(use-modules (rnrs bytevectors)
             (oop goops)
             (png)
             (png graphics))

(define (main args)
  (let* ((image  (png->scm))
         (color  (u8-list->bytevector '(255 255 255 0)))
         (circle (make <circle>
                   #:radius 200
                   #:center (make <point>
                              #:x 250
                              #:y 250)
                   #:color color)))

    (draw! image circle)

    (scm->png image)))

$ guix install guile-png