A package riddle, part IV (28 Aug 2009)

(defun test()
  (test_dialog))

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

(sd-defdialog 'test_dialog
  :ok-action '(display "test_dialog"))

In part 3 of this mini-series, we figured out that the #: 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:

(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

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 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 shadowing-import:

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 uninterned from package.

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

(defun test()
  (test_dialog))

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.

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

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

(sd-defdialog 'test_dialog
  :ok-action '(display "test_dialog"))

sd-defdialog performs (shadowing-import 'test_dialog :cl-user), 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.