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From: eb@lucid.com (Eric Benson)
Newsgroups: comp.lang.lisp
Subject: Re: shadowing defstruct print-function
Message-ID: <EB.91May31142722@watergate.lucid.com>
Date: 31 May 91 20:27:22 GMT
References: <1991May31.183953.29579@jpl-devvax.jpl.nasa.gov>
Sender: usenet@lucid.com
Organization: Lucid, Inc., Menlo Park, CA
Lines: 40
In-Reply-To: charest@ai-cyclops.jpl.nasa.gov's message of 31 May 91 18:39:53 GMT

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>
> Now I want to locally redefine PRINT-FOO as in:
> (flet ((print-foo (foo stream depth)
>          (declare (ignore foo depth))
>          (format stream "{Foo}")))
>   (format t "~a" foo))
> => {Foo}
> execpt that FLET only has lexical scope and the ~a FORMAT directive
> calls PRINT-FOO at a nested level where the global definition is
> active. So #<FOO> will really be printed. Any pointers?

The simplest way to do this is:

(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))

eb@lucid.com 	           	 	Eric Benson
415/329-8400 x5523                      Lucid, Inc.
Telex 3791739 LUCID                     707 Laurel Street
Fax 415/329-8480                        Menlo Park, CA 94025