Question about Presentations

[RWK@fuji.ila.com (Robert W. Kerns)]

    Date: Fri, 12 Oct 90 14:50:08+0900
    From: kddlab!harl.hitachi.co.jp!mklein@uunet.uu.net
    I am trying to "present" an object to a screen with a label different
    from the usual printed representation of the object. When I mouse-select
    the label during an "accept" call, I want to have accept return the
    presented object.

    I tried the following simple function:

    (defun present-as (object label &key (stream *standard-output*))
      "Presents object to stream with given label"
      (dw:with-output-as-presentation (:single-box t
				       :type (type-of object)
				       :stream stream
				       :object object)
	(format stream "~a" label)))

    When I present a structure instance, typing (accept 'sys:expression) and 
    clicking on the screen representation returns the original instance.

    However, when I present an instance of flavor "frame" (a flavor I define
    in my code), I get the following curious behaviour:

    (1) using (accept 'frame) and clicking returns the original instance
    (2) using (accept 'sys:expression) and clicking returns the label!

    I want to use (accept 'sys:expression) because I have several different
    kinds of presented objects that I want all to be mouse-selectable.

    I imagine this is a failure in my understanding of presentations rather
    than a bug. Could someone suggest how I can achieve the behaviour I desire?

	    Thanks very much,

		    Mark Klein
		    Hitachi Ltd.
		    Advanced Research Lab
		    Hatoyama, Saitama 350
		    Japan
Konban wa, Kurain-san.

The problem is your use of (FORMAT STREAM "~A" LABEL), if label
is not a string, creates a presentation, because FORMAT calls
WRITE in that case.  This presentation will be a SYMBOL presentation,
and since SYMBOL is a subtype of EXPRESSION, it will satisfy your
call to ACCEPT.  (When multiple presentations could apply, the system
always chooses the innermost).

Here are a couple ways to do what you want:

(defun present-as (object label &key (stream *standard-output*))
  "Presents object to stream with given label"
  (dw:with-output-as-presentation (:single-box t
				   :type (type-of object)
				   :stream stream
				   :object object)
    (format stream "~a" (string label))))

Or more efficiently:

(defun present-as (object label &key (stream *standard-output*))
  "Presents object to stream with given label"
  (dw:with-output-as-presentation (:single-box t
				   :type (type-of object)
				   :stream stream
				   :object object)
    (write-string (string label) stream)))