Tezos Protocol Implementation - Error Monad

type error_category = [
  1. | `Branch
    (*

    Errors that may not happen in another context

    *)
  2. | `Temporary
    (*

    Errors that may not happen in a later context

    *)
  3. | `Permanent
    (*

    Errors that will happen no matter the context

    *)
]

Categories of error

type error = TzCore.error = ..
include Sig.CORE with type error := error
val error_encoding : error Data_encoding.t
val pp : Format.formatter -> error -> unit
include Sig.EXT with type error := error
val register_error_kind : Sig.error_category -> id:string -> title:string -> description:string -> ?pp:(Format.formatter -> 'err -> unit) -> 'err Data_encoding.t -> (error -> 'err option) -> ('err -> error) -> unit

The error data type is extensible. Each module can register specialized error serializers id unique name of this error. Ex.: overflow_time_counter title more readable name. Ex.: Overflow of time counter description human readable description. Ex.: The time counter overflowed while computing delta increase pp formatter used to pretty print additional arguments. Ex.: The time counter overflowed while computing delta increase. Previous value %d. Delta: %d encoder decoder data encoding for this error. If the error has no value, specify Data_encoding.empty

val register_recursive_error_kind : Sig.error_category -> id:string -> title:string -> description:string -> pp:(Format.formatter -> 'err -> unit) -> (error Data_encoding.t -> 'err Data_encoding.t) -> (error -> 'err option) -> ('err -> error) -> unit

Same as register_error_kind but allow errors to wrap other errors.

The encoding argument is a function which will be given the encoding of errors as argument so that you can encode errors in errors using a fixpoint.

Another difference with register_error_kind is that pp is mandatory.

val classify_error : error -> Sig.error_category

Classify an error using the registered kinds

type error +=
  1. | Unclassified of string
    (*

    Catch all error when 'deserializing' an error.

    *)

Catch all error when 'serializing' an error.

type error +=
  1. | Unregistered_error of Data_encoding.json
val json_of_error : error -> Data_encoding.json

An error serializer

val error_of_json : Data_encoding.json -> error

Error documentation

type error_info = {
  1. category : Sig.error_category;
  2. id : string;
  3. title : string;
  4. description : string;
  5. schema : Data_encoding.json_schema;
}

Error information

val pp_info : Format.formatter -> error_info -> unit
val find_info_of_error : error -> error_info

find_info_of_error e retrieves the `error_info` associated with the given error `e`.

  • raises [Invalid_argument]

    if the error is a wrapped error from another monad

  • raises [Not_found]

    if the error's constructor has not been registered

val get_registered_errors : unit -> error_info list

Retrieves information of registered errors

include Sig.WITH_WRAPPED with type error := error
module type Wrapped_error_monad = sig ... end

The purpose of this module is to wrap a specific error monad E into a more general error monad Eg.

val register_wrapped_error_kind : (module Wrapped_error_monad) -> id:string -> title:string -> description:string -> unit

Same as register_error_kind but for a wrapped error monad. The codec is defined in the module parameter. It makes the category of the error Wrapped instead of Main.

module TzTrace : Sig.TRACE with type 'error trace = 'error list
type 'error trace = 'error TzTrace.trace
include Sig.MONAD with type 'error trace := 'error TzTrace.trace
val ok : 'a -> ('a, 'trace) result

Successful result

val ok_unit : (unit, 'trace) result
val ok_none : ('a option, 'trace) result
val ok_some : 'a -> ('a option, 'trace) result
val ok_nil : ('a list, 'trace) result
val ok_true : (bool, 'trace) result
val ok_false : (bool, 'trace) result
val return : 'a -> ('a, 'trace) result Lwt.t

