And... Action! (31 Aug 2009)

Duck and cover! Another mini-series approaching!

My apologies to users of CoCreate Modeling who tried to find some meat for them in the package riddle series for them - there wasn't any, as that series was strictly meant for Lisp geeks. Sorry!

This new series covers programming fundamentals as well. If you ever wanted to understand how Common Lisp functions like print and CoCreate Modeling commands such as extrude differ and how they interact, you've come to the right place.

Reader highway45 recently came up with a very interesting observation (abridged and translated from German):

Usually, I call a dialog like this: (set_pers_context "Toolbox-Context" function)

Or like this: function

As soon as I add parentheses, however, the "ok action" will be called: (function)

extrude.png When highway45 talks of "functions" here, he actually means commands like extrude or turn. So, (set_pers_context "Toolbox-Context" extrude)? Really? Wow!

set_pers_context is an internal CoCreate Modeling function dealing with how UI elements for a given command are displayed and where. I was floored - first, by the fact that an end user found a need to call an internal function like this, and second, because that magic incantation indeed works "as advertised" by highway45. For example, try entering the following in CoCreate Modeling's user input line:

(set_pers_context "Toolbox-Context" extrude)

Lo and behold, this will indeed open the Extrude dialog, and CoCreate Modeling now prompts for more input, such as extrusion distances or angles.

What's so surprising about this, you ask? If you've used CoCreate Modeling for a while, then you'll know that, as a rule of thumb, code enclosed in parentheses won't prompt for more input, but will instead expect additional parameters in the command line itself.

For example, if you run (extrude) (with parentheses!) from the user input line, Lisp will complain that the parameter "DISTANCE is not specified". But in highway45's example, there clearly was a closing parenthesis after extrude, and yet the Extrude command started to prompt!

So is set_pers_context some kind of magic potion? Try this:

  (print extrude)

The Extrude dialog opens and prompts for input! Seems like even print has magic powers, even though it's a plain ol' Common Lisp standard function!

Well, maybe there is something special about all built-in functions? Let's test this out and try a trivial function of our own:

  (defun foobar() 42)
  (foobar extrude)

Once more, the dialog opens and awaits user input!

So maybe it is neither of set_pers_context, print or foobar that is magic - but instead extrude. We'll tumble down that rabbit hole next time.

To be continued...



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