Lwtreslib: an Lwt- and Result-friendly addition/replacement for the Stdlib

The OCaml's Stdlib modules are orthogonal: each define their own datatype and a set of functions operating on this datatype. Result for result, Option for option, List for list, etc. This orthogonality provides a high expressive power for a low lines-of-code count. E.g.,

let fold f init xs =
      (fun acc x -> Result.bind acc (fun acc -> f acc x))
      (Result.ok init)

However, in code-bases that make heavy uses of some datatypes, a little more integration is welcome. For example, in code bases that use the result type pervasively, the fold function above should be available in a module of list-traversing functions.

Lwtreslib is a library that supplement some of the OCaml's Stdlib modules with a tight integration of Lwt and Result. It focuses on data-structures that can be traversed (iterated, mapped, folded, what have you).

Design principles

  1. Exception-safety

    The functions exported by Lwtreslib do not raise exceptions. These functions may return option or result to indicate that some error happened during traversal, and they may propagate result.

    (For convenience, the module WithExceptions provides a few exception-raising functions which are convenient in specific contexts.)

  2. Coverage

    As much as it makes sense, for each function foo, Lwtreslib also provides

    • foo_e: a variant operating on result,
    • foo_s: a variant operating on Lwt promises, sequentially,
    • foo_es: a variant operating on Lwt promises of results, sequentially,
    • foo_p: a variant operating on Lwt promises, concurrently, and
    • foo_ep: a variant operating on Lwt promises of results, concurrently.

    As mentioned, this is only applied when it makes sense. E.g., some traversals cannot happen concurrently in which case the _p and _ep are not provided.

  3. Semantic consistency

    Exported functions and values have consistent names that reflect their consistent semantic. For example, all _e and _es functions have a fail-early semantic wherein the traversal is interrupted as soon as an Error occurs.


The Tezos_lwt_result_stdlib library exports a single top-level module Lwtreslib. This module exports Bare for simple traversal functions, and Traced for traversal with automatic composition of errors.

Reading guide

It is recommended to read the entirety of the documentation of the entry-point Lwtreslib module. It contains a high-level overview as well as some details about notable features of the library.