OCaml Weekly News

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Here is the latest OCaml Weekly News, for the week of March 03 to 10, 2020.

Table of Contents

Non opam workflows

Manas asked

Very recently, I learnt that there is a significant chunk of users in the OCaml community that does not use opam to install packages. As a small initiative to contribute to tooling, I want to ensure what I build is compatible with these workflows - workflows I'm not familiar with myself.

I'd love to learn more - what does it look like? How do you setup your compiler, dune and merlin (and/or soon ocamllsp)? How do you configure your editor to find them and what would make it easier to do so?

I'm told of Duniverse as one tool that being used in these non-opam workflows. Are there any more popular ones out there?

Théo Zimmermann replied

I am one of these people. I mostly rely on Nix, whose package repository nixpkgs provides package sets for all (relatively recent) versions of OCaml. These package sets are not generally as complete as what you can find on opam, so it sometimes happens that I open a PR on the nixpkgs repository to add a new package (and in the meantime I use my local updated copy of the nixpkgs repo).

You can see the list of available OCaml packages at: https://nixos.org/nixos/packages.html?channel=nixpkgs-unstable&query=ocamlPackages (This is for the default OCaml version, currently 4.07 in nixpkgs-unstable. Other package sets are called ocaml-ng.ocamlPackages_4_0X but are not shown in this web search.)

Most OCaml packages are available at a single version in nixpkgs (even though you can choose your version of OCaml). To gain more flexibility on the exact version I use in one of my project, I am planning to test Duniverse. At that point, I would rely on Duniverse for library dependencies, but I would still rely on Nix to install OCaml, findlib, Dune, Duniverse (I'll have to take care of packaging it), utop, merlin, or ocamlformat.

Nix is pretty straightforward to use. You generally provide a default.nix at the root of your repository, and it will list the dependencies that you use. When you want to go develop your project, you just enter a special shell (with the nix-shell command) and you are in an environment where the tools you need are in PATH and the libraries you need are in OCAMLPATH.

There's just one tool that I needed special configuration for: ocamlformat (especially because some projects use it and some do not). When I use it, my default.nix contains:

shellHook = ''
  export OCAMLFORMAT_LOCATION=${ocamlformat}

which will export an environment variable when I enter the shell.

And my .emacs contains:

(setq ocamlformat-location (getenv "OCAMLFORMAT_LOCATION"))
(when (> (length ocamlformat-location) 0)
 (add-to-list 'load-path (concat ocamlformat-location "/share/emacs/site-lisp"))
 (require 'ocamlformat)
 (add-hook 'tuareg-mode-hook
           (lambda () (add-hook 'before-save-hook 'ocamlformat-before-save))))

I want to ensure what I build is compatible with these workflows

If you mean as a library author, then all you have to ensure is that you use Dune as the build system (makes the Duniverse workflow better, and makes it easier to package your library in nixpkgs, cf. buildDunePackage documented at https://nixos.org/nixpkgs/manual/#sec-language-ocaml).

Rwmjones also replied

You might want to check out the Fedora OCaml packages.

Unfortunately I don't have a convenient way to link to the whole list, but if you look at all the OCaml packages here: https://koji.fedoraproject.org/koji/search?match=glob&type=package&terms=ocaml* and then if you substitute the ocaml-<packagename> in two places in this URL: https://src.fedoraproject.org/rpms/ocaml-re/blob/master/f/ocaml-re.spec (example showing ocaml-re package), you can see how we build and package them in the %prep, %build and %install sections.

And yes, please make sure your software doesn't depend on opam. Building everything in your home directory is not suitable for enterprise software distribution.

First release of metapp

Thierry Martinez announced

I am happy to announce the first release of metapp, yet another preprocessor for OCaml. Similarly to ppx_optcomp, metapp is a PPX rewriter. But instead of introducing a specific DSL for preprocessor directives, metapp provides a [%meta ...] extension, where the dots ... are arbitrary OCaml expressions that are substituted at compile-time by the AST nodes they evaluate into. These expressions build AST nodes either by (anti-)quoting some code directly, or by using compiler-libs (Parsetree, Ast_helper, …).

In particular, this preprocessor is easy to use for conditional compilation, and is an alternative to cppo and ppx_optcomp.

let option_get o =
  [%meta if Sys.ocaml_version >= "4.08.0" then
     [%e Option.get o]
     [%e match o with
     | None -> invalid_arg "option_get"
     | Some x -> x]]

In this example, the code between [%e ... ] is "anti-quoted": it is the code that is inserted (conditionally) in the rewritten module. Of course, the anti-quoted code can contain itself some [%meta ...] code. [%meta ...] can even itself contain other levels of [%meta ...] code for multi-stage programming.

