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 Octez that you also have access to an extension of the
Stdlib dedicated to these monads: the
tezos-lwt-result-stdlib package exports an
which is made available, through
tezos-base, to the whole of the codebase. Specifically, within the
codebase of Octez the following modules of OCaml’s
shadowed by Lwtreslib’s:
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.,
(* 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
result-aware variant of
Seq_sfor Lwt-aware variant of
result-aware variant of
WithExceptionsfor unsafe accesses such as
Whenever you need to traverse a standard data structure with some
result or Lwt or Lwt-
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
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.
%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.
%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
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
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 traceuseful to convert an
error trace. By extension, this is useful to convert an
('a, error) resultinto an
TzTrace.cons : 'e -> 'e trace -> 'e traceis the low-level combinators that builds-up traces. In most cases, you’ll want to use
record_traceinstead, 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 traceis the parallel composition of two traces. Unlike
cons, the traces composed by
conpare 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
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
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
Error, but not the
payload of the
Error. This part of the legacy code has already been
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
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:
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 Octez. However, from within Octez, 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
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
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.try_bind rather than
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
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.