It's used to implement a generalization of assignment. If you have a special form that can tell you where something is stored, you can make macros to set it. E.g.

  > (set x '(a b c))
  (a b c)
  > (set (2 x) 'z)
  z
  > x
  (a z c)

which you can do in Common Lisp, and

  > (set ((if (coin) 1 3) x) 'y)
  y
  > x
  (y z c)

which you can't.

I am diliriously happy to see you here commenting about lisp. I grew up with your writing and your work and... I don’t know, I just wanted to express excitement.

Thanks for the new dialect!

Thanks for sharing, Paul.

Still knee-deep in the source. Gonna steal some of this.

What made you settle on (coin), is that a LISP trope? I flopped back and forth between naming it (coin) and (flip) in my own LISP before finally settling on (flip). I'd honestly like to divorce the name entirely from its physical counterpart.

I originally did call it flip, but then I took that name for the current flip:

  (def flip (f)
    (fn args (apply f (rev args))))

> I'd honestly like to divorce the name entirely from its physical counterpart.

How about (bit) or (randbit)

(randbit) isn't bad. My implementation also takes a float 0 <= x <= 1 to determine the probability of the outcome, so (bit) would probably be too ambiguous. I do like the brevity of a 4-letter function, though. A lot of my lisp coding is genetic and probabilistic so it gets used a lot.

dice?

Would be the result of (where (cadr x)) still be ((a b c) d)? Is where basically tracking the most recent traversal?

(where (cadr x)) would be ((b c) a).

where tells you what to set, and setting the cadr of (a b c) means setting the car of (b c).

Ah, now it's crystal clear.