# Module Doubly_linked

    module Doubly_linked: sig .. end

doubly-linked lists

Compared to other doubly-linked lists, in this one:

1. Calls to modification functions (insert*, move*, ...) detect if the list is being iterated over (iter, fold, ...), and if so raise an exception. For example, a use like the following would raise.

          iter t ~f:(fun _ -> ... remove t e ...)

2. There is a designated "front" and "back" of each list, rather than viewing each element as an equal in a ring.

3. Elements know which list they're in. Each operation that takes an `Elt.t` also takes a `t`, first checks that the `Elt` belongs to the `t`, and if not, raises.

4. Related to (3), lists cannot be split, though a sort of splicing is available as `transfer`. In other words, no operation will cause one list to become two. This makes this module unsuitable for maintaining the faces of a planar graph under edge insertion and deletion, for example.

5. Another property permitted by (3) and (4) is that `length` is O(1).

    module Elt: sig .. end

    type 'a t

    include Container.S1

    val invariant : 'a t -> unit

    val create : unit -> 'a t

creating doubly-linked lists

    val of_list : 'a list -> 'a t

`of_list l` returns a doubly-linked list `t` with the same elements as `l` and in the same order (i.e. the first element of `l` is the first element of `t`). It is always the case that `l = to_list (of_list l)`.

    val equal : 'a t -> 'a t -> bool

predicates

pointer equality

    val is_first : 'a t -> 'a Elt.t -> bool

    val is_last : 'a t -> 'a Elt.t -> bool

    val first_elt : 'a t -> 'a Elt.t option

constant-time extraction of first and last elements.

    val last_elt : 'a t -> 'a Elt.t option

    val first : 'a t -> 'a option

    val last : 'a t -> 'a option

    val next : 'a t -> 'a Elt.t -> 'a Elt.t option

constant-time retrieval of next or previous element.

    val prev : 'a t -> 'a Elt.t -> 'a Elt.t option

    val insert_before : 'a t -> 'a Elt.t -> 'a -> 'a Elt.t

constant-time insertion of a new element.

    val insert_after : 'a t -> 'a Elt.t -> 'a -> 'a Elt.t

    val insert_first : 'a t -> 'a -> 'a Elt.t

    val insert_last : 'a t -> 'a -> 'a Elt.t

    val move_to_front : 'a t -> 'a Elt.t -> unit

constant-time move of an element from and to positions in the same list. An exception is raised if `elt` is equal to `anchor`.

    val move_to_back : 'a t -> 'a Elt.t -> unit

    val move_after : 'a t ->
           'a Elt.t -> anchor:'a Elt.t -> unit

    val move_before : 'a t ->
           'a Elt.t -> anchor:'a Elt.t -> unit

    val remove : 'a t -> 'a Elt.t -> unit

constant-time removal of an element.

    val remove_first : 'a t -> 'a option

    val remove_last : 'a t -> 'a option

    val fold_elt : 'a t -> init:'b -> f:('b -> 'a Elt.t -> 'b) -> 'b

`fold_elt t ~init ~f` is the same as fold, except `f` is called with the `'a Elt.t`'s from the list instead of the contained `'a` values.

Note that like other iteration functions, it is an error to mutate `t` inside the fold. If you'd like to call `remove` on any of the `'a Elt.t`'s, use `filter_inplace`.

    val iter_elt : 'a t -> f:('a Elt.t -> unit) -> unit

    val fold_right : 'a t -> init:'b -> f:('a -> 'b -> 'b) -> 'b

    val find_elt : 'a t -> f:('a -> bool) -> 'a Elt.t option

`find_elt t ~f` finds the first element in `t` that satisfies `f`, by testing each of element of `t` in turn until `f` succeeds.

    val clear : 'a t -> unit

`clear t` removes all elements from the list in constant time.

    val copy : 'a t -> 'a t

    val transfer : src:'a t -> dst:'a t -> unit

`transfer ~src ~dst` has the same behavior as `iter src ~f:(insert_last dst); clear src` except that it runs in constant time.

If `s = to_list src` and `d = to_list dst`, then after `transfer ~src ~dst`: `to_list src = []` `to_list dst = d @ s`

    val filter_inplace : 'a t -> f:('a -> bool) -> unit

`filter_inplace t ~f` removes all elements of `t` that don't satisfy `f`.

    val unchecked_iter : 'a t -> f:('a -> unit) -> unit

`unchecked_iter t ~f` behaves like `iter t ~f` except that `f` is allowed to modify `t`. Adding or removing elements before the element currently being visited has no effect on the traversal. Elements added after the element currently being visited will be traversed. Elements deleted after the element currently being visited will not be traversed. Deleting the element currently visited is an error that is not detected (presumably leading to an infinite loop) .

    val to_sequence : 'a t -> 'a Sequence.t

    val t_of_sexp : (Sexplib.Sexp.t -> 'a) -> Sexplib.Sexp.t -> 'a t

    val sexp_of_t : ('a -> Sexplib.Sexp.t) -> 'a t -> Sexplib.Sexp.t

creating doubly-linked lists
`of_list l` returns a doubly-linked list `t` with the same elements as `l` and in the same order (i.e. the first element of `l` is the first element of `t`). It is always the case that `l = to_list (of_list l)`.
predicates
pointer equality
constant-time extraction of first and last elements.
constant-time retrieval of next or previous element.
constant-time insertion of a new element.
constant-time move of an element from and to positions in the same list. An exception is raised if `elt` is equal to `anchor`.
constant-time removal of an element.
`fold_elt t ~init ~f` is the same as fold, except `f` is called with the `'a Elt.t`'s from the list instead of the contained `'a` values.

Note that like other iteration functions, it is an error to mutate `t` inside the fold. If you'd like to call `remove` on any of the `'a Elt.t`'s, use `filter_inplace`.
`find_elt t ~f` finds the first element in `t` that satisfies `f`, by testing each of element of `t` in turn until `f` succeeds.
`clear t` removes all elements from the list in constant time.
`transfer ~src ~dst` has the same behavior as `iter src ~f:(insert_last dst); clear src` except that it runs in constant time.

If `s = to_list src` and `d = to_list dst`, then after `transfer ~src ~dst`: `to_list src = []` `to_list dst = d @ s`
`filter_inplace t ~f` removes all elements of `t` that don't satisfy `f`.
`unchecked_iter t ~f` behaves like `iter t ~f` except that `f` is allowed to modify `t`. Adding or removing elements before the element currently being visited has no effect on the traversal. Elements added after the element currently being visited will be traversed. Elements deleted after the element currently being visited will not be traversed. Deleting the element currently visited is an error that is not detected (presumably leading to an infinite loop) .