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Comment by rntz | Hacker News Reader

> 5. (where x)

> Evaluates x. If its value comes from a pair, returns a list of that pair and either a or d depending on whether the value is stored in the car or cdr. Signals an error if the value of x doesn't come from a pair.

> For example, if x is (a b c), > > > (where (cdr x)) > ((a b c) d)

That is one zany form.

1. How is this implemented?

2. What is the use of this?

3. What does (where x) do if x is both the car of one pair and the cdr of another, eg. let a be 'foo, define x to be (join a 'bar), let y be (join 'baz a), and run (where a).

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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.

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'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?

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 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!

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.

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