Batteries user guide : Batteries.Enum

# Module Batteries.Enum

    module Enum: BatEnum

    type 'a t

    module type Enumerable = sig .. end

A signature for data structures which may be converted to and from `enum`.

    include BatEnum.Enumerable

    include BatInterfaces.Mappable

###### 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 `e1` and `e2` repeatedly until one of the two enumerations ends.

    val exists : ('a -> bool) -> 'a t -> bool

`exists f e` returns `true` if there is some `x` in `e` such that `f x`

    val for_all : ('a -> bool) -> 'a t -> bool

`for_all f e` returns `true` if for every `x` in `e`, `f x` is true

    val fold : ('b -> 'a -> 'b) -> 'b -> 'a t -> 'b

A general loop on an enumeration.

If `e` is empty, `fold f v e` returns `v`. Otherwise, `fold v e` returns `f (... (f (f v a0) a1) ...) aN` where `a0,a1..aN` are the elements of `e`. This function may be used, for instance, to compute the sum of all elements of an enumeration `e` as follows: `fold ( + ) 0 e`. Eager.

    val reduce : ('a -> 'a -> 'a) -> 'a t -> 'a

A simplified version of `fold`, which uses the first element of the enumeration as a default value.

`reduce f e` throws `Not_found` if `e` is empty, returns its only element if e is a singleton, otherwise `f (... (f (f a0 a1) a2)...) aN` where `a0,a1..aN` are the elements of `e`.

    val sum : int t -> int

`sum` returns the sum of the given int enum. If the argument is empty, returns 0. Eager

    val fsum : float t -> float

Since 2.0

    val kahan_sum : float t -> float

`kahan_sum l` returns a numerically-accurate sum of the floats of `l`. See `BatArray.fsum` for more details.
Since 2.2.0

    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.

    val scanl : ('b -> 'a -> 'b) -> 'b -> 'a t -> 'b t

A variant of `fold` producing an enumeration of its intermediate values. If `e` contains `x0`, `x1`, ..., `scanl f init e` is the enumeration containing `init`, `f init x0`, `f (f init x0) x1`... Lazy.

    val scan : ('a -> 'a -> 'a) -> 'a t -> 'a t

`scan` is similar to `scanl` but without the `init` value: if `e` contains `x0`, `x1`, `x2` ..., `scan f e` is the enumeration containing `x0`, `f x0 x1`, `f (f x0 x1) x2`...

For instance, `scan ( * ) (1 -- 10)` will produce an enumeration containing the successive values of the factorial function.

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.
Raises `Not_found` if no such element exists in the enumeration, consuming the whole enumeration in the search.

Since `find` (eagerly) consumes a prefix of the enumeration, it can be used several times on the same enumeration to find the next element.

    val find_map : ('a -> 'b option) -> 'a t -> 'b

`find_map f e` finds the first element `x` of `e` such that `f x` returns `Some r`, then returns r. It consumes the enumeration up to and including the found element.
Since 2.0
Raises `Not_found` if no such element exists in the enumeration, consuming the whole enumeration in the search.

Since `find_map` (eagerly) 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. Forces at most one 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 get_exn : 'a t -> 'a

`get_exn e` returns the first element of `e`.
Since 2.0
Raises `No_more_elements` if `e` 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.

    val take : int -> 'a t -> 'a t

`take n e` returns the prefix of `e` of length `n`, or `e` itself if `n` is greater than the length of `e`

    val drop : int -> 'a t -> unit

`drop n e` removes the first `n` element from the enumeration, if any.

    val skip : int -> 'a t -> 'a t

`skip n e` removes the first `n` element from the enumeration, if any, then returns `e`.

This function has the same behavior as `drop` but is often easier to compose with, e.g., `skip 5 %> take 3` is a new function which skips 5 elements and then returns the next 3 elements.

    val take_while : ('a -> bool) -> 'a t -> 'a t

`take_while f e` produces a new enumeration in which only remain the first few elements `x` of `e` such that `f x`

    val drop_while : ('a -> bool) -> 'a t -> 'a t

`drop_while p e` produces a new enumeration in which only all the first elements such that `f e` have been junked.

    val span : ('a -> bool) -> 'a t -> 'a t * 'a t

`span test e` produces two enumerations `(hd, tl)`, such that `hd` is the same as `take_while test e` and `tl` is the same as `drop_while test e`.

    val break : ('a -> bool) -> 'a t -> 'a t * 'a t

Negated span. `break test e` is equivalent to `span (fun x -> not (test x)) e`

    val group : ('a -> 'b) -> 'a t -> 'a t t

`group test e` divides `e` into an enumeration of enumerations, where each sub-enumeration is the longest continuous enumeration of elements whose `test` results are the same.

