(define gen-current-default-size (make-parameter 3))

;; simplified to illustrate
(define (gen-char) #\a)

(define gen-string-of
  (case-lambda
    (()    (gen-string-of (gen-current-default-size) gen-char))
    ((gen) (gen-string-of (gen-current-default-size) gen))
    ((size gen)
     (with-output-to-string
       (do ((i 0 (add1 i)))
           ((>= i size))
         (display (gen)))))))



;; error
Error: call of non-procedure: #<unspecified>

        Call history:

        <eval>    [gen-string-of] (>= i size)
        <eval>    [gen-string-of] (display (gen))
        <eval>    [gen-string-of] (gen)
        <eval>    [gen-string-of] (doloop4588 (add1 i))
        <eval>    [gen-string-of] (add1 i)
        <eval>    [gen-string-of] (>= i size)
        <eval>    [gen-string-of] (display (gen))
        <eval>    [gen-string-of] (gen)
        <eval>    [gen-string-of] (doloop4588 (add1 i))
        <eval>    [gen-string-of] (add1 i)
        <eval>    [gen-string-of] (>= i size)
        <eval>    [gen-string-of] (display (gen))
        <eval>    [gen-string-of] (gen)
        <eval>    [gen-string-of] (doloop4588 (add1 i))
        <eval>    [gen-string-of] (add1 i)
        <eval>    [gen-string-of] (>= i size)   <--

;; I invoked it with:

(gen-string-of)