Part 3: Advanced topics

This is Part 3 of 4 of the The Error Monad tutorial.

Working with standard data-structures: Lwtreslib

Handling values within the Lwt, result, and Lwt-result monads is so common in Mavkit that you also have access to an extension of the Stdlib dedicated to these monads: the Lwtreslib library.

The mavryk-lwt-result-stdlib package exports an Lwtreslib module which is made available, through mavryk-error-monad and mavryk-base, to the whole of the codebase. Specifically, within the codebase of Mavkit the following modules of OCaml’s Stdlib are shadowed by Lwtreslib’s:

• List,

• Result,

• Option,

• Seq,

• Map,

• Set, and

• Hashtbl.

In all those modules, the underlying data structures are compatible with those of the Stdlib and thus with the rest of the OCaml ecosystem. However, the primitives in these modules are extended to support Lwt, result and the combination of the two. Specifically, for each function that traverses the data structure, the module also contains variants that perform the same traversal within each of the monad. E.g., for List.map

(* vanilla map *)
val map : ('a -> 'b) -> 'a list -> 'b list

(* [result]-aware map: stops at the first error *)
val map_e : ('a -> ('b, 'trace) result) -> 'a list -> ('b list, 'trace) result

(* sequential Lwt map: treats each element after the previous one *)
val map_s : ('a -> 'b Lwt.t) -> 'a list -> 'b list Lwt.t

(* sequential Lwt-[result] map:
   - treats each element after the previous one
   - stops at the first error *)
val map_es :
  ('a -> ('b, 'trace) result Lwt.t) ->
  'a list ->
  ('b list, 'trace) result Lwt.t

(* concurrent Lwt map: treats all the elements concurrently *)
val map_p : ('a -> 'b Lwt.t) -> 'a list -> 'b list Lwt.t

(* concurrent Lwt-[result] map:
   - treats all the elements concurrently
   - treats the whole list no matter the success/errors *)
val map_ep :
  ('a -> ('b, 'trace) result Lwt.t) ->
  'a list ->
  ('b list, 'trace list) result Lwt.t

Check out the online documentation of Lwtreslib for a description of the semantic and naming convention.

In addition to shadowing existing modules, Lwtreslib also exports new modules:

• Seq_e for result-aware variant of Seq,

• Seq_s for Lwt-aware variant of Seq,

• Seq_es for Lwt-result-aware variant of Seq, and

• WithExceptions for unsafe accesses such as Option.get.

Whenever you need to traverse a standard data structure with some result or Lwt or Lwt-result function, Lwtreslib should have that function ready for you. You should never fold over a data structure with a promise or result accumulator. E.g., you should do

List.rev_map_es fetch keys

and you shouldn’t do

let open Lwt_result_syntax in
List.fold_left
  (fun resources key ->
    let* resources = resources in
    let* resource = fetch key in
    return (resource :: resources))
  (return [])
  keys

If you do not find the traversal function you need, do not hesitate to contribute to Lwtreslib.

Working with traces and errors and such directly

Occasionally, you may need more interaction with traces than the primitives presented thus far.

For error reporting or debugging purpose, you may need to show traces to users. You can do so with the following values.

• pp_print_trace: a %a-compatible formatter. Note that the trace formatting is unspecified and subject to change. Also be aware that it generally prints the trace over multiple lines.

• pp_print_top_error_of_trace: a %a-compatible formatter that only shows the most recent error in the trace (or one of the most recent errors if there are several). This is useful to get shorter error messages. Most often used for the declaration of logging events in Internal_event.Simple.

• trace_encoding: an encoding for traces. Useful to combine into encoding of data structures that contain traces. Most often used for the declaration of logging events in Internal_event.Simple.

If you are working with non-standard data structures and if you need to define monad-aware traversors for these data structures, you may need to build some traces by hand. You can do so with the following values.

• TzTrace.make : 'e -> 'e trace useful to convert an error into an error   trace. By extension, this is useful to convert an ('a, error) result into an 'a tzresult.

• TzTrace.cons : 'e -> 'e trace -> 'e trace is the low-level combinators that builds-up traces. In most cases, you’ll want to use trace or record_trace instead, but you might need it when you are defining a low-level traversal function for some data structure.

  let iter_with_bounded_errors bound f xs =
    (* we rely on syntax for Lwt, we handle results by hand *)
    let open Lwt_syntax in
    let rec aux_all_ok = function
      | [] -> return_ok ()
      | x :: xs ->
        let* r = f x in
        match r with
        | Ok () -> aux_all_ok xs
        | Error e -> aux_some_error 1 (TzTrace.make e) xs
    and aux_some_error num_errors trace xs =
      if num_errors > bound then
        return_error (TzTrace.cons (Exceeded_error_limit bound) trace)
      else
        match xs with
        | [] -> return_ok ()
        | x :: xs ->
          let* r = f x in
          match r with
          | Ok () -> aux_some_error num_errors trace xs
          | Error e -> aux_some_error (num_errors + 1) (TzTrace.cons e trace) xs
    in
    aux_all_ok xs
  

• TzTrace.conp : 'e trace -> 'e trace -> 'e trace is the parallel composition of two traces. Unlike cons, the traces composed by conp are not organised hierarchically. The errors are presented as having happened side-by-side.

