[pro] The Best Examples of "Code is Data"

[Scott McKay]

I recently wrote a macro that expands forms like this into
an airline flight itinerary.  It's used in test code, in which
we don't want to hard-wire the itineraries because we want to
be able to test multiple airlines.

What's interesting about this is that it's actually a constraint
language where the variables (like $A, $B, $DATE) get filled in
and tested to see if they meet the constraints.  This requires
Prolog-like backtracking, and the final example uses a lazy-
evaluation-like technique so that $S1 can be "bound" and used
in the same clause.

(Heh, trying doing this in Java.)


;; Binds $S1, $S2, $S3 to specific flights
(with-test-itinerary
   (($S1 (:AC 8334 "2007-11-13" :BOS :CHI))
    ($S2 (:AC 8470 "2007-11-13" :CHI :SFO))
    ($S3 (:AC 1156 "2007-11-14" :SFO :BOS)))
 (values $S1 $S2 $S3))

;; Makes a 3-segment itinerary from A to B to C,
;; with the third segment one day later than the first two
(with-test-itinerary
   (($S1 (:hosted :* $DATE $A $B))
    ($S2 (:hosted :* $DATE $B $C) )
    ($S3 (:hosted :* (plus-days $DATE 1) $C $A)))
 (values $S1 $S2 $S3))

;; Makes a 3-segment itinerary from A to B to C,
;; with the third segment one day later than the first two,
;; and the first two segments have the same flight number
(with-test-itinerary
   (($S1 (:hosted $F1 $DATE $A $B))
    ($S2 (:hosted $F1 $DATE $B $C))
    ($S3 (:hosted $F2 (plus-days $DATE 1) $C $A)))
 (values $S1 $S2 $S3))

;; Makes a 3-segment itinerary with different flight numbers
;; for each segment
(with-test-itinerary
   (($S1 (:hosted $F1 :* :* :*))
    ($S2 (:hosted $F2 :* :* :*))
    ($S3 (:hosted $F3 :* :* :*)))
 (values $S1 $S2 $S3))

;; Makes a 4-segment itinerary from A to B to C to D,
;; the last two segments being two days later, and the
;; final airport satisfies the predicate "far away from C"
(with-test-itinerary
   (($S1 (:hosted :* $DATE :* $A))
    ($S2 (:hosted :* $DATE $A $B) )
    ($S3 (:hosted :* ($DATE2 (plus-days $DATE 1)) $B $C))
    ($S4 (:hosted :* $DATE2 (far-away-airport $C) $D) ))
 (values $S1 $S2 $S3 $S4))

;; See if you can figure this one out
(with-test-itinerary
   (($S1 (($C1 (and (segment-hosted-carrier-p $S1) 
                    (OA-operated-p $S1)
                    $C1)) :* :* :* :*))
    ($S2 (($C2 (and (segment-hosted-carrier-p $S2)
                    (not (OA-operated-p $S2))
                    $C2)) :* :* :* :*)))
 (values $S1 $S2))



On Sep 5, 2010, at 8:24 AM, Kazimir Majorinc wrote:

> As first, good luck with this list!
> 
> I'm in search for best examples of "code is data" paradigm in Common
> Lisp. For most CL-ers, it probably means "macros", but eval, backquote,
> anything that processes the code as data is of interest. As "best" I
> think on the most surprising, powerful, sophisticated examples, not
> necessarily of a pedagogical value.
> 
> Imagine that someone invited you to write the presentation "Five best CL
> macros ... I seen" or "Five best CL macros ... I wrote." What would you
> chose and why?
> 
> Kazimir Majorinc
> 
> 
> 
> 
> 
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