#define private public - A package riddle, part IV (28 Aug 2009)

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---+++ [[DefinePrivatePublic20090828PackageRiddle4][A package riddle, part IV]] (28 Aug 2009)

<summary>
I'll bore you just one more time with this: When executing <tt>(test)</tt> as defined
in the following code, Lisp claimed that <i>the function #:TEST_DIALOG is undefined</i>.
</summary>

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

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

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

In [[DefinePrivatePublic20090822PackageRiddlePart3][part 3]] of this mini-series,
we figured out that the <tt>#:</tt> prefix indicates an _uninterned symbol_ - and now
we can solve the puzzle!

Earlier, I had indicated that =sd-defdialog= automatically exports dialog
names into the default package. To perform this trick, somewhere in the bowels of
the =sd-defdialog= macro, the following code is generated and executed:

<pre>
(shadowing-import ',name :cl-user)  ;; import dialog name into cl-user package
(export ',name)                     ;; export dialog name in current package
(import ',name :oli)                ;; import dialog name into oli package
(export ',name :oli)                ;; export dialog name from the oli package
</pre>

As a consequence, the dialog's name is now visible in three packages:
   * The default package (=cl-user=)
   * Our Lisp API package (=oli=)
   * The package in which the dialog was defined (here: =clausbrod.de=)

This is quite convenient for <nop>CoCreate Modeling users - typically
mechanical engineers, not Lisp programmers. They don't want to deal with the 
intricacies of Lisp's package handling, but instead simply assume that the 
command (dialog) will be at their disposal whenever they need it.

Let's look up what the Common Lisp standard has to say on
[[http://l1sp.org/cl/shadowing-import][shadowing-import]]:

<blockquote><i>shadowing-import inserts each of symbols into package as an internal symbol, regardless of whether another symbol of the same name is shadowed by this action. If a different symbol of the same name is already present in package, that symbol is first <b>uninterned</b> from package.</i></blockquote>

That's our answer! With this newly-acquired knowledge, let's go through our 
code example one more and final time:

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

Upon loading this code, the Lisp reader will intern a symbol 
called =test_dialog= into the current (default) package. As =test_dialog= has not
been defined yet, the symbol =test_dialog= does not have a value; it's just
a placeholder for things to come.

<pre>
(in-package :clausbrod.de)
(use-package :oli)
</pre>

We're no longer in the default package, and can freely use =oli:sd-defdialog= without
a package prefix.

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

=sd-defdialog= performs <tt>(shadowing-import 'test_dialog :cl-user)</tt>,
thereby shadowing (hiding) _and_ uninterning the previously interned =test_dialog= symbol.

Until we re-evaluate the definition for =(test)=, it will still refer to the
old definition of the symbol =test_dialog=, which - by now - is a) still without
a value and b) uninterned, i.e. homeless.

Lessons learned:
   * Pay attention to the exact wording of Lisp error messages.
   * The Common Lisp standard is your friend.
   * Those Lisp package problems can be pesky critters.

The good news: If you follow a few rules of thumb, you'll probably never run into 
complex package problems like this. One such simple rule is to define your 
functions first before referring to them. So in our code example, defining
the dialog first before loading/defining the =(test)= function would have saved
us all that hassle.

Phew.

---

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