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Here is the latest OCaml Weekly News, for the week of September 24 to October 01, 2019.

Table of Contents

How does one print any type?

Continuing this thread, Rwmjones announced

One way to do this is to iterate over the runtime structure. You can get quite good information out of this, although it's not a complete view of the compile time types by any means. I wrote a utility to do this called Dumper: https://web.archive.org/web/20101101234304/http://merjis.com/developers/dumper

It's pure OCaml and doesn't require any libraries, weird syntax, modifications to your source code or anything like that.

Category theory for Programmers book - OCaml flavor

Continuing this thread, Anton Kochkov said

Update : work-in-progress pull request - #201 to the main repository - completely translated Parts 1 and 2. Only one, last, part remained.

Today's trick : memory limits with Gc alarms

Guillaume Munch-Maccagnoni announced

Hello, today's little known OCaml feature we will learn about are memory limits using Gc alarms.

Imagine you want to perform a memory-heavy computation, whose size of data set is not known in advance. (This post is inspired by the implementation of the SAT/SMT solver mSAT.) You would like this computation to fail gracefully if memory becomes insufficient. How can you do it?

The Out_of_memory exception

The first thing you might think about is the Out_of_memory exception. You simply run the computation inside an exception handler, expecting to receive this exception if memory ever becomes exhausted. However, for several reasons this is not reliable. First, the program is not always notified of failing allocations and gets killed instead much after, when it tries to access the memory for the first time (this behaviour of the operating system is called overcommitting). But this is the least problem, because you quickly figure out how to get rid of overcommitting, for instance by setting a hard memory limit to your process using ulimit.

However, you then encounter the second issue: in OCaml, if a minor allocation fails, then the program will issue a fatal error instead of raising Out_of_memory. That's it: Out_of_memory is only reliable for allocations that are guaranteed to go directly on the major heap (by default those bigger than 256 words, or those performed for bigarrays or unmarshalling). Not for repeated small allocations on the minor heap, which would represent most allocations in a functional program. (There are bug reports about this, but making Out_of_memory reliable is reported to be non-trivial.)

GC alarms

GC alarms are callbacks which run at the end of major GC cycles. They are registered with Gc.create_alarm. Now observe this cool trick:

let run_with_memory_limit limit f =
  let limit_memory () =
    let mem = Gc.(quick_stat ()).heap_words in
    if mem > limit / (Sys.word_size / 8) then raise Out_of_memory
  let alarm = Gc.create_alarm limit_memory in
  Fun.protect f ~finally:(fun () -> Gc.delete_alarm alarm ; Gc.compact ())

That's it, we simply raise Out_of_memory from the Gc alarm. You can try it in the toplevel on a memory-hungry function:

let f () =
  let x = ref [] in
  let rec f () = x := ()::!x ; f () in
  f ();;
run_with_memory_limit 1000000000 f;;


This is not reliable with multi-threaded programs, because we do not know in which thread the exception must be raised. This is fine for programs like mSAT and many logic/language tools, because such tools are rarely limited by blocking IO, so they do not lose much by remaining single-threaded in the absence of true parallelism. (Haskell proposes per-thread allocation limits as a built-in feature, which manifests itself likewise as an asynchronous exception.)


Behind each Gc alarm, there is a finalised block. The finaliser calls the callback, and sets itself to run again at the next cycle. Thus, the memory limit (thus implemented) is an example of asynchronous exception usefully arising from a finaliser. Was such a use of Gc alarms expected when they were conceived? History did not tell.

Guillaume Bury then said

As the author of mSAT, I'd just like to say the trick of using gc alarms to implement a memory limit in mSAT was actually taken from the theorem prover zenon, where if I remember correctly it was written by @damiendoligez , which might explain the subtle use of alarms/finalisers ^^

Also, as a side note, for people who would like to implement a similar limit but for computation time, I'd advise against using alarms (as is done in the code of zenon iirc) as they have proven to be somewhat not reliable, and I've found using Unix timers to raise an Out_of_time exception much better:

let run_with_time_limit limit f =
  (* create a Unix timer timer *)
  let _ = Unix.setitimer Unix.ITIMER_REAL Unix.{it_value = limit; it_interval = 0.01 } in
  (* The Unix.timer works by sending a Sys.sigalrm, so in order to use it,
     we catch it and raise the Out_of_time exception. *)
  let () =
    Sys.set_signal Sys.sigalrm (
      Sys.Signal_handle (fun _ ->
          raise Out_of_time)
      ) in
  Fun.protect f ~finally:(fun () ->
    Unix.setitimer Unix.ITIMER_REAL Unix.{it_value = 0.; it_interval = 0. }

Arch Linux installer written in OCaml

Darren announced

Static binary builds are now available here (renamed repo as well).

