Re: Unified project interface

[Dmitry Gutov <dgutov@yandex.ru>]

On 07/29/2015 04:36 AM, Stephen Leake wrote:

> So you would keep project-roots, but not project-root. I can live with
> that (Ada mode needs neither :).

It's fine if some projects define project-roots to nil. Unfortunately, 
as I imagine it, xref-query-replace won't work for them.

Of course, we could add a preference (turned off by default) that 
xref-query-replace walks the matches in the search-path, too, not just 
in project-roots.

> How will the elisp backend define project-roots?

It won't. Creating a project backend for Elisp seems pretty much 
impossible (or rather useless), from where I'm standing.

That's why emacs-lisp-mode sets project-search-path-function instead: 
the VC project backend uses it automatically.

> Since it doesn't know anything about project-roots, it should either
> throw an error, or return nil.

But that's exactly the problem: if I'm working on a third-party Elisp 
project, it has its own repository, there's no guarantee that its root 
directory is added to load-path. If all Elisp code is in the lisp 
subdirectory (and it's in load-path), if I call xref-find-regexp, it 
would miss all files on the level above.

> On the other hand, (project-search-path (current-project)) in a .el
> buffer on my machine returns:
>
> ("c:/Projects/emacs/master-build-mingw64/lisp/"
>   "c:/Projects/emacs/master/"
>   "c:/Projects/emacs_stephe.main/emacs_stephe/"
>   "c:/Projects/emacs_stephe.main/emacs_stephe_site_lisp/"
>   "c:/Projects/org.emacs.ada-mode/"
>   "c:/Projects/org.emacs.dvc/lisp/"
>   "c:/home/stephe/.emacs.d/elpa/")
>
> Those are all 'project-roots' as far as I can tell; they each represent
> a logically and physically separate collection of elisp source files
> (except that master-build-mingw64/lisp/ actually has no elisp source files).

No, these are just search-path. We don't know where the roots of most of 
these "projects" are.

> On the other hand, "(project-directories (project-current))" returns the
> single root that contains the current .el file; the same as
> "(project-root (project-current))". Why does project-directories not
> return a list?

It returns a one-element list.

> Currently, the only code that uses project-directories is
> xref-find-regexp, and it concats it with project-search-path, so it
> confuses the two.

Right. Sorry, that must indeed be confusing. xref-query-replace does not 
discriminate against project-search-path yet.

> Ada mode has no need of project-roots. It seems elisp also has no need
> of project-roots. JDE (Java Development Environment) has no need of
> project-roots.

A common Java project usually has one root: the directory containing its 
project file (build.xml, pom.xml, or so on).

Even so the project implementation could provide a way to open several 
projects together, if explicitly told so by the user. Then it would 
return them all in project-roots.

>> Some caller up the stack will have to call project--prune-directories
>> anyway, because we don't want to guarantee that search-path and
>> project-roots don't mix (see above).
>
> The top-level generic API should be agnostic about that; only the
> backend knows precisely what project-search-path and project-roots are.
> In elisp and Ada, project-roots is simply undefined.

So?

The caller would do what xref-find-regexp does now:

(project--prune-directories
   (nconc
     (project-roots proj)
     (project-search-path proj)))

> Ada mode wants search-path to be flat, _not_ containing roots. So it
> does _not_ what something to call project--prune-directories.

It wouldn't hurt anyway.

>> However, we could provide a
>> public function in project.el that will combine both.
>
> Why?

So that the caller doesn't need to call project--prune-directories itself.

> I still have not seen an actual use case for project-roots; it seems the
> simplest thing is to simply drop it.

I have provided a few, along the way of this discussion.