An example of usage of metapp is the metaquot package, which implements the same quoters as ppx_tools.metaquot: [%expr ...], [%type: ...], etc. These quoters are implemented by meta-programming: the meta-code introspects Parsetree.cmi from compiler-libs to generate the code matching the current OCaml version.

Raphaël Proust added

To potentially save a few second to the next readers: https://github.com/thierry-martinez/metapp seems to be the repo where it is hosted.

Thierry Martinez then said

Thanks, @raphael-proust! The package is also available via opam: opam install metapp (and metaquot is available via opam as well).

OCaml 4.10 released

octachron continued this thread

The Merlin team has just released a preview version of Merlin which is compatible with 4.10.0 (Merlin is an editor service that provides modern IDE features for OCaml) .

This is a preview version:

  • the support for short-path is disabled
  • only OCaml 4.10.0 is supported in this preview

It can be installed via opam with the usual

opam install merlin

Transept 0.1.0: Generalised Parser Combinators

Didier Plaindoux announced

I’m happy to announce the first release of Transept an OCaml implementation of generalized parsers combinators.

This implementation has been inspired by a 19 years old paper - written by Daan Leijen and Erik Meijer - titled “Parsec: Direct Style Monadic Parser Combinators For The Real World” [1]. The current implementation provides basic combinators dedicated to char, chars recognition but also conjunction, sequence, repetition and more. Since the current design relies on the abstract definition of manipulated element most of the parsers are generic and can be used with streams of chars or something else.

Finally, with this library, I wanted to share my love of OCaml modules 🤗

Opam: https://opam.ocaml.org/packages/transept/transept.0.1.0/

[1] https://www.microsoft.com/en-us/research/wp-content/uploads/2016/02/parsec-paper-letter.pdf

Didier Wenzek then said

It good to see yet another parser combinator for OCaml, even if this makes more difficult the choice of one of them. I believe this highlights how well OCaml shines for this kind of applications where both high-level expressiveness and performance matter.

angstrom is one the alternatives and provides a comparison with others. It would be good to position transept here.

There is also a more recent article with a radically new approach: A Typed, Algebraic Approach to Parsing by Neelakantan R. Krishnaswami and Jeremy Yallop - PLDI 2019. This paper proposes a library of parser combinators for context-free expressions, an algebraic presentation of the context-free languages. The key points are

  • the use of types to statically reject any language which cannot be parsed unambiguously and linearly;
  • the use of staging, with OcamlBER, to produce parsers which performance are close to those of hand-written code.

Multicore OCaml: Feb 2020 update

Continuing this thread, Rwmjones asked

Hi Anil (or anyone!). Is there a place I can find more about breaking changes that might be made to C extensions? As you may know we have a lot of C code which interfaces with OCaml, both as ordinary extensions written in C, but also embedding OCaml in C programs (although that's much more rare), and I'd like a heads up about what's likely to change.

Anil Madhavapeddy replied

Hi @rwmjones! In a nutshell: no breaking C changes. The longer version is that we implemented two different minor collectors in order to evaluate various tradeoffs systematically:

  • a concurrent minor collector that requires a read barrier and some C API changes in order to create more safe points
  • a stop-the-world minor collector that doesn't require a read barrier and no extra C API changes, but would probably cause longer pauses

The good news is that our STW collector scales up much better than we expected (tested to 24 cores), and so our first domains patchset will almost certainly use that version now. We expect to shift to a concurrent (and possibly pauseless) collection algorithm at some future point, but in terms of upstreaming it looks like we should be able to delay any C API changes until after the first version of multicore has landed.

Do you have any nice standalone candidate programs using the C FFI we could add to Sandmark?

owl 0.8.0 and 0.9.0 released

Marcello Seri announced

We are happy to announce two new releases of owl: a dedicated system for scientific and engineering computing in OCaml.

Since our previous announcement in July last year, there has been an enormous amount of work going on to cleanup and extend owl's internals and its interfaces.

In this period we have been trying to release often and keep disruption to a minimum. Owl 0.8.0 and 0.9.0 are exceptional in this respect:

  • owl.0.8.0:
    • the discrepancy between owl-base (pure ocaml) and owl (links cblas/lapacke) interfaces started becoming a problem in few places. In this release many interfaces have been unified and reused. The algodiff module has undergone a similar refactoring. Although most users should be shielded from these changes, they may break existing code, requiring an upper bound on owl and some localized updates. This should mostly boil down to changes like

      -module CGraph_D = Owl_computation_engine.Make_Graph (Owl_computation_cpu_device.Make (Dense.Ndarray.D))
      +module CGraph_D = Owl_computation_engine.Make_Graph (Owl_computation_cpu_device.Make (Owl_algodiff_primal_ops.D))
    • this is the last edition supporting OCaml compiler versions < 4.10.0 (more on this later).
  • owl.0.9.0: the main difference between 0.8.0 and 0.9.0 is that owl now requires OCaml 4.10.0. This release of OCaml introduces extended indexing operators. With them we can now write things like x.%{0;3} (for indexing) and x.${[0:2];[2;4]} (for slicing) instead of the more cumbersome x.%{[|0;3|]} and x.${[[0:2];[2;4]]}.

