Kenneth Tilton  <kentilton@gmail.com> wrote:
+---------------
| Slobodan Blazeski wrote:
| > Mirko <mirko.vuko...@gmail.com> wrote:
| >> Is there a way to rewrite
| >>> (let ((a nil))
| >>    `(5 6 ,(when a a)))
| >> (5 6 NIL)
| >>
| >> I would like to get rid of the NIL.
...
| > What's the point of this, since a is always nil why don't you just use
| > (5 6) ?
| 
| The example was distilled from a real-world case in which a might not be 
| nil.
+---------------

I actually run into this quite frequently with macros that call
functions that use a SUPPLIED-P arg to distinguish explicit NIL
values from "missing" NIL values in &OPTIONAL and &KEY args.
The ,@(WHEN FOO-P (LIST FOO)) and ,@(WHEN FOO-P (LIST :FOO FOO))
idioms handle this nicly.


-Rob

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Rob Warnock			<rpw3@rpw3.org>
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