Hooking gambit's ##wr.

cb.ss

;; cb.ss

(defstruct ugly-struct (a b c))
(defstruct wonderful-struct (a b c))
(defmethod {:wr wonderful-struct}
  (lambda (self we obj)
    (with ((wonderful-struct a b c) self)
      (##wr-str we "🦄<")
      (##wr-str we " 🌈🌈🌈 ")
      (##wr-str we (string-append
                    "a: " (number->string a)
                    " b: " (number->string a)
                    " c: " (number->string a)))
      (##wr-str we " 🌈🌈🌈 ")
      (##wr-str we ">🦄"))))

(def us (ugly-struct 1 2 3))
(def ws (wonderful-struct 1 2 3))

(def (gerbil-##wr we obj)
  (let (wr (find-method (object-type obj) ':wr))
    (if wr
      (wr obj we obj)
      (##default-wr we obj))))

(##wr-set! gerbil-##wr)

;; > us
;; #<ugly-struct #2>
;; > ws
;; 🦄< 🌈🌈🌈 a: 1 b: 1 c: 1 🌈🌈🌈 >🦄🦄< 🌈🌈🌈 a: 1 b: 1 c: 1 🌈🌈🌈 >🦄
;; > 

Looks like in current Gerbil, the method has arguments (self we) not (self we obj)—indeed the obj was redundant.