  Note that currently there is little difference between cons and conp traces. But the difference will be more marked in the future.

  You should use conp (rather than cons) when you are gathering errors and traces from two or more concurrent processes.

Working within the protocol

If you are working on the protocol, things are slightly different for you. This is because the protocol has a restricted access to external resources and libraries. You can find more details in the dedicated documentation. This section focuses on the error-monad within the protocol.

The protocol environment libraries evolve at a slightly different pace than the underlying library. You need to check the mli files within src/lib_protocol_environment/sigs/.

Note that unlike in the shell, the traces in the protocol are already abstract. As a result there is no matching of traces (and thus errors) within the protocol: you can match Ok and Error, but not the payload of the Error. This part of the legacy code has already been removed.

The main difference between the protocol and the shell is that the category parameter of the register_error_kind function is meaningful. You must pass a category which is appropriate for the error you are registering:

• Branch: is for branch-specific failures, i.e., failures that happen in the current branch (of the chain) but maybe wouldn’t happen in a different branch. E.g., a reference to an unknown block is invalid, but it might become valid once the head block has changed. This category is then used by the shell to retry after the branch changes.

• Temporary: is for transient failures, i.e., failures that happen but may not always happen. This category is used by the shell to retry at some later time.

• Permanent: is for irremediable failures, i.e., failures that happen and will always happen whatever the context. E.g., originating a contract that does not type-check is a permanent error. This is used by the shell to mark the data as invalid.

• Outdated: is for failures that happen when some data is too old.

Another thing to consider is that errors from the protocol can reach the shell. However, because the error type of the protocol is distinct from that of the shell, the protocol errors are wrapped inside a shell error constructor.

This has no impact within the protocol (where shell errors don’t exist) nor within the shell (where protocol errors are automatically wrapped inside a shell error). However, it can have an impact in the spaces in between. Most typically, this matters in the unit-tests of the protocol (src/proto_alpha/lib_protocol/test/unit/) where you call some protocol functions directly. In this case, you need to wrap the errors yourself, using the wrapping functions provided by the environment: Environment.wrap_tzresult, Environment.wrap_tztrace, and Environment.wrap_tzerror.

Working below the error-monad

If you are working on some low-level libraries (e.g., src/lib_stdlib) or the external dependencies (e.g., data-encoding) you don’t have access to the error monad at all.

In this case, you can still use the result type but you need to define your own let* binding operator: let ( let* ) = Result.bind.

You can also use Lwt which provides its own Lwt.Syntax module.

Finally, the Lwt_result module (provided as part of Lwt) can help you deal with result-Lwt combinations, including via its Lwt_result.Syntax module.

Working with external libraries

This tutorial covers error-management techniques in Mavkit. However, from within Mavkit, you may need to call external libraries for cryptography or RPCs or data-encoding or what have you.

The first thing you do is to carefully read the documentation of the external library you are using. You should check the overview documentation with a look out for comments on error management.

Then, you also need to read the documentation of each function that you are calling. This documentation may explain how errors are handled: does the function return a result? does it raise and exception? is it unspecified?

If the function you are calling may raise exceptions, you should catch these exceptions. You can either do so at the level of the call itself or, if you are calling multiple functions that can all raise similar exceptions, around a whole block of calls.

When you catch an exception, the most common thing to do is to translate it or wrap it into a result or a tzresult.

try
  let v1 = Data_encoding.Json.destruct e1 j1 in
  let v2 = Data_encoding.Json.destruct e2 j2 in
  Ok (v1, v2)
with
  | exc -> Error (Cannot_destruct_json_value exc)

Note that if you are calling an Lwt function, you have to use Lwt.catch or Lwt.try_bind rather than try-with.

Lwt.catch
  (fun () ->
    let open Lwt_syntax in
    let* () = Lwt_unix.mkdir d1 perm in
    let* () = Lwt_unix.mkdir d2 perm in
    Lwt_result_syntax.return_unit)
  (function
    | exc -> Lwt_result_syntax.fail (Cannot_destruct_json_value exc))

The error monad provides several helpers functions for catching exceptions .

val catch : ?catch_only:(exn -> bool) -> (unit -> 'a) -> 'a tzresult

If the function you are calling may raise exceptions only under well-defined conditions on the parameters, then you can also check those conditions yourself and ignore the exceptions. When doing so, please add a comment to explain it.

let get_or_defaults low_default high_default array offset =
  if offset < 0 then
    low_default
  else if offset >= Array.length array then
    high_default
  else
    (* This cannot raise because of checks on offset above *)
    Array.get array offset

If the function may fail with result, you can map the error directly or simply continue with it. If it may fail with option, you can translate None into an appropriate error.

match find k kvs with
| None -> Error "cannot find key"
| Some v -> Ok v

If the function’s documentation specifies some pre-conditions but doesn’t explain what happens if those aren’t met, then you must check those pre-conditions.