Successful return

val return_unit : (unit, 'trace) result Lwt.t

Successful return of ()

val return_none : ('a option, 'trace) result Lwt.t

Successful return of None

val return_some : 'a -> ('a option, 'trace) result Lwt.t

return_some x is a successful return of Some x

val return_nil : ('a list, 'trace) result Lwt.t

Successful return of []

val return_true : (bool, 'trace) result Lwt.t

Successful return of true

val return_false : (bool, 'trace) result Lwt.t

Successful return of false

val error : 'err -> ('a, 'err TzTrace.trace) result

Erroneous result

val fail : 'err -> ('a, 'err TzTrace.trace) result Lwt.t

Erroneous return

Infix operators for monadic binds/maps. All operators follow this naming convention:

  • the first character is >
  • the second character is > for bind and | for map
  • the next character is = for Lwt or ? for Error
  • the next character (if present) is = for Lwt or ? for Error, it is only used for operator that are within both monads.
val (>>=) : 'a Lwt.t -> ('a -> 'b Lwt.t) -> 'b Lwt.t

Lwt's bind reexported. Following Lwt's convention, in this operator and the ones below, = indicate we operate within Lwt.

val (>|=) : 'a Lwt.t -> ('a -> 'b) -> 'b Lwt.t

Lwt's map reexported. The | indicates a map rather than a bind.

val (>>?) : ('a, 'trace) result -> ('a -> ('b, 'trace) result) -> ('b, 'trace) result

Non-Lwt bind operator. In this operator and the ones below, ? indicates that we operate within the error monad.

val (>|?) : ('a, 'trace) result -> ('a -> 'b) -> ('b, 'trace) result

Non-Lwt map operator.

val (>>=?) : ('a, 'trace) result Lwt.t -> ('a -> ('b, 'trace) result Lwt.t) -> ('b, 'trace) result Lwt.t

Combined bind operator. The =? indicates that the operator acts within the combined error-lwt monad.

val (>|=?) : ('a, 'trace) result Lwt.t -> ('a -> 'b) -> ('b, 'trace) result Lwt.t

Combined map operator.

val (>>?=) : ('a, 'trace) result -> ('a -> ('b, 'trace) result Lwt.t) -> ('b, 'trace) result Lwt.t

Injecting bind operator. This is for transitioning from the simple Error monad to the combined Error-Lwt monad.

Note the order of the character: it starts with the error monad marker ? and has the Lwt monad marker later. This hints at the role of the operator to transition into Lwt.

val (>|?=) : ('a, 'trace) result -> ('a -> 'b Lwt.t) -> ('b, 'trace) result Lwt.t

Injecting map operator.

val record_trace : 'err -> ('a, 'err TzTrace.trace) result -> ('a, 'err TzTrace.trace) result

Enrich an error report (or do nothing on a successful result) manually

val trace : 'err -> ('b, 'err TzTrace.trace) result Lwt.t -> ('b, 'err TzTrace.trace) result Lwt.t

Automatically enrich error reporting on stack rewind

val record_trace_eval : (unit -> ('err, 'err TzTrace.trace) result) -> ('a, 'err TzTrace.trace) result -> ('a, 'err TzTrace.trace) result

Same as record_trace, for unevaluated error

val trace_eval : (unit -> ('err, 'err TzTrace.trace) result Lwt.t) -> ('b, 'err TzTrace.trace) result Lwt.t -> ('b, 'err TzTrace.trace) result Lwt.t

Same as trace, for unevaluated Lwt error

val error_unless : bool -> 'err -> (unit, 'err TzTrace.trace) result

Error on failed assertion

val error_when : bool -> 'err -> (unit, 'err TzTrace.trace) result
val fail_unless : bool -> 'err -> (unit, 'err TzTrace.trace) result Lwt.t

Erroneous return on failed assertion

val fail_when : bool -> 'err -> (unit, 'err TzTrace.trace) result Lwt.t
val unless : bool -> (unit -> (unit, 'trace) result Lwt.t) -> (unit, 'trace) result Lwt.t
val when_ : bool -> (unit -> (unit, 'trace) result Lwt.t) -> (unit, 'trace) result Lwt.t
val dont_wait : (exn -> unit) -> ('trace -> unit) -> (unit -> (unit, 'trace) result Lwt.t) -> unit

