Result is often used to handle error messages.
type ('ok, 'err) t = 
|Ok of 'ok
|Error of 'err
'a is a function's expected return type, and 'b is often an error message string.
let ric_of_ticker = function
    | "IBM" -> Ok "IBM.N"
    | "MSFT" -> Ok "MSFT.OQ"
    | "AA" -> Ok "AA.N"
    | "CSCO" -> Ok "CSCO.OQ"
    | _ as ticker -> Error (sprintf "can't find ric of %s" ticker)
The return type of ric_of_ticker could be string option, but (string, string) Result.t gives more control over the error message.
include Monad.S2 with type t := ('a, 'err) t
val fail : 'err -> ('a, 'err) t
val failf : ('a, unit, string, ('b, string) t) format4 -> 'a
e.g. failf "Couldn't find bloogle %s" (Bloogle.to_string b)
val is_ok : ('a, 'b) t -> bool
val is_error : ('a, 'b) t -> bool
val ok : ('ok, 'a) t -> 'ok option
val error : ('a, 'err) t -> 'err option
val of_option : 'ok option -> error:'err -> ('ok, 'err) t
val iter : ('ok, 'a) t -> f:('ok -> unit) -> unit
val iter_error : ('a, 'err) t -> f:('err -> unit) -> unit
val map : ('ok, 'err) t -> f:('ok -> 'c) -> ('c, 'err) t
val map_error : ('ok, 'err) t -> f:('err -> 'c) -> ('ok, 'c) t
val combine : ('ok1, 'err) t ->
('ok2, 'err) t ->
ok:('ok1 -> 'ok2 -> 'ok3) -> err:('err -> 'err -> 'err) -> ('ok3, 'err) t
val call : f:('a -> unit, 'b) t -> 'a -> unit
val apply : f:('a -> 'b, 'err) t -> 'a -> ('b, 'err) t
val ok_fst : ('ok, 'err) t -> [ `Fst of 'ok | `Snd of 'err ]
ok_fst is useful with List.partition_map. Continuing the above example:
    let rics, errors = List.partition_map ~f:Result.ok_fst
      (List.map ~f:ric_of_ticker ["AA"; "F"; "CSCO"; "AAPL"])
val ok_if_true : bool -> error:'err -> (unit, 'err) t
val try_with : (unit -> 'a) -> ('a, exn) t
val ok_exn : ('ok, exn) t -> 'ok
ok_exn t returns x if t = Ok x, and raises exn if t = Error exn
val ok_or_failwith : ('ok, string) t -> 'ok
val ok_unit : (unit, 'a) t
ok_unit = Ok (), used to avoid allocation as a performance hack
module Export : sig .. end
type ('ok, 'err) _result = ('ok, 'err) t = 
|Ok of 'ok
|Error of 'err
val is_ok : ('a, 'b) t -> bool
val is_error : ('a, 'b) t -> bool
module Stable : sig .. end
module V1 : sig .. end
include sig .. end with type __pa_nonrec_0 := ('a0, 'a1) t
type ('ok, 'err) __pa_nonrec_0 = ('ok, 'err) t = 
|Ok of 'ok
|Error of 'err
type ('ok, 'err) t = ('ok, 'err) __pa_nonrec_0 = 
|Ok of 'ok
|Error of 'err
val compare : ('ok -> 'ok -> int) ->
('err -> 'err -> int) -> ('ok, 'err) t -> ('ok, 'err) t -> int
val bin_t : 'ok Bin_prot.Type_class.t ->
'err Bin_prot.Type_class.t -> ('ok, 'err) t Bin_prot.Type_class.t
val bin_read_t : 'ok Bin_prot.Read.reader ->
'err Bin_prot.Read.reader -> ('ok, 'err) t Bin_prot.Read.reader
val __bin_read_t__ : 'ok Bin_prot.Read.reader ->
'err Bin_prot.Read.reader -> (int -> ('ok, 'err) t) Bin_prot.Read.reader
val bin_reader_t : 'ok Bin_prot.Type_class.reader ->
'err Bin_prot.Type_class.reader -> ('ok, 'err) t Bin_prot.Type_class.reader
val bin_size_t : 'ok Bin_prot.Size.sizer ->
'err Bin_prot.Size.sizer -> ('ok, 'err) t Bin_prot.Size.sizer
val bin_write_t : 'ok Bin_prot.Write.writer ->
'err Bin_prot.Write.writer -> ('ok, 'err) t Bin_prot.Write.writer
val bin_writer_t : 'ok Bin_prot.Type_class.writer ->
'err Bin_prot.Type_class.writer -> ('ok, 'err) t Bin_prot.Type_class.writer
val t_of_sexp : (Sexplib.Sexp.t -> 'ok) ->
(Sexplib.Sexp.t -> 'err) -> Sexplib.Sexp.t -> ('ok, 'err) t
val sexp_of_t : ('ok -> Sexplib.Sexp.t) ->
('err -> Sexplib.Sexp.t) -> ('ok, 'err) t -> Sexplib.Sexp.t
val compare : ('ok -> 'ok -> int) ->
('err -> 'err -> int) -> ('ok, 'err) t -> ('ok, 'err) t -> int
val t_of_sexp : (Sexplib.Sexp.t -> 'ok) ->
(Sexplib.Sexp.t -> 'err) -> Sexplib.Sexp.t -> ('ok, 'err) t
val sexp_of_t : ('ok -> Sexplib.Sexp.t) ->
('err -> Sexplib.Sexp.t) -> ('ok, 'err) t -> Sexplib.Sexp.t
val bin_t : 'ok Bin_prot.Type_class.t ->
'err Bin_prot.Type_class.t -> ('ok, 'err) t Bin_prot.Type_class.t
val bin_read_t : 'ok Bin_prot.Read.reader ->
'err Bin_prot.Read.reader -> ('ok, 'err) t Bin_prot.Read.reader
val __bin_read_t__ : 'ok Bin_prot.Read.reader ->
'err Bin_prot.Read.reader -> (int -> ('ok, 'err) t) Bin_prot.Read.reader
val bin_reader_t : 'ok Bin_prot.Type_class.reader ->
'err Bin_prot.Type_class.reader -> ('ok, 'err) t Bin_prot.Type_class.reader
val bin_size_t : 'ok Bin_prot.Size.sizer ->
'err Bin_prot.Size.sizer -> ('ok, 'err) t Bin_prot.Size.sizer
val bin_write_t : 'ok Bin_prot.Write.writer ->
'err Bin_prot.Write.writer -> ('ok, 'err) t Bin_prot.Write.writer
val bin_writer_t : 'ok Bin_prot.Type_class.writer ->
'err Bin_prot.Type_class.writer -> ('ok, 'err) t Bin_prot.Type_class.writer