Welcome to the CHICKEN Scheme pasting service

(import (chicken pathname))

(define home-dir (make-parameter "/home"))
(define username (make-parameter "mario"))
(define user-dir (make-parameter (make-pathname (home-dir) (username))))

(print "### With SRFI-39 parameters:")
(print "user-dir before rebinding home-dir: " (user-dir))
(home-dir "/mnt/storage/home")
(print "user-dir after rebinding home-dir: " (user-dir))

(print "----------------------------------------------------")

(define-syntax define-lazy-parameter
  (syntax-rules ()
    ((_ var value)
     (define var
       (let ((state (lambda () value)))
         (lambda args
           (if (null? args)
               (state)
               (let ((val (car args)))
                 (set! state (lambda () val))
                 val))))))))

(define-lazy-parameter home-dir "/home")
(define-lazy-parameter username "mario")
(define-lazy-parameter user-dir (make-pathname (home-dir) (username)))

(print "### With lazy parameters:")
(print "user-dir before rebinding home-dir: " (user-dir))
(home-dir "/mnt/storage/home")
(print "user-dir after rebinding home-dir: " (user-dir))

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

$ csi -s lazy-parameters.scm 
### With SRFI-39 parameters:
user-dir before rebinding home-dir: /home/mario
user-dir after rebinding home-dir: /home/mario
----------------------------------------------------
### With lazy parameters:
user-dir before rebinding home-dir: /home/mario
user-dir after rebinding home-dir: /mnt/storage/home/mario