Welcome to the CHICKEN Scheme pasting service

$ cat opt.scm
(module foo ()

(import chicken scheme)

(define foo
  (let ((buf '()))
    (lambda (#!optional bar)
      (if bar
          (set! buf (cons bar buf))
          buf))))

(define xxx (append (foo) '(1)))

)

$ csc -S opt.scm

Warning: at toplevel:
  (opt.scm:12) in procedure call to `append', expected argument #1 of type `list', but was given an argument of type `undefined'

(module foo ()

(import chicken scheme extras)

(define foo
  (let ((buf '()))
    (lambda ()
      (if (random 2)
          (set! buf '())
          buf))))

(define xxx (append (foo) '(1)))

)

(module foo ()

(import chicken scheme extras)

(define foo
  (let ((buf '()))
    (lambda ()
      (if (< (random 2) 1)
          (set! buf '())
          buf))))

(define xxx (append (foo) '(1)))

)