I remember seeing tricks for cons counting, profiling (http://arclanguage.org/item?id=11556, another story about forgetting the existence of a key tool), tracing, visualizing macroexpansion.. the list goes on. Lots of stuff in the arc2 branches never made it to arc3. I'm certain there's gems there.

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I posted a few at http://arclanguage.org/item?id=11111

REPL tricks at http://arclanguage.org/item?id=11103

Then there's defgeneric/defmethod. I'm still proud of the writeup on how I rediscovered defgeneric for myself: http://arclanguage.org/item?id=11779

And serialize/unserialize/persisted: http://arclanguage.org/item?id=11865

I like these one-liners:

  (= be is)
  (= neither nor)
  (= redef =)

  (= const =)

I don't want to use init (http://arclanguage.org/item?id=11103) because I do want the value to be updated when I reload the file. init is for global state in data structures which shouldn't be reset when I reload the file, and const is for global parameters that influence the behavior of my app. Those I often do want to update on reload.

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Another recent macro encapsulates a common pattern for initialization code:

  (mac firsttime(place . body)
    `(unless ,place
       ,@body
       (set ,place)))

A workhorse of readwarp: pick a random feed from a set and check if it satisfies certain properties (e.g. has an unread item, is recent, is well-cleaned, etc., etc.)

  (mac findg(generator test)
    (w/uniq (ans count)
      `(ret ,ans ,generator
         (let ,count 0
           (until (or (,test ,ans) (> (++ ,count) 10))
              (= ,ans ,generator))
           (unless (,test ,ans)
             (= ,ans nil))))))

  (def randpos(l)
    (when l
      (l (rand:len l))))