Module Stdlib.Either

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.

type ('a, 'b) t =
  1. | Left of 'a
  2. | 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.