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 listIf 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.
A value of ('a, 'b) Either.t contains either a value of 'a or a value of 'b
val left : 'a -> ('a, 'b) tleft v is Left v.
val right : 'b -> ('a, 'b) tright v is Right v.
val is_left : ('a, 'b) t -> boolis_left (Left v) is true, is_left (Right v) is false.
val is_right : ('a, 'b) t -> boolis_right (Left v) is false, is_right (Right v) is true.
val find_left : ('a, 'b) t -> 'a optionfind_left (Left v) is Some v, find_left (Right _) is None
val find_right : ('a, 'b) t -> 'b optionfind_right (Right v) is Some v, find_right (Left _) is None
map_left f e is Left (f v) if e is Left v and e if e is Right _.
map_right f e is Right (f v) if e is Right v and e if e is Left _.
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 -> 'cfold ~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 -> unititer ~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 -> boolfor_all ~left ~right (Left v) is left v, and for_all ~left ~right (Right v) is right v.
equal ~left ~right e0 e1 tests equality of e0 and e1 using left and right to respectively compare values wrapped by Left _ and Right _.