Pattern-matching over objects using views and coviews

gistfile1.rkt

(define-language <list>
  (Nil)
  (Cons first rest))

(define-language <maybe>
  (Nothing)
  (Just value))

(define-language <tsil>
  (Snoc butlast last))

(extend-behaviour <list>
  (object self
    ((meta (coerce '<tsil>))
     (ocase self
	    ((Cons a (Nil))      (Just (Snoc (Nil) a)))
	    ((Cons a (Snoc b c)) (Just (Snoc (Cons a b) c)))
	    (_                   (Nothing))))))

(extend-behaviour <tsil>
  (object self
    ((meta (coerce '<list>))
     (ocase self
	    ((Snoc (Nil) a)      (Just (Cons a (Nil))))
	    ((Snoc (Cons a b) c) (Just (Cons a (Snoc b c))))
	    (_                   (Nothing))))))