`Enum.group (x -> x mod 2) [1;2;4;1] = [[1];[2;4];[1]]`

`Enum.group (fun x -> x mod 3) [1;2;4;1] = [[1];[2];[4;1]]`

`Enum.group (fun s -> s.[0]) ["cat"; "canary"; "dog"; "dodo"; "ant"; "cow"] = [["cat"; "canary"];["dog";"dodo"];["ant"];["cow"]]`

Warning: The result of this operation cannot be directly cloned safely; instead, reify to a non-lazy structure and read from that structure multiple times.

    val group_by : ('a -> 'a -> bool) -> 'a t -> 'a t t

`group_by eq e` divides `e` into an enumeration of enumerations, where each sub-enumeration is the longest continuous enumeration of elements that are equal, as judged by `eq`.

Warning: The result of this operation cannot be directly cloned safely; instead, reify to a non-lazy structure and read from that structure multiple times.

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

`clump size add get e` runs `add` on `size` (or less at the end) elements of `e` and then runs `get` to produce value for the result enumeration. Useful to convert a char enum into string enum.

    val cartesian_product : 'a t -> 'b t -> ('a * 'b) t

`cartesian_product e1 e2` computes the cartesian product of `e1` and `e2`. Pairs are enumerated in a non-specified order, but in fair enough an order so that it works on infinite enums (i.e. even then, any pair is eventually returned)
Since 2.2.0

###### 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 a0, f a1, ...)` where `a0,a1...` are the elements of `e`. Lazy.

    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 : mapi f e returns an enumeration over `(f 0 a0, f 1 a1, ...)` where `a0,a1...` are the elements of `e`.

    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`. Lazy.

Note filter is lazy in that it returns a lazy enumeration, but each element in the result is eagerly searched in the input enumeration. Therefore, the access to a given element in the result will diverge if it is preceded, in the input enumeration, by infinitely many false elements (elements on which the predicate `p` returns `false`).

Other functions that may drop an unbound number of elements (`filter_map`, `take_while`, etc.) have the same behavior.

    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`.

`filter_map` works on infinite enumerations; see `filter`.

    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`. Lazy.

Note The behavior of appending `e` to itself or to something derived from `e` is not specified. In particular, cloning `append e e` may destroy any sharing between the first and the second argument.

    val prefix_action : (unit -> unit) -> 'a t -> 'a t

`prefix_action f e` will behave as `e` but guarantees that `f ()` will be invoked exactly once before the current first element of `e` is read.

If `prefix_action f e` is cloned, `f` is invoked only once, during the cloning. If `prefix_action f e` is counted, `f` is invoked only once, during the counting.

May be used for signalling that reading starts or for performing delayed evaluations.

    val suffix_action : (unit -> unit) -> 'a t -> 'a t

`suffix_action f e` will behave as `e` but guarantees that `f ()` will be invoked after the contents of `e` are exhausted.

If `suffix_action f e` is cloned, `f` is invoked only once, when the original enumeration is exhausted. If `suffix_action f e` is counted, `f` is only invoked if the act of counting requires a call to `force`.

May be used for signalling that reading stopped or for performing delayed evaluations.

    val concat : 'a t t -> 'a t

`concat e` returns an enumeration over all elements of all enumerations of `e`.

    val flatten : 'a t t -> 'a t

Synonym of `BatEnum.concat`

    val concat_map : ('a -> 'b t) -> 'a t -> 'b t

Synonym of `BatEnum.Monad.bind`, with flipped arguments. `concat_map f e` is the same as `concat (map f e)`.
Since 2.2.0

###### 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.

    exception Infinite_enum

As a convenience for debugging, this exception may be raised by the `count` function of `make` when attempting to count an infinite enum.

    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 or may raise `Infinite_enum` if it is known that the enumeration is infinite.
- 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 `BatList.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 `count` will result in a call to `force` that will enumerate all elements in order to return a correct value.

    val from_while : (unit -> 'a option) -> 'a t

`from_while next` creates an enumeration from the `next` function. `next` shall return `Some x` where `x` is the next element of the enumeration or `None` 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 from_loop : 'b -> ('b -> 'a * 'b) -> 'a t

`from_loop data next` creates a (possibly infinite) enumeration from the successive results of applying `next` to `data`, then to the result, etc. The list ends whenever the function raises `BatEnum.No_more_elements`.

    val seq : 'a -> ('a -> 'a) -> ('a -> bool) -> 'a t

`seq init step cond` creates a sequence of data, which starts from `init`, extends by `step`, until the condition `cond` fails. E.g. `seq 1 ((+) 1) ((>) 100)` returns `1, 2, ... 99`. If `cond init` is false, the result is empty.

    val unfold : 'b -> ('b -> ('a * 'b) option) -> 'a t

As `from_loop`, except uses option type to signal the end of the enumeration.

