Project Management in Emacs

Over the years, I have tried many different “project management” libraries for emacs – enough that I do not even remember which ones. I do not really remember finding one that did what I wanted (or could be coaxed into it within any reasonable amount of time). Sometime last year I stitched together something rudimentary that serves my workflow very well. With my recent emacs configuration rewrite, I took the opportunity to refine my solution into something slightly nicer. I am documenting it here partly so I do not forget what I did, but it may be useful to someone else.

Requirements

First, let me outline my simple requirements for project management:

Note that my projects do not necessarily have a one-to-one correspondence with source code repositories. I have a few large projects split over multiple repositories that I would prefer to treat as a single projects. In particular, I want a single TAGS file that has tags spanning multiple repositories for easy source code navigation.

Solution

I ended up using simple directory local variables. This emacs feature lets you put a file named .dir-locals.el at the root of a source tree. When a file is loaded, the first .dir-locals.el above it in the file system is loaded. Variables defined in .dir-locals.el are defined as buffer locals in the opened file. I define two directory local variables for each project: tag-builder and build-method. I use the following elisp to build my project management functionality:

;;; Tag support
(defun make-tags-hook ()
  (when (boundp 'tag-builder)
    (let ((project-root (locate-dominating-file buffer-file-name ".dir-locals.el")))
      (let ((cmd (format "cd %s ; %s &" project-root tag-builder)))
        (call-process "bash" nil 0 nil "-c" cmd)))))

(add-hook 'after-save-hook 'make-tags-hook)

(defun set-project-tags ()
  (let ((project-root (locate-dominating-file buffer-file-name ".dir-locals.el")))
    (let ((tags-file (format "%s/TAGS" project-root)))
      (when (file-readable-p tags-file)
        (visit-tags-table tags-file)))))
(add-hook 'find-file-hook 'set-project-tags)

;;; Compile support
(defun set-compile-command ()
  "Set the compile command based on a `.dir-locals.el' build-method"
  (when (boundp 'build-method)
    (let ((project-root (locate-dominating-file buffer-file-name ".dir-locals.el")))
      (setq compile-command (format "cd %s ; %s" project-root build-method)))))
(add-hook 'find-file-hook 'set-compile-command)

The make-tags-hook function rebuilds the tags for an entire project and is run after each file is saved. In practice this has been fast enough for me. I have only tried it on ~20,000 lines of Haskell code, though. Simply, it finds the closest .dir-locals.el file and assumes that is the project root. It changes to that directory and runs the tag building command from the directory local variable file.

The next function, set-project-tags selects the appropriate TAGS file every time a new file is opened (using visit-tags-table). This is not exactly ideal. Only one TAGS file can be loaded at a time, normally, so working with multiple projects in one emacs session would not work perfectly. Luckily, I do not do that very often. This shortcoming could be fixed with something like etags-table. I might try to do that soon.

The last function deals with compilation support. It uses the build-method directory local to define an appropriate compile-command for the project. The key functionality here is that the computed compile-command changes directory to the root of the project before running the build command. Again, the project root is determined based on the location of .dir-locals.el. Since this just modifies compile-command, the normal build infrastructure (like M-x compile) Just Works.

Notes

I used unconventional names for my directory local variables. I wanted to use build-command and tag-builder-command, but there is an interesting quirk in directory local variables. For security purposes, only variables explicitly marked “safe” in emacs are loaded without user interaction. The user is interactively prompted before so-called “risky” variables are loaded. These “risky” variables can be trusted (and recorded in custom.el) easily. Variables that are fundamentally unsafe, or ending in “-command” (or a few other suffixes) can be loaded explicitly by the user, but cannot be marked as safe in custom.el. This was very annoying, so I just chose some unconventional names to work around it. This is in some sense “unsafe”, but you are still prompted to accept new values of these variables the first time they are encountered. This is a fine tradeoff for simplicity and convenience, for me.