# Module Either

    module Either: sig .. end

Either type.

Either is the simplest and most generic sum/variant type: a value of `('a, 'b) Either.t` is either a `Left (v : 'a)` or a `Right (v : 'b)`.

It is a natural choice in the API of generic functions where values could fall in two different cases, possibly at different types, without assigning a specific meaning to what each case should be.

For example:

    List.partition_map:
        ('a -> ('b, 'c) Either.t) -> 'a list -> 'b list * 'c list

If you are looking for a parametrized type where one alternative means success and the other means failure, you should use the more specific type `Result.t`.

- Since 4.12

    type ('a, 'b) t =

`|` | `Left of `'a``
`|` | `Right of `'b``

A value of `('a, 'b) Either.t` contains either a value of `'a` or a value of `'b`

    val left : 'a -> ('a, 'b) t

`left v` is `Left v`.

    val right : 'b -> ('a, 'b) t

`right v` is `Right v`.

    val is_left : ('a, 'b) t -> bool

`is_left (Left v)` is `true`, `is_left (Right v)` is `false`.

    val is_right : ('a, 'b) t -> bool

`is_right (Left v)` is `false`, `is_right (Right v)` is `true`.

    val find_left : ('a, 'b) t -> 'a option

`find_left (Left v)` is `Some v`, `find_left (Right _)` is `None`

    val find_right : ('a, 'b) t -> 'b option

`find_right (Right v)` is `Some v`, `find_right (Left _)` is `None`

    val map_left : ('a1 -> 'a2) -> ('a1, 'b) t -> ('a2, 'b) t

`map_left f e` is `Left (f v)` if `e` is `Left v` and `e` if `e` is `Right _`.

    val map_right : ('b1 -> 'b2) -> ('a, 'b1) t -> ('a, 'b2) t

`map_right f e` is `Right (f v)` if `e` is `Right v` and `e` if `e` is `Left _`.

    val map : left:('a1 -> 'a2) ->
           right:('b1 -> 'b2) -> ('a1, 'b1) t -> ('a2, 'b2) t

`map ~left ~right (Left v)` is `Left (left v)`, `map ~left ~right (Right v)` is `Right (right v)`.

    val fold : left:('a -> 'c) -> right:('b -> 'c) -> ('a, 'b) t -> 'c

`fold ~left ~right (Left v)` is `left v`, and `fold ~left ~right (Right v)` is `right v`.

    val iter : left:('a -> unit) -> right:('b -> unit) -> ('a, 'b) t -> unit

`iter ~left ~right (Left v)` is `left v`, and `iter ~left ~right (Right v)` is `right v`.

    val for_all : left:('a -> bool) -> right:('b -> bool) -> ('a, 'b) t -> bool

`for_all ~left ~right (Left v)` is `left v`, and `for_all ~left ~right (Right v)` is `right v`.

    val equal : left:('a -> 'a -> bool) ->
           right:('b -> 'b -> bool) -> ('a, 'b) t -> ('a, 'b) t -> bool

`equal ~left ~right e0 e1` tests equality of `e0` and `e1` using `left` and `right` to respectively compare values wrapped by `Left _` and `Right _`.

    val compare : left:('a -> 'a -> int) ->
           right:('b -> 'b -> int) -> ('a, 'b) t -> ('a, 'b) t -> int

`compare ~left ~right e0 e1` totally orders `e0` and `e1` using `left` and `right` to respectively compare values wrapped by `Left _` and `Right _`. `Left _` values are smaller than `Right _` values.