They are built via Travis CI using the ocaml/opam2:alpine docker image, so the binary should be fully static and run fine on the live CD.

Added other stuff

  • Added disk layout for system partition + USB key
  • Code for installing hardened kernel

Note that it's not well tested at all, so don't use it for anything too serious yet.

First release of mrmime, parser and generator of emails

Calascibetta Romain announced

We're glad to announce the first release of mrmime, a parser and a generator of emails. This library provides an OCaml way to analyze and craft an email. The eventual goal is to build an entire unikernel-compatible stack for email (such as SMTP or IMAP).

In this article, we will show what is currently possible with mrmime and present a few of the useful libraries that we developed along the way.

An email parser

Some years ago, Romain gave a talk about what an email really is. Behind the human-comprehensible format (or rich-document as we said a long time ago), there are several details of emails which complicate the process of analyzing them (and can be prone to security lapses). These details are mostly described by three RFCs:

Even though they are cross-compatible, providing full legacy email parsing is an archaeological exercise: each RFC retains support for the older design decisions (which were not recognized as bad or ugly in 1970 when they were first standardized).

The latest email-related RFC (RFC5322) tried to fix the issue and provide a better formal specification of the email format – but of course, it comes with plenty of obsolete rules which need to be implemented. In the standard, you find both the current grammar rule and its obsolete equivalent.

  • An extended email parser

    Even if the email format can defined by "only" 3 RFCs, you will miss email internationalization (RFC6532), the MIME format (RFC2045, RFC2046, RFC2047, RFC2049), or certain details needed to be interoperable with SMTP (RFC5321). There are still more RFCs which add extra features to the email format such as S/MIME or the Content-Disposition field.

    Given this complexity, we took the most general RFCs and tried to provide an easy way to deal with them. The main difficulty is the multipart parser, which deals with email attachments (anyone who has tried to make an HTTP 1.1 parser knows about this).

  • A realistic email parser

    Respecting the rules described by RFCs is not enough to be able to analyze any email from the real world: existing email generators can, and do, produce non-compliant email. We stress-tested mrmime by feeding it a batch of 2 billion emails taken from the wild, to see if it could parse everything (even if it does not produce the expected result). Whenever we noticed a recurring formatting mistake, we updated the details of the ABNF to enable mrmime to parse it anyway.

  • A parser usable by others

    One demonstration of the usability of mrmime is ocaml-dkim, which wants to extract a specific field from your mail and then verify that the hash and signature are as expected.

    ocaml-dkim is used by the latest implementation of ocaml-dns to request public keys in order to verify email.

    The most important question about ocaml-dkim is: is it able to verify your email in one pass? Indeed, currently some implementations of DKIM need 2 passes to verify your email (one to extract the DKIM signature, the other to digest some fields and bodies). We focused on verifying in a single pass in order to provide a unikernel SMTP relay with no need to store your email between verification passes.

An email generator

OCaml is a good language for making little DSLs for specialized use-cases. In this case, we took advantage of OCaml to allow the user to easily craft an email from nothing.

