#define private public - And... Action! (Part 2)

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---+++ [[DefinePrivatePublic20090908AndActionPart2][And... Action! (Part 2, 08 Sep 2009)]]

<summary>
You may have guessed it: The whole <tt>set_pers_context</tt> business in the 
<a href="DefinePrivatePublic20090831AndAction">first part</a> of this mini-series
was actually a red herring. I promise I won't mislead you this time -
and I'll even reveal the reason why the series is titled "And...Action!"
</summary>

<img src="%PUBURL%/Blog/DefinePrivatePublic20090831AndAction/extrude.png" style="float:right;" />
No, we don't need contrived constructs like <tt> (print extrude)</tt> to show that
=extrude= is somehow... different from all the other kids. All we need is a simple experiment.

First, enter =extrude= in 
<nop>CoCreate Modeling's user input line: The <tt>Extrude</tt> dialog 
unfolds in all its glory, and patiently awaits your input.

Now try the same with =print=: All you get is an uncooperative
<i>"Lisp error: The variable PRINT is unbound"</i>. How disappointing.

But then, the behavior for =print= is expected, considering the usual
[[http://www.psg.com/~dlamkins/sl/chapter03-02.html][evaluation rules]] for Common Lisp, 
particularly for symbols. As a quick reminder:
   * If the symbol refers to a variable, the value of the variable is returned.
   * If the symbol refers to a function and occurs in the first position of a list,
     the function is executed.

=extrude= &amp; friends belong to the symbol jet-set in <nop>CoCreate Modeling. For them, 
the usual evaluation rules for functions don't apply (pun intended).
Using [[http://www.cs.cmu.edu/Groups/AI/html/cltl/clm/node108.html][symbol properties]]
as markers, they carry a backstage pass and can party anywhere.
For members of the =extrude= posse, it doesn't really matter if you use them as an 
atom, in the first position of a list, or anywhere else: In all cases, the function which 
they refer to will be executed right away - by virtue of an extension to the evaluator 
which is unique to <nop>CoCreate Modeling's implementation of Common Lisp.

You can create such upper-class symbols yourself - using a macro called =defaction=.
This macro is also unique to <nop>CoCreate Modeling. Functions 
defined by =defaction= are called, you guessed it, <i>action routines</i>.

But why, you ask, would I want such a feature, particularly if I know that it breaks with 
established conventions for Lisp evaluation?

Well, precisely _because_ this feature breaks with the established rules.

[[DefinePrivatePublic20090919AndActionPart3][<i>To be continued...</i>]]


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r1.3 - 15 Jul 2010 - 06:50 - ClausBrod to top
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