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Re: Unified project interface

From: Dmitry Gutov
Subject: Re: Unified project interface
Date: Wed, 29 Jul 2015 05:10:09 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.0

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:


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

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.


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

    (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.


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.

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