`unfold data next` creates a (possibly infinite) enumeration from the successive results of applying `next` to `data`, then to the result, etc. The enumeration ends whenever the function returns `None`

Example: `Enum.unfold n (fun x -> if x = 1 then None else Some (x, if x land 1 = 1 then 3 * x + 1 else x / 2))` returns the hailstone sequence starting at `n`.

    val init : int -> (int -> 'a) -> 'a t

`init n f` creates a new enumeration over elements `f 0, f 1, ..., f (n-1)`

    val singleton : 'a -> 'a t

Create an enumeration consisting of exactly one element.

    val repeat : ?times:int -> 'a -> 'a t

`repeat ~times:n x` creates a enum sequence filled with `n` times of `x`. It return infinite enum when `~times` is absent. It returns empty enum when `times <= 0`

    val cycle : ?times:int -> 'a t -> 'a t

`cycle` is similar to `repeat`, except that the content to fill is a subenum rather than a single element. Note that `times` represents the times of repeating not the length of enum. When `~times` is absent the result is an infinite enum.

    val delay : (unit -> 'a t) -> 'a t

`delay (fun () -> e)` produces an enumeration which behaves as `e`. The enumeration itself will only be computed when consumed.

A typical use of this function is to explore lazily non-trivial data structures, as follows:

`type 'a tree = Leaf | Node of 'a * 'a tree * 'a tree let enum_tree = let rec aux = function | Leaf -> BatEnum.empty () | Node (n, l, r) -> BatEnum.append (BatEnum.singleton n) (BatEnum.append (delay (fun () -> aux l)) (delay (fun () -> aux r)))`

    val to_object : 'a t -> (< clone : 'b; count : int; next : 'a > as 'b)

`to_object e` returns a representation of `e` as an object.

    val of_object : (< clone : 'b; count : int; next : 'a > as 'b) -> 'a t

`of_object e` returns a representation of an object as an enumeration

    val enum : 'a t -> 'a t

identity : added for consistency with the other data structures

    val of_enum : 'a t -> 'a t

identity : added for consistency with the other data structures

###### 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.

    val hard_count : 'a t -> int

`hard_count` returns the number of remaining in elements in `e`, consuming the whole enumeration somewhere along the way. This function is always at least as fast as the fastest of either `count` or a `fold` on the elements of `t`.

This function is useful when you have opened an enumeration for the sole purpose of counting its elements (e.g. the number of lines in a file).

###### Utilities

    val range : ?until:int -> int -> int t

`range p until:q` creates an enumeration of integers `[p, p+1, ..., q]`. If `until` is omitted, the enumeration is not bounded. Behaviour is not-specified once `max_int` has been reached.

    val dup : 'a t -> 'a t * 'a t

`dup stream` returns a pair of streams which are identical to `stream`. Note that stream is a destructive data structure, the point of `dup` is to return two streams can be used independently.

    val combine : 'a t * 'b t -> ('a * 'b) t

`combine` transform a pair of stream into a stream of pairs of corresponding elements. If one stream is short, excess elements of the longer stream are ignored.

    val uncombine : ('a * 'b) t -> 'a t * 'b t

`uncombine` is the opposite of `combine`

    val merge : ('a -> 'a -> bool) -> 'a t -> 'a t -> 'a t

`merge test a b` merge the elements from `a` and `b` into a single enumeration. At each step, `test` is applied to the first element `xa` of `a` and the first element `xb` of `b` to determine which should get first into resulting enumeration. If `test xa xb` returns `true`, `xa` (the first element of `a`) is used, otherwise `xb` is used. If `a` or `b` runs out of elements, the process will append all elements of the other enumeration to the result.

For example, if `a` and `b` are enumerations of integers sorted in increasing order, then `merge (<) a b` will also be sorted.

    val interleave : 'a t array -> 'a t

`interleave enums` creates a new enumeration from an array of enumerations. The new enumeration first yields the first elements of the enumerations in the supplied order, then second elements, etc. Thus, a sequence `[| [x11 ; x12 ; ...] ; [x21 ; x22, ...] , ... [xN1 ; xN2 ; ...] |]` becomes `[ x11 ; x12 ; ... ; xN1 ; x21 ; x22 ; ... ; xN2 ; x31 ; ... ]`.

    val uniq : 'a t -> 'a t

`uniq e` returns a duplicate of `e` with repeated values omitted (similar to unix's `uniq` command). It uses structural equality to compare consecutive elements.

