(module opcache mzscheme
  (require (lib "list.ss"))
  (provide (all-defined))
  
  ; cache-op : proc? path? -> proc?
  (define (cache-op op cache-path)
    (define the-ht (make-hash-table 'equal))
    (define (write-cache!)
      (with-output-to-file cache-path
        (lambda ()
          (write (hash-table-map the-ht list)))
        'truncate/replace))
    (for-each (lambda (k+v)
                (hash-table-put! the-ht (first k+v) (second k+v)))
              (if (file-exists? cache-path)
                  (with-input-from-file cache-path read)
                  empty))                    
    (lambda key
      (hash-table-get the-ht key
                      (lambda ()
                        (define value (apply op key))
                        (hash-table-put! the-ht key value)
                        (write-cache!)
                        value)))))