Module `Rresult.R`

Result value combinators.

Results

type ('a, 'b) t = ('a, 'b) Result.result: The type for results.

val ok : 'a -> ('a, 'b) Result.result: ok v is Ok v.

val error : 'b -> ('a, 'b) Result.result: error e is Error e.

val reword_error : ('b -> 'c) -> ('a, 'b) Result.result -> ('a, 'c) Result.result

reword_error reword r is:

r if r = Ok v
Error (reword e) if r = Error e

val get_ok : ('a, 'b) Result.result -> 'a

get r is v if r = Ok v and

raises Invalid_argument: otherwise.

val get_error : ('a, 'b) Result.result -> 'b

get_error r is e if r = Error e and

raises Invalid_argument: otherwise.

Composing results

val bind : ('a, 'b) Result.result -> ('a -> ('c, 'b) Result.result) -> ('c, 'b) Result.result: bind r f is f v if r = Ok v and r if r = Error _.

val map : ('a -> 'c) -> ('a, 'b) Result.result -> ('c, 'b) Result.result: map f r is bind (fun v -> ret (f v)) r.

val join : (('a, 'b) Result.result, 'b) Result.result -> ('a, 'b) Result.result: join r is v if r = Ok v and r otherwise.

val (>>=) : ('a, 'b) Result.result -> ('a -> ('c, 'b) Result.result) -> ('c, 'b) Result.result: r >>= f is bind r f.

val (>>|) : ('a, 'b) Result.result -> ('a -> 'c) -> ('c, 'b) Result.result: r >>| f is map r f.

module Infix : sig ... end: Infix operators.

Error messages

type msg = [ | `Msg of string ]: The type for (error) messages.

val msg : string -> [> msg ]: msg s is `Msg s.

val msgf : ('a, Stdlib.Format.formatter, unit, [> msg ]) Stdlib.format4 -> 'a: msgf fmt ... formats a message according to fmt.

val pp_msg : Stdlib.Format.formatter -> msg -> unit: pp_msg ppf m prints m on ppf.

val error_msg : string -> ('a, [> msg ]) Result.result: error_msg s is error (`Msg s).

val error_msgf : ('a, Stdlib.Format.formatter, unit, ('b, [> msg ]) Result.result) Stdlib.format4 -> 'a: error_msgf fmt ... is an error message formatted according to fmt.

val reword_error_msg : ?⁠replace:bool -> (string -> msg) -> ('a, msg) Result.result -> ('a, [> msg ]) Result.result: reword_error_msg ~replace reword r is like reword_error except if replace is false (default), the result of reword old_msg is concatened, on a new line to the old message.

val error_to_msg : pp_error:(Stdlib.Format.formatter -> 'b -> unit) -> ('a, 'b) Result.result -> ('a, [> msg ]) Result.result: error_to_msg pp_error r converts errors in r with pp_error to an error message.

val error_msg_to_invalid_arg : ('a, msg) Result.result -> 'a

err_msg_to_invalid_arg r is v if r = Ok v and

raises Invalid_argument: with the error message otherwise.

val open_error_msg : ('a, msg) Result.result -> ('a, [> msg ]) Result.result: open_error_msg r allows to combine a closed error message variant with other variants.

val failwith_error_msg : ('a, msg) Result.result -> 'a: failwith_error_msg r raises Failure m if r is Error (`Msg m).

Trapping unexpected exceptions

Getting rid of null was not enough.

type exn_trap = [ | `Exn_trap of exn * Stdlib.Printexc.raw_backtrace ]: The type for exception traps.

val pp_exn_trap : Stdlib.Format.formatter -> exn_trap -> unit: pp_exn_trap ppf bt prints bt on ppf.

val trap_exn : ('a -> 'b) -> 'a -> ('b, [> exn_trap ]) Result.result: trap_exn f v is f v and traps any exception that may occur as an exception trap error.

val error_exn_trap_to_msg : ('a, exn_trap) Result.result -> ('a, [> msg ]) Result.result: error_exn_trap_to_msg r converts exception trap errors in r to an error message.

val open_error_exn_trap : ('a, exn_trap) Result.result -> ('a, [> exn_trap ]) Result.result: open_error_exn_trap r allows to combine a closed exception trap error variant with other variants.

Pretty printing

val pp : ok:(Stdlib.Format.formatter -> 'a -> unit) -> error:(Stdlib.Format.formatter -> 'b -> unit) -> Stdlib.Format.formatter -> ('a, 'b) Result.result -> unit: pp ok error ppf r prints r on ppf using ok and error according to r.

val dump : ok:(Stdlib.Format.formatter -> 'a -> unit) -> error:(Stdlib.Format.formatter -> 'b -> unit) -> Stdlib.Format.formatter -> ('a, 'b) Result.result -> unit: dump ~ok ~error formats an OCaml result value using ok or error according to case, no parentheses are added.

Predicates and comparison

val is_ok : ('a, 'b) Result.result -> bool: is_ok r is true iff r = Ok _.

val is_error : ('a, 'b) Result.result -> bool: is_error r is true iff r = Error _.

val equal : ok:('a -> 'a -> bool) -> error:('b -> 'b -> bool) -> ('a, 'b) Result.result -> ('a, 'b) Result.result -> bool: equal ~ok ~error r r' tests r and r' for equality using ok and error.

val compare : ok:('a -> 'a -> int) -> error:('b -> 'b -> int) -> ('a, 'b) Result.result -> ('a, 'b) Result.result -> int: compare ~ok ~error r r' totally orders r and r' using ok and error.

Converting

val to_option : ('a, 'b) Result.result -> 'a option: to_option r is Some v if r = Ok v and None otherwise.

val of_option : none:(unit -> ('a, 'b) Result.result) -> 'a option -> ('a, 'b) Result.result: of_option ~none r is Ok v if r = Some v and none () otherwise.

val to_presult : ('a, 'b) Result.result -> [> `Ok of 'a | `Error of 'b ]: to_presult r is r as a polymorphic variant result value.

val of_presult : [< `Ok of 'a | `Error of 'b ] -> ('a, 'b) Result.result: of_presult pr is pr as a result value.

Ignoring errors

Warning. Using these functions is, most of the time, a bad idea.

val ignore_error : use:('b -> 'a) -> ('a, 'b) Result.result -> 'a: ignore_error ~use r is v if r = Ok v and use e if r = Error e.

val kignore_error : use:('b -> ('a, 'c) Result.result) -> ('a, 'b) Result.result -> ('a, 'c) Result.result: kignore_error ~use r is r if r = Ok v and use e if r = Error e.