Welcome to the CHICKEN Scheme pasting service

(require-extension coops)
(define-class <a>)
 (define-method (foo (i <a>))
   (print "foo v1"))

(define obj (make <a>))
(foo obj)

(define-class <a>)
 (define-method (foo (i <a>))
   (print "foo v2"))

(foo obj)

;; Output:
;; foo v1
;; foo v1 <- this is OK

;; Now, same with modules:

(module a
 *
 (import chicken scheme)
 (use coops)
 (define-class <a>)
 (define-method (foo (i <a>))
   (print "foo v1"))
 )

(import a)

(define obj (make <a>))
(foo obj)

(module a
 *
 (import chicken scheme)
 (use coops)
 (define-class <a>)
 (define-method (foo (i <a>))
   (print "foo v2"))
 )

(foo obj)

;; Output:
;; foo v1
;; Error: (foo) no method defined for given argument classes: (#<coops standard-class `<a>'>)