Module Stdlib.Either

Either type.

Either is the simplest and most generic sum/variant type: a value of ('a, 'b) Either.tEither.t('a, 'b) 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 =

type t('a, 'b) =

| Left of 'a

| Left('a)

| Right of 'b

| Right('b)

;

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

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

let left: 'a => t('a, 'b);

left v is Left v.

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

let right: 'b => t('a, 'b);

right v is Right v.

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

let is_left: t('a, 'b) => bool;

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

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

let is_right: t('a, 'b) => bool;

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

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

let get_left: t('a, 'b) => 'a;

get_left e is v if e is Left v and raise otherwise.

raises Invalid_argument if e is Right _. since 5.4

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

let get_right: t('a, 'b) => 'b;

get_right e is v if e is Right v and raise otherwise.

raises Invalid_argument if e is Left _. since 5.4

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

let find_left: t('a, 'b) => option('a);

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

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

let find_right: t('a, 'b) => option('b);

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

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

let map_left: ('a1 => 'a2) => t('a1, 'b) => t('a2, 'b);

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

let map_right: ('b1 => 'b2) => t('a, 'b1) => t('a, 'b2);

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

let map: left:('a1 => 'a2) => right:('b1 => 'b2) => t('a1, 'b1) => t('a2, 'b2);

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

let fold: left:('a => 'c) => right:('b => 'c) => t('a, 'b) => 'c;

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

val retract : ('a, 'a) t -> 'a

let retract: t('a, 'a) => 'a;

retract (Left v) is v, and retract (Right v) is v.

since 5.4

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

let iter: left:('a => unit) => right:('b => unit) => t('a, 'b) => 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

let for_all: left:('a => bool) => right:('b => bool) => t('a, 'b) => 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

let equal: 
  left:('a => 'a => bool) =>
  right:('b => 'b => bool) =>
  t('a, 'b) =>
  t('a, 'b) =>
  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

let compare: 
  left:('a => 'a => int) =>
  right:('b => 'b => int) =>
  t('a, 'b) =>
  t('a, 'b) =>
  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.