Writing structures to a file

[Robert W. Kerns <RWK at MIT-MC>]

From:     BYRON@MIT-ML
Date: Tue, 21 Jan 81 17:15:37 GMT
Original-Date: 01/21/81 13:15:37 EDT
Subject: Re: Writing structures to a file
    To: (BUG LISP) at MIT-ML
    What is the canonical way to save DEFVST structures in a file.
    Does code exist to parse the #{} syntax?

No code currently exists to parse the #{} syntax.  However, it
wouldn't be that hard to write, and if you do so you should let
us know so we can steal it.

Another way of dealing with the problem is to define a method
to output code to produce the objects.  For example:

;; Everything not handled specially, just print
(DEFMETHOD* (:OUTPUT OBJECT-CLASS) (SELF FILE)
  (PRINT SELF FILE))

;; Unaesthetic, but workable
(DEFMETHOD* (:OUTPUT PAIR-CLASS) (SELF FILE)
  (PRINC "(" FILE)
  (SEND (CAR SELF) ':OUTPUT FILE)
  (PRINC " . " FILE)
  (SEND (CDR SELF) ':OUTPUT FILE)
  (PRINC ") " FILE))


(DEFVST MYFROB SLOT-A SLOT-B)

(DEFMETHOD* (:OUTPUT MYFROB-CLASS) (SELF FILE)
  (PRINC "#.(CONS-A-MYFROB SLOT-A  '" FILE)
  (SEND (MYFROB-SLOT-A SELF) ':OUTPUT FILE)
  (PRINC " SLOT-B '" FILE)
  (SEND (MYFROB-SLOT-B SELF) ':OUTPUT FILE)
  (PRINC ")
" FILE))


and then to output stuff to the file, instead of using
(PRINT <stuff> <file>), do

(SEND <stuff> ':OUTPUT <file>).  This will print code to
the file to reproduce <stuff> when read back in.  If you don't
mind always seeing your objects looking like this, you can just do

(DEFMETHOD* (:PRINT-SELF MYFROB-CLASS) (SELF FILE LEVEL SLASHIFY-P)
  (IF (NOT SLASHIFY-P)				;PRINC case, do as usual
      (SEND-AS STRUCT-CLASS SELF FILE LEVEL SLASHIFY-P)
      ;; Otherwise we output a form to be evaluated to get our object
      (PRINC "#." FILE)
      (PRINT-OBJECT `(CONS-A-MYFROB SLOT-A ,(MYFROB-SLOT-A SELF)
				    SLOT-B ,(MYFROB-SLOT-B SELF))
		    LEVEL SLASHIFY-P FILE)))

Thus, PRINC will print using the #{} syntax, but PRINT will print using
the #.(CONS-A-MYFROB ....) syntax.