gimp 関数の一部が一貫して値を返さないことを説明しているこのページを見つけたので、 do while ループを実装して、 を使用する前に関数が確実にペアを返すようにしcarました。それでもエラーError: ( : 1) car: argument 1 must be: pairが発生しますが、ペアが返されるまで関数を実行し続ける必要があるため、それがどのように可能かはわかりません。
(define (script-fu-scratchpad drawable)
  (let* ((imgHeight 0)
         (imgWidth)
         (bpp)
         (pixel))
    (set! imgHeight (gimp-drawable-height drawable))
    (do ()
        [(pair? imgHeight)]
      (set! imgHeight (gimp-drawable-height drawable)))
    (set! imgHeight (car imgHeight))
    (set! imgWidth (gimp-drawable-width drawable))
    (do ()
        [(pair? imgWidth)]
      (set! imgWidth (gimp-drawable-width drawable)))
    (set! imgWidth (car imgWidth))
    (set! bpp (gimp-drawable-bpp drawable))
    (do ()
        [(pair? bpp)]
      (set! bpp (gimp-drawable-bpp drawable)))
    (set! bpp (car bpp))
    (display bpp) (newline)
    (set! pixel (cons-array bpp 'byte))
    (aset pixel 0 150)
    (aset pixel 1 150)
    (aset pixel 2 150)
    (aset pixel 3 0)
    (gimp-drawable-set-pixel drawable (/ imgHeight 2) (/ imgWidth 2) bpp pixel)
    (gimp-context-set-background '(100 100 100))
    (define county 0)
    (define countx 0)
    (do ()
        [(= countx imgWidth)]
      (do ()
          [(= county imgHeight)]
        (gimp-drawable-set-pixel drawable county countx bpp pixel)
        (set! county (+ county 1)))
      (set! countx (+ countx 1)))))
GoZoner に応答して編集したところ、次のエラーが表示されました。Error: (:1) car: argument 1 must be: pair
(define
    (script-fu-scratchpad drawable)
    (let*
        ( 
            (imgHeight 0)
            (imgWidth 0)
            (bpp 0)
            (pixel 0)
        )
        (set! imgHeight (gimp-drawable-height drawable))    
        (set! imgWidth (gimp-drawable-width drawable))
        (set! bpp (gimp-drawable-bpp drawable))
        (do ()
            [(pair? bpp)]
            (set! bpp (gimp-drawable-bpp drawable))
        )
        (set! bpp (car bpp))
        (display bpp) (newline)
        (set! pixel (cons-array bpp 'byte))
        (aset pixel 0 150)
        (aset pixel 1 150)
        (aset pixel 2 150)
        (aset pixel 3 0)
        (gimp-drawable-set-pixel drawable (/ imgHeight 2) (/ imgWidth 2) bpp pixel)
        (gimp-context-set-background '(100 100 100))
        (define county 0)
        (define countx 0)
        (do ()
            [(= countx imgWidth)]
            (do ()
                [(= county imgHeight)]
                (gimp-drawable-set-pixel drawable county countx bpp pixel)
                (set! county (+ county 1))
            )
            (set! countx (+ countx 1))
        )
    )
)