[Merlin-discuss] Merlin's configuration

[Thomas Refis]

Hi!

There have been some changes recently on merlin's configuration, and
we're also considering some more heavy changes on the syntax for the
.merlin file.  We'd like to have people's input on this.

   * * *

First we've added two new "directives": CMT and CMI, these are
refinements of the "B" directive, which was used to add directories to
the build path. The need for this is primarily felt at JaneStreet.

At janestreet, there is one big repository including, among others,
core and async.  Every source directory is also a build directory but
the cmi for packed modules are afterwards copied to a particular lib/
directory at the root of this repo.
You then have two options to set-up your build path in merlin: add
only lib/, or add every source directory.
The first option is unsatisfactory since you won't have access to cmt
files (that implies no fancy source browsing), but the second is also
problematic.  Indeed there is a lot of module name conflicts, and
there's no predictable way to tell which one will be choosen. That
results in the frequent presence of the error message "Module Foo and
Bar make inconsistent assumptions over interface Baz".

With the new directives, one can now add lib/ to the "CMI" dirs (i.e.
the buildpath) and every other directory to the "CMT" dirs.

On simpler setups, one can of course still simply use "B".

NB: This is not on the opam package yet, only on the git repository.
We should however make a new release soon (in the next two weeks I'd
say).

   * * *

We are considering moving to a more expressive syntax for the .merlin
files, the goal is to be more precise about which files/directories
require which packages/compiler flags and which do not.
There is two different paths we can follow here:

  The first one is to adopt the syntax ocamlbuild uses in its _tags
file. This should be precise enough to express everything one would
ever need. Also it has the advantage of being a syntax everyone
already knows, and one can also imagine that at some point in the
future, merlin would be able to understand (at least partially) _tags
files whenever there is no .merlin.

  The second one, a bit more ambitious, consist of finding a way to
delegate the work to external tools. That is: merlin would call an
(arbitrary) external process, giving it the path of the file currently
under edition. And that process would then output merlin's
configuration.
That way, people could first have the configuration file in their
language of choice, or call a tool to generate a configuration from an
_tags, _oasis, ?, but would also be able to have merlin integrated in
their build process. For example people using a build system running
in "daemon" mode (jenga, OMake) could then feed it the path of the
file being edited, and would then receive a list of the dependencies
of that file, as well as compiler flags required.

We would like to know if you think either of this approach is useful,
or interesting... or not.

   * * *

That's about it.

Cheers,

Frederic and Thomas.