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From: jeff@aiai.ed.ac.uk (Jeff Dalton)
Newsgroups: comp.lang.lisp
Subject: Re: STRUCTUREP
Message-ID: <3533@skye.ed.ac.uk>
Date: 12 Oct 90 14:01:43 GMT
References: <753@forsight.Jpl.Nasa.Gov> <27133D6E.715C@marob.masa.com>
Reply-To: jeff@aiai.UUCP (Jeff Dalton)
Organization: AIAI, University of Edinburgh, Scotland
Lines: 23

In article <27133D6E.715C@marob.masa.com> cowan@marob.masa.com (John Cowan) writes:
>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.

>No, there isn't, and that's intentional.  Some of the Common Lisp standard
>types, notably STREAM, READTABLE, RANDOM-STATE, PATHNAME, and PACKAGE
>may be implemented using the structure mechanism, but this fact is hidden
>from the user.  A STRUCTUREP function would be unable to discriminate
>between these implementation-defined structures and user-defined ones.

So?  Is that supposed to be a bad thing (that some built-in types
might be structures)?

Note that in Common Lisps with an integrated CLOS (which is how
they'll probably all be after a while) you *can* find out if a 
type is a structure.

>There's very little you can say about a structure qua structure, anyway.
>The names of the {access, constructor} functions aren't deducible from the
>outside, given that the default names can be overridden at DEFSTRUCT time.

That's why you need more than STRUCTUREP.