[no subject]

[mike%acorn@oak.lcs.mit.edu (mike@gold-hill.com any day now)]

    Date:	  Tue, 5 Apr 88 11:29:13 PDT
    From:     POTHIERS%TUVA.SAINET.MFENET@NMFECC.ARPA
    
    Subject: Defstructs
    Date:    Tue,  5-APR-1988 10:55 MST
    X-VMS-Mail-To: @COMMON-LISP
    
    I'm trying to create structures on the fly. I have a problem with:
    
    (defun test (&aux x)
      (defstruct foo a b c)
      (setf x (make-foo))
      (setf (foo-a x) 1)
      )
    
    When TEST is interpreted, all is fine. When it is compiled the
    function (foo-a) is undefined since it isn't created by defstruct 
    until run time. In my actual code FOO is a symbol that's build based
    upon some args to TEST, therefore I can't simply put the defstruct at 
    the top level.  Any ideas??

There are a few problems here. First, to compile (setf (foo-a x) 1)
you would have to have the structure predefined, since "setters" are
not even required to be callable functions by common lisp. 

I have to ask why you have to do this in this fashion. It appears
you need to dynamically create named record "types". The way to do this
is not necessarily to evaluate a defstruct at run time. 
I'd do it by inventing a meta-structure type.

(defstruct record 
   (name (gensym) :type symbol)
   (slots #() :type vector))

or something like that. Then I'd use the copier to create new
instances, the constructor to create the initial "prototype" instance,
etc. You'd have to create your own predicates for recognizing
them and relating them.

This is unfortunately, exactly the way you'd have to do it
in a statically typed programming language.

...mike beckerle
Gold Hill