# Module Enum

    module Enum: sig .. end

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

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

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).

- Raises `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.

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`.

Constructors

In this section the word 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 shall be raised by the `next` function of `make` or `from` when no more elements can be enumerated, it 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.

- the `next` function 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.
- the `count` function shall return the actual number of remaining elements in the enumeration.
- the `clone` function 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` 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)`

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.