Re: how to read "structs" from disk file ?

[Matthias Felleisen <matthias@cs.rice.edu>]

Here is about the only way to read and write structures: 

  (print-struct #t) 

  (define-struct posn (x y))

  (define (vector->posn v)
    (apply make-posn (cdr (vector->list v))))

  (with-output-to-file "foo.txt"
    (lambda ()
      (printf "~a~n" (list (make-posn 1 2) (make-posn 3 4)))))

  (define x (map vector->posn (with-input-from-file "foo.txt" read)))

That's what I do in a similar context (maintaining "Daddy's bank accounts"
for my kids :-). 

Explanation: It is impossible to read structures back properly because
define-struct is generative. That is, every evaluation of define-struct
creates a different class of structured data. Furthermore, reading may
happen in a context where the structure hasn't been defined, so read cannot
call make-s.

-- Matthias