ('a -> 'b) -> 'a t -> 'b t
(** [map f e] returns an enumeration over [(f a1, f a2, ... , f aN)] where
 a1...N are the elements of [e]. *)

val mapi : (int -> 'a -> 'b) -> 'a t -> 'b t
(** [mapi] is similar to [map] except that [f] is passed one extra argument
 which is the index of the element in the enumeration, starting from 0. *)

val filter : ('a -> bool) -> 'a t -> 'a t
(** [filter f e] returns an enumeration over all elements [x] of [e] such
 as [f x] returns [true]. *)

val filter_map : ('a -> 'b option) -> 'a t -> 'b t
(** [filter_map f e] returns an enumeration over all elements [x] such as
 [f y] returns [Some x] , where [y] is an element of [e]. *)

val append : 'a t -> 'a t -> 'a t
(** [append e1 e2] returns an enumeration that will enumerate over all
 elements of [e1] followed by all elements of [e2]. *)

val concat : 'a t t -> 'a t
(** [concat e] returns an enumeration over all elements of all enumerations
 of [e]. *)

(** {6 Constructors} 

 In this section the word {i shall} denotes a semantic
 requirement. The correct operation
 of the functions in this interface are conditional
 on the client meeting these requirements.
*)

exception No_more_elements
(** This exception {i shall} be raised by the [next] function of [make] 
  or [from] when no more elements can be enumerated, it {i shall not}
  be raised by any function which is an argument to any
  other function specified in the interface.
*)

val empty : unit -> 'a t
(** The empty enumeration : contains no element *)

val make : next:(unit -> 'a) -> count:(unit -> int) -> clone:(unit -> 'a t) -> 'a t
(** This function creates a fully defined enumeration.
  {ul {li the [next] function {i shall} return the next element of the
  enumeration or raise [No_more_elements] if the underlying data structure
  does not have any more elements to enumerate.}
  {li the [count] function {i shall} return the actual number of remaining
  elements in the enumeration.}
  {li the [clone] function {i shall} create a clone of the enumeration
  such as operations on the original enumeration will not affect the
  clone. }}
 
  For some samples on how to correctly use [make], you can have a look
    at implementation of [ExtList.enum]. 
*)

val from : (unit -> 'a) -> 'a t
(** [from next] creates an enumeration from the [next] function.
 [next] {i shall} return the next element of the enumeration or raise
 [No_more_elements] when no more elements can be enumerated. Since the
 enumeration definition is incomplete, a call to [clone] or [count] will
 result in a call to [force] that will enumerate all elements in order to
 return a correct value. *)

val init : int -> (int -> 'a) -> 'a t
(** [init n f] creates a new enumeration over elements
  [f 0, f 1, ..., f (n-1)] *)

(** {6 Counting} *)

val count : 'a t -> int
(** [count e] returns the number of remaining elements in [e] without
  consuming the enumeration.

Depending of the underlying data structure that is implementing the
enumeration functions, the count operation can be costly, and even sometimes
can cause a call to [force]. *)

val fast_count : 'a t -> bool
(** For users worried about the speed of [count] you can call the [fast_count]
    function that will give an hint about [count] implementation. Basically, if
    the enumeration has been created with [make] or [init] or if [force] has
    been called on it, then [fast_count] will return true. *)