    val uniqq : 'a t -> 'a t

`uniqq e` behaves as `uniq e` except it uses physical equality to compare consecutive elements.
Since 2.4.0

    val uniq_by : ('a -> 'a -> bool) -> 'a t -> 'a t

`uniqq cmp e` behaves as `uniq e` except it allows to specify a comparison function.
Since 2.4.0

    val switch : ('a -> bool) -> 'a t -> 'a t * 'a t

`switch test enum` splits `enum` into two enums, where the first enum have all the elements satisfying `test`, the second enum is opposite. The order of elements in the source enum is preserved.

    val partition : ('a -> bool) -> 'a t -> 'a t * 'a t

as `switch`

    val arg_min : ('a -> 'b) -> 'a t -> 'a

    val arg_max : ('a -> 'b) -> 'a t -> 'a

`arg_min f xs` returns the `x` in `xs` for which `f x` is minimum. Similarly for `arg_max`, except it returns the maximum. If multiple values reach the maximum, one of them is returned. (currently the first, but this is not guaranteed)

Example: `-5 -- 5 |> arg_min (fun x -> x * x + 6 * x - 5) = -3` Example: `List.enum ["cat"; "canary"; "dog"; "dodo"; "ant"; "cow"] |> arg_max String.length = "canary"`
Raises `Invalid_argument` if the input enum is empty

###### Trampolining

    val while_do : ('a -> bool) ->
           ('a t -> 'a t) -> 'a t -> 'a t

`while_do cont f e` is a loop on `e` using `f` as body and `cont` as condition for continuing.

If `e` contains elements `x0`, `x1`, `x2`..., then if `cont x0` is `false`, `x0` is returned as such and treatment stops. On the other hand, if `cont x0` is `true`, `f x0` is returned and the loop proceeds with `x1`...

Note that f is used as halting condition after the corresponding element has been added to the result stream.

###### Infix operators

Infix versions of some functions

This module groups together all infix operators so that you can open it without opening the whole batEnum module.

    module Infix: sig .. end

    val (--) : int -> int -> int t

    val (--^) : int -> int -> int t

    val (--.) : float * float -> float -> float t

    val (---) : int -> int -> int t

    val (--~) : char -> char -> char t

    val (//) : 'a t -> ('a -> bool) -> 'a t

    val (/@) : 'a t -> ('a -> 'b) -> 'b t

    val (@/) : ('a -> 'b) -> 'a t -> 'b t

    val (//@) : 'a t -> ('a -> 'b option) -> 'b t

    val (@//) : ('a -> 'b option) -> 'a t -> 'b t

###### Monad related modules

    module WithMonad: functor (Mon : BatInterfaces.Monad) -> sig .. end

Monadic operations on Enumerations containing monadic elements

    module Monad: sig .. end

The BatEnum Monad

###### Boilerplate code

    val print : ?first:string ->
           ?last:string ->
           ?sep:string ->
           ('a BatInnerIO.output -> 'b -> unit) ->
           'a BatInnerIO.output -> 'b t -> unit

Print and consume the contents of an enumeration.

    val print_at_most : ?first:string ->
           ?last:string ->
           ?sep:string ->
           limit:int ->
           ('a BatInnerIO.output -> 'b -> unit) ->
           'a BatInnerIO.output -> 'b t -> unit

`print_at_most pp limit out enum` consumes `enum` to print its elements into `out` (using `pp` to print individual elements). At most `limit` arguments are printed, if more elements are available an ellipsis "..." is added.
Since 2.2.0
Raises `Invalid_argument` if the limit is <= 0.

    val compare : ('a -> 'a -> int) -> 'a t -> 'a t -> int

`compare cmp a b` compares enumerations `a` and `b` by lexicographical order using comparison `cmp`.
Returns `compare cmp a' b'`, where `a'` and `b'` are respectively equal to `a` and `b` without their first element, if both `a` and `b` are non-empty and `cmp x y = 0`, where `x` is the first element of `a` and `y` is the first element of `b`

    val ord : ('a -> 'a -> BatOrd.order) -> 'a t -> 'a t -> BatOrd.order

Same as `compare` but returning a `BatOrd.order` instead of an interger.

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

`equal eq a b` returns `true` when `a` and `b` contain the same sequence of elements.

###### Override modules

The following modules replace functions defined in `BatEnum` with functions behaving slightly differently but having the same name. This is by design: the functions meant to override the corresponding functions of `BatEnum`.

    module Exceptionless: sig .. end

Operations on `BatEnum` without exceptions.

    module Labels: sig .. end

Operations on `BatEnum` with labels.