transcendental-lisp/lisp/finance/interest-compounder.lisp

(load "../lang/dlambda.lisp")

(let ((static))

  (setq static
    (eval
      (dlambda

        (:percent-of-number (n percentage)
          (if (> percentage 0)
            (/ (+ (* n percentage) 50) 100)
            (/ (- (* n percentage) 50) 100))))))

  (defun interest-compounder (initial-principal initial-interest-rate)
    (let ((private) (public)
          (principal initial-principal)
          (interest-rate initial-interest-rate)
          (years-passed 0))

      (setq private
        (eval
          (dlambda

            (:add-years (years)
              (if (> years 0)
                (setq years-passed (+ years-passed years))))

           (:compound-interest (years)
              (if (> years 0)
                (begin
                  (setq principal
                    (+ principal
                      (call static :percent-of-number principal interest-rate)))
                  (call private :compound-interest (- years 1))))))))

      (setq public
        (eval
          (dlambda

            (:get-years-passed ()
              years-passed)

            (:get-principal ()
              principal)

            (:get-interest-rate ()
              interest-rate)

            (:set-interest-rate (new-interest-rate)
              (setq interest-rate new-interest-rate))

            (:make-contribution (contribution)
              (setq principal (+ principal contribution)))

            (:move-forward-years (years)
              (begin
                (call private :compound-interest years)
                (call private :add-years years)))))))))