(require 'smie)

(defvar foo-smie-grammar
  (smie-prec2->grammar
   (smie-bnf->prec2
    '((statements (statement)
                  (statement ";" statements))
      (statement ("def" non-block-expr "do" statements "end")
                 ("def" non-block-expr "COMMA" "dop" non-block-expr))
      (non-block-expr ("STRING"))))))

(define-derived-mode foo-mode prog-mode "Foo"
  (set (make-local-variable 'comment-start) "# ")
  (smie-setup foo-smie-grammar #'ignore))