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dev meeting 20200513
Present at the meeting:
- Andrey Mokhov (@snowleopard)
- Anil Madhavapeddy (@avsm)
- Emilio Jesús Gallego Arias (@ejgallego)
- François Bobot (@bobot)
- Jérémie Dimino (@jeremiedimino)
- Rudi Grinberg (@rgrinberg)
In the past we have had a few issues related to dune-configurator:
- old versions of
dune-configuratordon't work with recent versions of Dune - new versions of
dune-configuratordon't work with older versions of OCaml
Which makes it impossible for a project to:
- use
dune-configurator - keep compatibility with older versions of OCaml
- use recent dune features
This is unfortunate because this scenario is not that rare. Indeed, a
lot of project want to support a wide range of OCaml versions in order
to maximise the reah of their software. They also want to use recent
dune features because we keep adding useful features. And if they need
to query some C related settings, dune-configurator comes in very
handy.
The reason old versions of dune-configurato don't work with recent
versions of Dune is because there is a bit of communication at runtime
between Dune and OCaml and this communication is not versioned.
Making dune-configurator work with older versions of OCaml is
difficult because it depends on the Dune private libraries and we'd
like the freedom to rely on recent OCaml features in the Dune code
base. This is usually fine given that we only provide a binary and the
OCaml version used to build it is just an implementation detail,
however this is not fine for libraries.
At a high-level, the relation ship between dune and
dune-configurator is not well defined and versioned. Fundamentally,
the only thing Configurator needs from Dune is the configuration of
the current build context. If we limit this relationship to just that,
it becomes easy to version it and add more flexibility between the
versions of Dune and Configurator.
To enforce this, we decided to treat Configurator as an external project and extract into a separate repository, forcing us to make the relationship between the two project more explicit and controllable.
Dropping the dependency between Configurator and the private libraries of Dune caused us to duplicate some code from Stdune into Configurator, and we discussed what we could do to avoid this duplication.
The obvious idea would be to extract Stdune into its own repository
and package. What is more, some other projects such as
OCaml-LSP are already vendoring
Stdune and sharing it more officially could be nice for such
projects. However, the API of Stdune is way too big for us to commit
to its stability, which is obviously a problem. We don't want to end
up in a situation where making a new release of dune-configurator
splits the universe because the API of Stdune broke.
Jérémie proposed to avoid such problems by putting the major version
of the library in the package name. i.e. we would have stdune1,
stdune2, and so on. This way, we could make new major release of
stdune as often as we would like without breaking the world.
Anil mentions that this is not going to be great for opam. Emilio replies that this is what Linux distributions are doing. After some more braistorming and various other solutions proposed there is no consensus. It is not clear whether some of the solution proposed will solve the original problem, and also what will be the damage in terms of workflow.
For now, we decided to write up a bit more about the various solutions so that we can reason about them.
There are various issues related to the way cmt files are produced:
Currently, cmt files are produced when comping the code in bytecode. This has a few consequences:
- for an executable or library that has
(modes native),dune build @allwill not build the cmt files :( - for a public executable or lilbrary that has
(modes native),dune build @allwill build the bytecode versions just for the cmt files
We could produce the cmt files using dedicated build rules that only
produce the cmt files by using -stop-after typing, however this
might make builds slower.
We decided to change our build rules so that when bytecode compilation
is not requested via the (modes ...) field, the cmt files are
produced by native compilation indeed. This should fix both problems.
Rudi also noted that it would be nice if we could restart compilation
from cmt files, which would both solve this problem in a nice way and
avoid the problem where we get warnings twice; once from ocamlc and
once from ocamlopt.
Jérémie pointed out that eventually cmt files won't be necessary for Merlin anymore. Indeed Thomas Refis and Leo White have a plan to make cmi files sufficient for Merlin. When this happens, we could reconsider building and installing cmt files given that they are the biggest kind of artifacts. Anil points out that this is not that simple because odig needs them for odoc.
François talked about his PR for sites and relocation:
https://github.com/ocaml/dune/pull/3104
This PR implements a feature needed by real world programs so this fits well into Dune. However, it is quite big and there are a lot of design questions. To proceed, we will need to have a few people look at the high level design and play with the feature. Once this review staged has finished and we have reached a consensus, we can proceed with the technical code review. Emilio says that he is willing to try out the feature and Jérémie suggests that David and Nicolás might be good persons to ask as well.
Because this feature is touching various part of Dune and adding some code for better interop with findlib, Jérémie asked François if he could send a small summary of the technical changes to the code base, in order to asses the change from a maintenance/sustainability point of view.
A few users are interested in using Rust code in their OCaml projects. Rust code builds with Cargo, which is the equivalent of opam+dune in the Rust world. There is a thread on discuss about this, and an example project on github.
Anil mentions the idea of treating the Rust build in a similar fashion to the foreign build sandboxing method.
Jérémie asked what the story will be when the user is in the editor and working on both the Rust and OCaml code. It feels like that there should be a bit more cooperation between Cargo and Dune in order for things to go smootlhy. Anil that he knows someone who should start working on this question soon.