The project is thoroughly documented at ocaml.xyz where you can find multiple examples of use.

A lot of work has (and is) been going into improving the documentation, you can find the results in the new owl book: https://ocaml.xyz/book/toc.html. This is currently targeting the development version of owl, so using master or 0.9.0 is the best bet if you want to try the examples out.

One of the issue of the old documentation was that it was getting stale very fast: the book is reusing some of the infrastructure of RWO, so all examples get recompiled and retested continuously to ensure their correctness.

As a final note, we would like to send a huge thank to the OCaml Software Foundation, see also the announcement made on this forum, which has given us some funding that will support a retreat of the maintainers and a development sprint that will take place at the end of March.

We meant to announce the retreat and sprint for some time now, but the size and publicity of the event may depend on updates to the various governmental and institutional recommendation in regards to COVID-19 spreading. If a public event will be possible, we will make a separate announce on this forum.

We want to also thank all the contributors for the increasing number of comments, fixes and discussions that are helping us shape the next releases of owl.

The Owl Dev Team

Parser combinators vs. parser preprocessors?

Continuing this thread, yallop said

Gasche said:

Combinators also describe a grammar; they can build a representation that is then processed. I think it would be perfectly reasonable to provide combinators to describe a L(AL)R grammar, and then a function from such a grammar to a parsing automaton, along with the result of various analyses. This would solve the “additional tooling” problem of typical parser generators, and also the “lack of conflict analysis” problem of typical parser combinator libraries. But it may require support for staging for performance reasons.

Readers of this thread may be interested in the asp (algebraic staged parsing) library (also described in the Transept post linked above), which is built on an approach along the lines @gasche describes:

  • combinators that describe a grammar (using context-free expressions)
  • an analysis (formulated as a type system) that ensures deterministic parsing
  • staging to eliminate performance overhead

The interface is pretty standard, with combinators for alternation, sequencing, etc., and performance is quite good (better than ocamlyacc on our benchmarks).

There's a paper, A typed algebraic approach to parsing, that describes the design in more detail.

Chet_Murthy said:

Also, I’m personally a massive LL(1) (over LALR) bigot

Grammars built using asp are essentially LL(1). (The weasel word "essentially" isn't hiding much here, but the paper has the details.)

Dune 2.4.0

Rudi Grinberg announced

On behalf of the dune team, I'm pleased to announce the release of dune 2.4.0. This releases features support for mdx, an interesting take on the notebook paradigm by the RWO team. This release also includes a crucial fix to polling mode which makes it usable in environments with finite memory :slight_smile:.

Happy hacking!

2.4.0 (06/03/2020)

  • Add mdx extension and stanza version 0.1 (#3094, @NathanReb)
  • Allow to make Odoc warnings fatal. This is configured from the (env ...) stanza. (#3029, @Julow)
  • Fix separate compilation of JS when findlib is not installed. (#3177, @nojb)
  • Add a dune describe command to obtain the topology of a dune workspace, for projects such as ROTOR. (#3128, @diml)
  • Add plugin linking mode for executables and the (embed_in_plugin_libraries ...) field. (#3141, @nojb)
  • Add an %{ext_plugin} variable (#3141, @nojb)
  • Dune will no longer build shared objects for stubs if supports_shared_libraries is false (#3225, fixes #3222, @rgrinberg)
  • Fix a memory leak in the file-watching mode (dune build -w) (#3220, @snowleopard and @aalekseyev)

Tyxml 4.4.0

Gabriel Radanne announced

I have the pleasure to announce the release of TyXML 4.4.0, with special Reason support!

TyXML is a library for building statically correct HTML and SVG documents. TyXML provides a set of combinators which use the OCaml type system to ensure the validity of the HTML. TyXML is now a stable library and this release comes with a few newly supported elements and attributes (such as ARIA elements) and associated bug fixes. However, the main novelty of this release is a long awaited feature: the support for Reason’s JSX syntax in the brand new tyxml-jsx package.

See the complete announcement for code examples and details: https://drup.github.io/2020/03/06/tyxml440/

first release of oplsr: an OCaml wrapper to the pls R package - Partial Least Squares (PLS) regression

UnixJunkie announced

It is my great pleasure to release one more hackish wrapper to use some R package from within OCaml:


For some background: https://en.wikipedia.org/wiki/Partial_least_squares_regression

Cf. test.ml in the sources for a usage example.


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