Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!think.com!barmar
From: barmar@think.com (Barry Margolin)
Newsgroups: comp.lang.lisp
Subject: Re: Problem with defstruct printing in Common Lisp
Message-ID: <1991Mar15.183935.16362@Think.COM>
Date: 15 Mar 91 18:39:35 GMT
References: <dsharma.669010518@kaffa>
Sender: news@Think.COM
Organization: Thinking Machines Corporation, Cambridge MA, USA
Lines: 36

In article <dsharma.669010518@kaffa> dsharma@kaffa.anu.oz.au (Dharmendra Sharma) writes:
>I would like to be able to print a defstruct object on more than 1
>stream i.e. on the screen and to a file. How can I do it with a
>predefined :print-function?

I have a feeling you're not describing your problem correctly, or you're
very confused.

To print something on more than one stream, you can call PRINT multiple
times, e.g.:

	(progn
	  (print my-object *standard-output*)
	  (print my-object file-stream))

If you want to print lots of stuff to both streams without having to double
all your PRINT forms, you can use a broadcast stream:

	(with-open-stream (bs (make-broadcast-stream *standard-output* file-stream))
	  (print my-object bs)
	  ...)

The fact that a defstruct type has a :PRINT-FUNCTION is immaterial.  The
:PRINT-FUNCTION specifies what the output looks like, but not where it
goes; the stream is a parameter to this function, and that's the only
stream it should perform any output to.  It's probably also a bad idea for
a :PRINT-FUNCTION to have *any* side effects other than output to the given
stream; however, CLtL doesn't specifically mention such a restriction (at
least not in the descriptions of :PRINT-FUNCTION and PRINT-OBJECT).


--
Barry Margolin, Thinking Machines Corp.

barmar@think.com
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