The idea is to build an OCaml value describing the desired email header, and then let the Mr. MIME generator transform this into a stream of characters that can be consumed by, for example, an SMTP implementation. The description step is quite simple:

#require "mrmime" ;;
#require "ptime.clock.os" ;;

open Mrmime

let romain_calascibetta =
  let open Mailbox in
  Local.[ w "romain"; w "calascibetta" ] @ Domain.(domain, [ a "gmail"; a "com" ])

let john_doe =
  let open Mailbox in
  Local.[ w "john" ] @ Domain.(domain, [ a "doe"; a "org" ])
  |> with_name Phrase.(v [ w "John"; w "D." ])

let now () =
  let open Date in
  of_ptime ~zone:Zone.GMT (Ptime_clock.now ())

let subject =
  Unstructured.[ v "A"; sp 1; v "Simple"; sp 1; v "Mail" ]

let header =
  let open Header in
  Field.(Subject $ subject)
  & Field.(Sender $ romain_calascibetta)
  & Field.(To $ Address.[ mailbox john_doe ])
  & Field.(Date $ now ())
  & empty

let stream = Header.to_stream header

let () =
  let rec go () =
    match stream () with
    | Some buf -> print_string buf; go ()
    | None -> ()
  go ()

This code produces the following header:

Date: 2 Aug 2019 14:10:10 GMT
To: John "D." <john@doe.org>
Sender: romain.calascibetta@gmail.com
Subject: A Simple Mail
  • 78-character rule

    One aspect about email and SMTP is about some historical rules of how to generate them. One of them is about the limitation of bytes per line. Indeed, a generator of mail should emit at most 80 bytes per line - and, of course, it should emits entirely the email line per line.

    So mrmime has his own encoder which tries to wrap your mail into this limit. It was mostly inspired by Faraday and Format powered with GADT to easily describe how to encode/generate parts of an email.

  • A multipart email generator

    Of course, the main point about email is to be able to generate a multipart email - just to be able to send file attachments. And, of course, a deep work was done about that to make parts, compose them into specific Content-Type fields and merge them into one email.

    Eventually, you can easily make a stream from it, which respects rules (78 bytes per line, stream line per line) and use it directly into an SMTP implementation.

    This is what we did with the project facteur. It's a little command-line tool to send with file attachement mails in pure OCaml - but it works only on an UNIX operating system for instance.

Behind the forest

Even if you are able to parse and generate an email, more work is needed to get the expected results.

Indeed, email is a exchange unit between people and the biggest deal on that is to find a common way to ensure a understable communication each others. About that, encoding is probably the most important piece and when a French person wants to communicate with a latin1 encoding, an American person can still use ASCII.

  • Rosetta

    So about this problem, the choice was made to unify any contents to UTF-8 as the most general encoding of the world. So, we did some libraries which map an encoding flow to Unicode code-point, and we use uutf (thanks to dbuenzli) to normalize it to UTF-8.

    The main goal is to avoid a headache to the user about that and even if contents of the mail is encoded with latin1 we ensure to translate it correctly (and according RFCs) to UTF-8.

    This project is rosetta and it comes with:

    • uuuu for ISO-8859 encoding
    • coin for KOI8-{R,U} encoding
    • yuscii for UTF-7 encoding
  • Pecu and Base64

    Then, bodies can be encoded in some ways, 2 precisely (if we took the main standard):

    • A base64 encoding, used to store your file
    • A quoted-printable encoding

    So, about the base64 package, it comes with a sub-package base64.rfc2045 which respects the special case to encode a body according RFC2045 and SMTP limitation.

    Then, pecu was made to encode and decode quoted-printable contents. It was tested and fuzzed of course like any others MirageOS's libraries.

    These libraries are needed for an other historical reason which is: bytes used to store mail should use only 7 bits instead of 8 bits. This is the purpose of the base64 and the quoted-printable encoding which uses only 127 possibilities of a byte. Again, this limitation comes with SMTP protocol.


mrmime is tackling the difficult task to parse and generate emails according to 50 years of usability, several RFCs and legacy rules. So, it still is an experimental project. We reach the first version of it because we are currently able to parse many mails and then generate them correctly.

Of course, a bug (a malformed mail, a server which does not respect standards or a bad use of our API) can appear easily where we did not test everything. But we have the feeling it it was the time to release it and let people to use it.

The best feedback about mrmime and the best improvement is yours. So don't be afraid to use it and start to hack your emails with it.

cross-post to Tarides's blog

First release of data-encoding, JSON and binary serialisation

Raphaël Proust announced

On behalf of Nomadic Labs, I'm pleased to announce the first public release of data-encoding: a library to encode and decode values to JSON or binary format. data-encoding provides fine grained control over the representation of the data, documentation generation, and detailed encoding/decoding errors.

