(defun normalize-wave-spec (wave)
  (etypecase wave
    (list wave)
    (number (list :frequency wave))
    (function (list :function wave))))

(defun generate-wave (waves array sample-rate)
  (loop with div = (/ (length waves))
        for spec in waves
        for wave = (normalize-wave-spec spec)
        for frequency = (getf wave :frequency 440)
        for function = (getf wave :function #'sine-wave)
        for amplitude = (min 1.0s0 (max 0.0s0 (getf wave :amplitude 1.0s0)))
        do (dotimes (i (length array))
             (incf (aref array i) (* div amplitude (funcall function i frequency sample-rate))))))