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 t('a, 'b) =
  1. | Left('a)
  2. | Right('b)
;

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

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

left v is Left v.

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

right v is Right v.

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

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

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

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

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

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

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

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

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 _.

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 _.

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).

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.

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.

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.

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 _.

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.