In the Tezos project, we use data-encoding for binary serialisation and deserialisation of data transported via the P2P layer and for JSON serialisation and deserialisation of configuration values stored on disk.

The library is available through opam (opam install data-encoding), hosted on Gitlab (https://gitlab.com/nomadic-labs/data-encoding), and distributed under MIT license.

This release was only possible following an effort to refactor our internal tools and libraries. Most of the credit for this effort goes to Pietro Abate and Pierre Boutillier. Additional thanks to Gabriel Scherer who discovered multiple bugs and contributed the original crowbar tests.

Planned future improvements of the library include:

  • splitting the library into smaller components (to minimise dependencies when using only a part of the library), and
  • providing multiple endianness (currently the library only provides big-endian binary encodings).

Release of Easy_logging v0.6

Sapristi announced

I'm glad to announce the release of Easy_logging v0.6.

Easy_logging is a logging library inspired from the python logging module, which allows fine grained tuning of log levels and log output, with relatively low configuration, and simple logger declaration and usage.

The most important new features are

  • auto-timestamp and/or versioning of file names when logging to a file
  • redefined handler type, so that custom handlers can easily be defined.
  • handlers support custom filters
  • loggers have tag generators (tags generated from callback functions)
  • log items now contain a timestamp

I also cleaned the API, so that the only exposed objects from opening Easy_logging are the Logging, Handlers and Logging_internals modules.

The documentation is still lagging a bit behind, and will be updated [insert a random date here].

What comes next

  • I plan to drop entirely the support for the MakeLogging functor (that creates a Logging module from a Handlers module), so if anyone depends on it please tell me.
  • Some still lacking options are file-rotation when logging to a file. I guess the saner way to do so would be using Lwt, so I might implement a Lwt version of the module.

PS: I try to maintain a relatively stable external API, but the internals might (and will) change whenever I feel like it.

soupault: a static website generator based on HTML rewriting

Daniil Baturin announced

1.3 release with some improvements.

  • Invalid config options cause warnings now. There are also "did you mean" suggestions for mistyped options, thanks to @c-cube's spelll library.
  • Footnotes now keep original id's for handy hotlinking, and you can add suffix/prefix to footnote ids to make a separate "namespace" for them.
  • Some minor bugfixes.

Update on the big ppx refactoring project

Jérémie Dimino announced

We wanted to give an update on the status of the big ppx refactoring project. At this point, we have settled on a technical solution and are working towards implementing it. In this post I'll describe briefly how it works and what the plan is. This post directly follows https://discuss.ocaml.org/t/the-future-of-ppx/3766 which gives an overview of the ppx history and the issues we want to solve.

We also gave a presentation explaining all this at the OCaml Users and Developers workshop at ICFP this year with @NathanReb, so if you'd like to see a live version of this post watch out for the video on youtube! :)

And before diving in the technicalities, on the social side we welcome @rgrinberg who joined the team! :dizzy:

The solution

It wasn't easy to came up with this solution, but now that we have it it is actually quite simple, which means that we can be confident it will work.

  • Abstracting the AST

    To solve the stability problem, we are not re-inventing the wheel. We are simply making use of a very well established method in the functional programming world: abstraction.

    Instead of exposing the raw data-types of the OCaml AST to authors of ppx rewriters, we are simply going to expose an API where all the types are abstract. Authors of ppx rewriters will need to use construction functions in order to construct the AST and one-level deconstruction functions to deconstruct it.

    Moreover, this API will follow closely the layout of the underlying AST so that we can mechanically follow its evolution. More precisely, there will be one module for every version of the AST with an API that matches the AST at this version.

    This is all still very fresh and experimental, but here is a sneaky peak of what this will look like: https://github.com/ocaml-ppx/ppx/blob/70c0bfd3b7a3e8a27e5ad890801d7c93f7dc69a7/astlib/src/stable.mli

    One important detail that will ensure good interoperability between ppx rewriters is that the types will be equal between versions. i.e. V4_07.Expression.t will be exposed as equal to V4_08.Expression.t.

    The deconstruction API will be a bit raw and in particular won't allow nested patterns. To help with this, we will provide view patterns implemented via a ppx rewriter shipped with the ppx package. And of course, meta-quotations will still be available.

