Newsgroups: comp.lang.lisp
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!think.com!barmar
From: barmar@think.com (Barry Margolin)
Subject: Re: shadowing defstruct print-function
Message-ID: <1991Jun1.203541.23931@Think.COM>
Sender: news@Think.COM
Reply-To: barmar@think.com
Organization: Thinking Machines Corporation, Cambridge MA, USA
References: <1991May31.183953.29579@jpl-devvax.jpl.nasa.gov> <EB.91May31142722@watergate.lucid.com> <13268@ptolemy-ri.arc.nasa.gov>
Date: Sat, 1 Jun 91 20:35:41 GMT
Lines: 30

In article <13268@ptolemy-ri.arc.nasa.gov> kthompso@ptolemy.arc.nasa.gov (Kevin Thompson) writes:
>Well this sounds great, and would be.  

For what the original poster requested, it *is* great.

>					But is there an
>implementation-independent way (or even implementation-dependent :) to 
>"<print foo the normal way>"?  

No, there's no portable way to print a structure the normal way, except by
writing a structure-specific function that has the structure slot names
hard-coded into it, e.g.

(defun print-foo (object stream depth)
  (if (< depth *print-level*)
      (format stream "#S(~S :A ~S :B ~S)"
	      'foo (foo-a object) (foo-b object))
      (write-char #\# stream)))

There isn't necessarily even an implementation-dependent way; an
implementation might define the above style of function as part of the
DEFSTRUCT expansion.  Since many implementations provide a way to discard
structure slot information at run time, a generic structure printer may not
be feasible.

-- 
Barry Margolin, Thinking Machines Corp.

barmar@think.com
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