I’ve started using lsp-mode in Emacs to add some smarter auto-completion and tooltips. I wanted to try and activate lsp-mode automatically when editing files in a Rails project, but only if solargraph is in the Gemfile.

I’m not sure this is actually something I’ll keep using, but it was a fun exercise.

(add-hook 'enh-ruby-mode-hook #'sodaware/enable-lsp-for-solargraph-projects)

(defun sodaware/enable-lsp-for-solargraph-projects ()
  "Enable lsp-mode if the current project has solargraph installed."
  (when (sodaware/has-solargraph-p)

(defun sodaware/has-solargraph-p ()
  "Check if the current Rails project has solargraph installed."
  (let* ((root    (sodaware/get-closest-gemfile-root))
         (gemfile (concat root "/Gemfile")))
    (unless nil
        (insert-file-contents gemfile)
        (search-forward "solargraph" nil t)))))

(defun* sodaware/get-closest-gemfile-root (&optional (file "Gemfile"))
  "Determine the pathname of the first instance of FILE starting from the current directory towards root.

This may not do the correct thing in presence of links. If it does not find
FILE, then it shall return the name of FILE in the current directory, suitable
for creation."
  (let ((root (expand-file-name "/")))
     for d = default-directory then (expand-file-name ".." d)
     if (file-exists-p (expand-file-name file d))
     return d
     if (equal d root)
     return nil)))

It’s not particularly elegant, and I think there will be performance problems for with larger Gemfiles.