#lang scheme/base
(require mzlib/serialize
         mzlib/contract)  

(define-serializable-struct internal-file-box (path))
(define file-box? internal-file-box?)

(define (file-box path default)
  (define fb (make-internal-file-box path))
  (unless (file-box-set? fb)
    (file-box-set! fb default))
  fb)

(define (file-box-set? fb)
  (with-handlers ([exn? (lambda _ #f)])
    (file-unbox fb)
    #t))

(define (file-unbox fb)
  (deserialize (call-with-input-file (internal-file-box-path fb) read)))
(define (file-box-set! fb v)
  (with-output-to-file (internal-file-box-path fb) (lambda () (write (serialize v)))
    #:exists 'replace))

(provide/contract 
 [file-box? (any/c . -> . boolean?)]
 [file-box (path? serializable? . -> . file-box?)]
 [file-unbox (file-box? . -> . serializable?)]
 [file-box-set? (file-box? . -> . boolean?)]
 [file-box-set! (file-box? serializable? . -> . void)])