Emacs helper to conditionally run `irb`, `pry`, `bundle console`, or `rails console` with respect to the current working directory.

conditionally-run-ruby.el

(when (not (boundp 'inf-ruby-implementations))
  (setq inf-ruby-implementations nil))

(defun ~/paths-to-root (file-name)
  (let* ((path-parts (s-split "/" (file-name-directory file-name) t))
	 (build-path (lambda (path-parts)
		       (concat "/" (s-join "/" path-parts))))
	 (paths (reduce (lambda (state _)
			  (let* ((old-path-parts (cdr state))
				 (new-path-parts (-butlast old-path-parts))
				 (old-paths (car state))
				 (new-paths (cons (funcall build-path new-path-parts) 
						  old-paths)))
			    `(,new-paths . ,new-path-parts)))
			path-parts
			:initial-value
			`(,(list (funcall build-path path-parts))
			  . ,path-parts))))
    (car paths)))

(defun ~/bundler-installed-p ()
  (stringp (executable-find "bundle")))

(defun ~/bundler-project-p ()
  (-any-p 'file-exists-p
	  (-map (lambda (path)
		  (concat path "/Gemfile"))
		(~/paths-to-root (buffer-file-name)))))

(defun ~/rbenv-installed-p ()
  (stringp (executable-find "rbenv")))

(defun ~/pry-installed-p ()
  (stringp (executable-find "pry")))

(defun ~/rails-project-p ()
  (-any-p 'file-exists-p
	  (-map (lambda (path)
		  (concat path "/bin/rails"))
		(~/paths-to-root (buffer-file-name)))))

(defun ~/conditionally-run-ruby ()
  (interactive)
  (let* ((ruby-command (if (and (~/bundler-installed-p) (~/bundler-project-p))
			   (if (~/rbenv-installed-p)
			       (if (~/rails-project-p)
				   "rbenv exec bundle exec rails console"
				 "rbenv exec bundle console")
			     "bundle console")
			 (if (~/pry-installed-p)
			     "pry"
			   "irb --prompt default --noreadline -r irb/completion"))))
    (setq inf-ruby-implementations
	  (cons `("ruby" . ,ruby-command)
		(delq (assoc "ruby" inf-ruby-implementations)
		      inf-ruby-implementations)))
    (setq inf-ruby-default-implementation "ruby")
    (run-ruby)))

(define-key ruby-mode-map (kbd "C-c M-j") '~/conditionally-run-ruby)