  • Using dynamic types under the hood

    The stable APIs are one thing, but we also want to keep the good composition semantic of ppxlib. Trying to compose things using the static types under these multiple APIs would be a bit of a nightmare. So instead we are going full dynamic. During the single-pass rewriting phase, the AST will be represented using dynamic types. Downgrades/upgrades will happen lazily and only at the edges as requested by individual ppx rewriters using a history of conversion functions provided by the compiler. In essence, these conversion functions will be very similar to the one we have in ocaml-migrate-parsetree, except that since they operate on the dynamic types they will be much smaller and focus on the interesting changes.

    And because the conversions will happen only when needed, in many cases we will be able to use new language features even if the various ppx rewriters in use are written against older versions of the AST.

The plan

The plan is to get all of this implemented, proof test it against a bunch of existing older version of the AST and finally eat our own dog food by porting a bunch of ppx rewriters to the new world, making sure that the port is as smooth as possible.

Once this is all done, we will release the 1.0.0 version of the ppx project for public consumption.

It will be possible to use ppx rewriter baseds on ppx in conjunction with ppx rewriters based on ppxlib or ocaml-migrate-parsetree, just so that we don't need to port everything at once.

Our expectation is that the next time the parsetree changes and authors of ppx rewriters need to update their code, they'll choose to migrate to ppx at the same time given that it will give them long term stability guarantees.

Replying to some questions, Jérémie Dimino said

What we call astlib won't be a user facing library. It will live in the compiler itself and be the smallest possible API that ensure that the ppx world keeps working with new compilers and even trunk. It will only contain:

  • the definition of the dynamic AST
  • functions to convert between the static and dynamic ASTs
  • the history of upgrade/downgrade functions

The user facing package will be ppx, which will be composed of

  • the ppx library, which in particular will expose the versioned AST interfaces, the view patterns, the driver and various modules ported from ppxlib. It will be approximately the same size as ppxlib except that it will have no dependency
  • ppx.metaquot for metaquotations
  • ppx.view for view patterns

If there is a need to have just the versioned AST interfaces on their own without the rest, we could certainly imagine distributing them as a separate library of even a separate package.

Regarding the driver, it will be similar to the ppxlib driver. i.e. the standard way to register a transformation will be by registering extension point expanders or derivers rather than whole AST mappers, though the latter will still be allowed for special cases. This is simply because such precise transformers have a better composition semantic and lead to faster rewriting, so we want to encourage ppx authors to use that.

Simplify roman utf8

sanette announced

If you are interested, I have released the library on github:


PR welcome!

Rfsm 1.6.0

jserot announced

This is to announce the 1.6.0 release of Rfsm, a package dedicated to the description, simulation and synthesis of StateChart-like state diagrams.

The Rfsm package includes both an OCaml library and a command-line compiler. Both take

  • a description of a system as a set of StateChart-like state diagrams
  • a description of stimuli to be used as input for this system

and can generate

  • graphical representations of the system
  • execution traces as .vcd files

Additionnaly, dedicated backends can generate system descriptions and testbenches in

  • CTask (a C dialect with primitives for describing tasks and event-based synchronisation)
  • SystemC
  • VHDL

More informations are available on the github page.

Rfsm is provided as a an opam package.

Graphical User interfaces to the command line compiler are provided separately.

Comments, feedback and bug reports welcome.

Ocaml-protoc-plugin 0.9: A protobuf compiler

Anders Fugmann announced

I am happy to announce the first release of ocaml-protoc-plugin.

The library implements the full proto3 specification, and aims to generate ocaml idiomatic bindings to protobuf types/mesages defined in .protoc files:

  • Messages are mapped to modules, with a type t
  • Enums are mapped to ADT's
  • Oneof types are mapped to polymorphic variants

All types are serialized using the proto3 specification (i.e. all repeated scalar types are packed).

All parts (package, service definitions, includes etc.) are supported.

The library is available through opam: opam install ocaml-protoc-plugin.

For more information, please visit the homepage of ocaml-protoc-plugin at: https://github.com/issuu/ocaml-protoc-plugin/

Comments and suggestions are more than welcome.


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