# Pastebin uAprf9SZ
  (defun nix-find-sandbox-file (path file)
    "Find the given FILE by walking to / starting from PATH"
    (let ((found
          (locate-dominating-file
           path (lambda (dir) (file-exists-p (concat dir (f-path-separator) file) )))))
      (and found (concat (expand-file-name found) file))))

  (defun nix-find-sandbox (path)
    "Find the closest nix-shell, prefer shell.nix but if none is found settle with default.nix"
    (and (file-exists-p path)
         (or (nix-find-sandbox-file path "shell.nix")
             (nix-find-sandbox-file path "default.nix"))))