A package riddle, part III (22 Aug 2009)

Lisp recently surprised me with an error message which I had not expected.

(defun test()
  (test_dialog))

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

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

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

In 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 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:

• Creating Symbols (from "Common Lisp the Language", 2nd edition)
• Programming in the Large: Packages and Symbols (from Peter Seibel's excellent "Practical Common Lisp")
• The Complete Idiot's Guide to Common Lisp Packages
• 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 - next time, we'll finally solve the puzzle!

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