#define private public - A package riddle, part III (22 Aug 2009)

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---+++ [[DefinePrivatePublic20090822PackageRiddlePart3][A package riddle, part III]] (22 Aug 2009)

<summary>
Lisp recently <a href="DefinePrivatePublic20090820PackageRiddlePart2">surprised me</a>
with an error message which I had not expected.
</summary>

<pre>
(defun test()
  (test_dialog))

(in-package :clausbrod.de)
(use-package :oli)

(sd-defdialog 'test_dialog
  :ok-action '(display "test_dialog"))
</pre>

Load the above code, run =(test)=, and you'll get:

<img src="%ATTACHURLPATH%/testdialog.png" alt="testdialog.png"  width="321" height="134"  />
<br clear="all" />

In <nop>CoCreate Modeling, the =sd-defdialog= macro automatically exports the name of the new
dialog (in this case, =test_dialog=) into the default package. Hence, you'd expect that
the function =(test)=, which is in the default package, would be able to call that dialog!

Astute readers (and <nop>CoCreate Modeling's Lisp compiler) will rightfully scold me for using 
=(in-package)= in the midst of a file. However, the error doesn't go away if you split up
the above code example into two files, the second of which then properly
starts with =(in-package)=. And in fact, the problem originally manifested itself in a 
multiple-file scenario. But to make it even easier for readers to run the test themselves,
I just folded the two files into one.

Lisp actually provides us with a subtle hint which I ignored so far: Did you notice 
that the complaint is about a symbol =#:TEST_DIALOG=, and not simply =TEST_DIALOG=? 

The =#:= prefix adds an important piece to the puzzle. Apparently, Lisp thinks 
that =TEST_DIALOG= is not a normal symbol,
but a so-called _uninterned symbol_. Uninterned symbols are symbols which don't 
belong to _any_ Lisp package - they are homeless. For details:
   * [[http://www.cs.cmu.edu/Groups/AI/html/cltl/clm/node110.html][Creating Symbols]] (from "Common Lisp the Language", 2nd edition)
   * [[http://gigamonkeys.com/book/programming-in-the-large-packages-and-symbols.html][Programming in the Large: Packages and Symbols]] (from Peter Seibel's excellent "Practical Common Lisp")
   * [[http://www.flownet.com/gat/packages.pdf][The Complete Idiot's Guide to Common Lisp Packages]]
   * [[http://osdir.com/ml/lisp.cl-gardeners/2006-05/msg00004.html][Potting Soil: Colons continued - uninterned symbols]]

Uninterned symbols are beasts which live in a slightly darker corner of Common Lisp, or 
at least you don't  run into them too often. And in our particular case, it isn't exactly obvious
how =TEST_DIALOG= turned into an uninterned symbol. We would have expected it to
be a symbol interned in the =clausbrod.de= package, which is where the dialog is defined!

Those who are still with me in this series will probably know where this is heading.
Anyway - [[DefinePrivatePublic20090828PackageRiddle4][next time]], we'll finally 
solve the puzzle!

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