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From: kthompso@ptolemy.arc.nasa.gov (Kevin Thompson)
Newsgroups: comp.lang.lisp
Subject: Re: shadowing defstruct print-function
Message-ID: <13268@ptolemy-ri.arc.nasa.gov>
Date: 31 May 91 22:11:11 GMT
References: <1991May31.183953.29579@jpl-devvax.jpl.nasa.gov> <EB.91May31142722@watergate.lucid.com>
Sender: usenet@ptolemy-ri.arc.nasa.gov
Organization: NASA-Ames Research Center, Moffett Field, CA
Lines: 37

In article <EB.91May31142722@watergate.lucid.com> eb@lucid.com (Eric Benson) writes:
>In article <1991May31.183953.29579@jpl-devvax.jpl.nasa.gov> charest@ai-cyclops.jpl.nasa.gov (Len Charest) wrote:
>> Is it possible to shadow the print-function used to print a
>> structure during a call to FORMAT? For example, suppose we have a
>> structure FOO that prints using the print-function PRINT-FOO, i.e.,:
>> (format t "~a" foo)
>> => #<FOO>

>(defstruct (foo (:print-function print-foo))
>  <etc>)
>
>(defvar *print-foo-hook*
>  #'(lambda (foo stream depth)
>      <print foo the normal way>))
>
>(defun print-foo (foo stream hook)
>  (funcall *print-foo-hook* foo stream hook))
>
>Then your local redefinition is:
>
>(let ((*print-foo-hook*
>	#'(lambda (foo stream depth)
>	    (declare (ignore foo depth))
>	    (format stream "{Foo}"))))
>  (format t "~a" foo))

Well this sounds great, and would be.  But is there an
implementation-independent way (or even implementation-dependent :) to 
"<print foo the normal way>"?  I poked around at great length in both Franz
& Lucid's system packages, and didn't find in either case something like
"generic-defstruct-printer", which took any struct and printed it out with
the #S (?) format.  Does such a beast exist?  It must in some ways, but I
don't know that it's accessible to the user.

-- Kevin Thompson
--
kthompso@ptolemy.arc.nasa.gov     NASA-Ames Research Center, Moffett Field, CA