[Merlin-discuss] Merlin's configuration

Thomas Refis thomas.refis
Mon Dec 2 18:43:43 GMT 2013


Thank you everyone for the quick answer, and sorry for the delay of ours.

  * * *

Gabriel said:
> I think trying to reimplement the build system's
> job in Merlin is a dead end. You must defer to the existing build
> system(s) to provide you this information.'

Yes, we think so too. We wanted to make sure others would agree.

  * * *

Sylvain said:
> I will be more than happy to help you on the second point to
> integrate an external process that can ask directly oasis to
> generate a configuration. I can help you on this project and
> even help you write the extension.

Thank you!
We will contact you when we start working on that.

  * * *

Roshan said:
> A. There should be a way to state this information per file.
> B. The merlin command should use the same syntax as an ocamlopt
>    call for expressing the information.
> C. The information from the command-line in B also be used to
>    determine other aspects of type checking, e.g. warnings.
> D. An auxiliary file (.merlin) be used for expressing only things
>    that the ocamlopt call doesn't express, e.g. (3).
> E. A running merlin process should accept a command that updates
>    this information.
> F. A running merlin process should accept a command that tells it
>    when a .cmt or .cmi changes, and should update its internal
>    cache.

A. Yes, that's what we want too, hence this thread.

B. Well, the command does use the same syntax as an ocamlopt call, for
   example you can call merlin like that :

      ocamlmerlin -I<some dir> -I<some other dir> -w +w32

   Except that's not the way you do it, because you (as an user) never
   call merlin, your editor does. And your editor just call merlin and
   then tell him the absolute path of the file currently under
   edition, and it's then to merlin to find the relevant configuration
   for this file (at the moment in the associated .merlin).
   It works that way because you don't want your editor to know the
   logic of your project.

C. You can already do that.

D. Right.

E. It's already the case.

F. It's already the case.

Roshan said:
> The thinking behind using an ocamlopt call as the mechanism of
> expression is that it is already present in all build systems,
> and is thus very likely to reflect what the build system is
> actually doing.
> [...]
> How should the compilation command be communicated to merlin?
> There are at least three approaches that one can think of, of
> which we think the third has several advantages:
> (i) Write things into files: As you proposed, ...
> (ii) Have merlin ask the build system: ...
> (iii) Have the build system notify merlin: ...

Ok so, I'll pass on the first part since (I think) we're more
interested by the global mechanisms than the concrete syntax.

So, one clarification, I think what we propose is closer to (ii) than
to (i) : what we want to store in a (unique) file is the information
merlin needs to retrieve informations from the build system (i.e.
"call 'oasis setup merlin'" or "send this query to jenga on this

I'll comment a bit on (iii), so let me quote it in full :

> Have the build system notify merlin: Merlin can provide a
> command that the build system calls to communicate the ocamlopt
> compilation command to the current running instance of
> merlin. This has the advantage that merlin gets updates whenever
> the build system needs to recompile files. Further, merlin can be
> notified of successful build completion of each .ml file.

That could be an option, we could have merlin "listening" for new
information about the "project state".
But I think there are several drawbacks to this:

1/ what if there are several instance of merlin running?
   You have emacs so you probably have only one instance of emacs
   running, which in turn has only one merlin subprocessed launched.
   Which is fine. But I use vim, I have several vim process launched,
   and with that several instances of merlin.
   It's gonna be a bit of a pain to tell your build system which
   instance of merlin it should be talking too.

2/ The build system has no way to know which file merlin is currently
   handling (except if you provide a way for the build system to ask
   that information to merlin, but then you need a real protocol and
   need to seriously extend the existing build systems), so it would
   feed merlin information about the entire project.
   Which is what we do at the moment, except this information are
   "global", "unprecise" if you want. But we are discussing having
   information on a "per file" basis, so that's gonna be a bit bigger.
   Which is fine for the project I write in my cellar. But that's
   gonna be a whole lot bigger at janestreet for example.
   How much memory does jenga need to stay in polling mode on your

3/ OMake and/or jenga can be run in polling mode, which is fine for
   that kind of usage, they can discuss with merlin without user
   interraction. But people using oasis, ocamlbuild, make would need
   to call their build system everytime they launch merlin, and
   everytime the project configuration changes; it's annoying.
   I'd rather have merlin ask the build system by itself.

As for notifying merlin when a successful build completed, there
already is a way to do that.

   * * *

There was one last point to Roshan's mail (the "best effort" behavior)
but that is off-topic. There are certainly things to say on that
topic, but I think it should be on a thread of its own.


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