Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!gatech!udel!princeton!phoenix!eliot
From: eliot@phoenix.Princeton.EDU (Eliot Handelman)
Newsgroups: comp.lang.lisp
Subject: Re: Question about INTERN
Message-ID: <5783@idunno.Princeton.EDU>
Date: 29 Jan 91 18:15:09 GMT
References: <1991Jan29.055536.1523@magnus.ircc.ohio-state.edu>
Sender: news@idunno.Princeton.EDU
Organization: Princeton University, New Jersey
Lines: 52

In article <1991Jan29.055536.1523@magnus.ircc.ohio-state.edu> murthy@magnus.ircc.ohio-state.edu (Murthy S Gandikota) writes:
;
;
;(setf my-house (make-house :rooms 4 :area 100))
;
;(setf any-house my-house)
;
;(defun get-slot-value (obj slot)
;;;this function returns the value of the slot in obj
;(PROG (obj1 value)
;	(setf obj1 obj)
;	(if (equal (type-of obj) 'symbol) (setf obj1 (eval obj)))
;	(setf value (eval `(,(intern (format nil "~a-~a" (type-of obj1) slot)) 
;		,obj)))
;	(RETURN value)))
;
;(get-slot-value my-house 'rooms) returns error


The problem is that TYPE-OF doesn't guarantee returning the most specific
type of an object, so if this used to work and now doesn't it's almost
certainly because your two TYPE-OF's aren't behaving consistently.
Common Lisp says that (TYPE-OF any-house) might return T.

There's no portable way to find out what the type of an object is.
But if you know the name of the slot then you might also know
the name of the object, which could reduce your overhead.

There's also no portable way to get the name of the structure slot-accessor,
given the structure and the slot. If I know that the structure is HOUSE
and the slot I'm looking for is ROOM, it would be nice to be able to 
get a handle on the accessor without having to build its name yourself,
but it can't be done. If you do go the way of concatenation and interning,
it's better to do this:

(defun build-room-slot-accessor (slot)
   (intern (concatenate 'string "HOUSE-" (symbol-name slot))))

Then rewrite your function as a macro:

(defmacro get-slot-value (obj slot)
  ;;this function returns the value of the slot in obj
  `(,(build-room-slot-accessor slot) ,obj)))

I don't understand what this is supposed to do:
;(if (equal (type-of obj) 'symbol) (setf obj1 (eval obj)))

If (setq a <a room>) and (setq b a) and (setq c b) then

A ==> <a room>
B ==> <a room> 
C ==> <a room> ;; that is, the same room.