Path: utzoo!attcan!uunet!snorkelwacker!usc!cs.utexas.edu!hellgate.utah.edu!cdr.utah.edu!moore
From: moore%cdr.utah.edu@cs.utah.edu (Tim Moore)
Newsgroups: comp.lang.lisp
Subject: Re: STRUCTUREP
Message-ID: <1990Oct10.091722.4495@hellgate.utah.edu>
Date: 10 Oct 90 15:17:22 GMT
References: <753@forsight.Jpl.Nasa.Gov>
Organization: University of Utah CS Dept
Lines: 36

In article <753@forsight.Jpl.Nasa.Gov> gat@robotics.Jpl.Nasa.Gov (Erann Gat) writes:
>Is there any way in Common Lisp to test if an object is a structure?  There
>doesn't seem to be a structurep function.
>
>E.

Not portably.

If your Common Lisp has a fully integrated CLOS, then

(defun structurep (x) (typep (class-of x) 'structure-class))

should work.

If not...

Most implementations leave clues around that a given type is a
structure; a glance at a structured type's property list can be
instructive. From these clues you can construct your own
structure-type-p, e.g. in Utah Common Lisp you could write

(defun structure-type-p (x) (get x 'lisp::structure-info))

Then structurep is

(defun structurep (x) (structure-type-p (type-of x)))

Dick Waters' XP pretty printer uses this technique.

Also, all implementations probably have a structure-p predicate that is
not documented; you can root around with apropos and use it if you
dare.

Tim Moore                    moore@cs.utah.edu {bellcore,hplabs}!utah-cs!moore
"Ah, youth. Ah, statute of limitations."
		-John Waters