Wrapper around Lwt_utils.dont_wait

val join_p : unit Lwt.t list -> unit Lwt.t

A few aliases for Lwt functions

val all_p : 'a Lwt.t list -> 'a list Lwt.t
val both_p : 'a Lwt.t -> 'b Lwt.t -> ('a * 'b) Lwt.t
val join_e : (unit, 'err TzTrace.trace) result list -> (unit, 'err TzTrace.trace) result

Similar functions in the error monad

val all_e : ('a, 'err TzTrace.trace) result list -> ('a list, 'err TzTrace.trace) result
val both_e : ('a, 'err TzTrace.trace) result -> ('b, 'err TzTrace.trace) result -> ('a * 'b, 'err TzTrace.trace) result
val join_ep : (unit, 'err TzTrace.trace) result Lwt.t list -> (unit, 'err TzTrace.trace) result Lwt.t

Similar functions in the combined monad

val all_ep : ('a, 'err TzTrace.trace) result Lwt.t list -> ('a list, 'err TzTrace.trace) result Lwt.t
val both_ep : ('a, 'err TzTrace.trace) result Lwt.t -> ('b, 'err TzTrace.trace) result Lwt.t -> ('a * 'b, 'err TzTrace.trace) result Lwt.t
include Sig.MONAD_EXT with type error := error and type 'error trace := 'error TzTrace.trace
type tztrace = error TzTrace.trace
type 'a tzresult = ('a, tztrace) result
val classify_errors : tztrace -> Sig.error_category
val pp_print_error : Format.formatter -> error TzTrace.trace -> unit
val pp_print_error_first : Format.formatter -> error TzTrace.trace -> unit

Pretty prints a trace as the message of its first error

val trace_encoding : error TzTrace.trace Data_encoding.t
val result_encoding : 'a Data_encoding.t -> 'a tzresult Data_encoding.t

A serializer for result of a given type

val generic_error : ('a, Format.formatter, unit, 'b tzresult) format4 -> 'a

Erroneous result (shortcut for generic errors)

val failwith : ('a, Format.formatter, unit, 'b tzresult Lwt.t) format4 -> 'a

Erroneous return (shortcut for generic errors)

val error_exn : exn -> 'a tzresult
val error_of_exn : exn -> error trace

error_of_exn e is a trace that carries the exception e. This function is intended to be used when interacting with a part of the code (most likely an external library) which uses exception. E.g.,

try
  Ok (make_some_call parameter)
with
  | (Not_found | Failure _) as e ->
    Error (error_of_exn e)
val record_trace_exn : exn -> 'a tzresult -> 'a tzresult
val trace_exn : exn -> 'b tzresult Lwt.t -> 'b tzresult Lwt.t
val generic_trace : ('a, Format.formatter, unit, ('b, error trace) result Lwt.t -> ('b, error trace) result Lwt.t) format4 -> 'a
val pp_exn : Format.formatter -> exn -> unit
val failure : ('a, Format.formatter, unit, error) format4 -> 'a
type error +=
  1. | Exn of exn

Wrapped OCaml/Lwt exception

type error +=
  1. | Canceled
val protect : ?on_error:(error trace -> 'a tzresult Lwt.t) -> ?canceler:Lwt_canceler.t -> (unit -> 'a tzresult Lwt.t) -> 'a tzresult Lwt.t

protect is a wrapper around Lwt.catch where the error handler operates over `trace` instead of `exn`. Besides, protect ~on_error ~canceler ~f may *cancel* f via a Lwt_canceler.t.

More precisely, protect ~on_error ~canceler f runs f (). An Lwt failure triggered by f () is wrapped into an Exn. If a canceler is given and Lwt_canceler.cancellation canceler is determined before f (), a Canceled error is returned.

Errors are caught by ~on_error (if given), otherwise the previous value is returned. An Lwt failure triggered by ~on_error is wrapped into an Exn

type error +=
  1. | Timeout
val with_timeout : ?canceler:Lwt_canceler.t -> unit Lwt.t -> (Lwt_canceler.t -> 'a tzresult Lwt.t) -> 'a tzresult Lwt.t