How do I use :iterate in loop?

[uvtc]

What is the purpose of :iterate in a general purpose loop, and how do I use it?

If I do:

(loop [n :range [1 4]
       m :iterate (fn [x] (* x x))]
  (pp (string n "--" m)))

it forever loops on n = 1 with m being a different function object each time. What's going on there?

Thanks!

[pepe]

Well I am not sure about your usecase, but there is a simple example:

(var a 0) (loop [i :iterate (and (> 10 a) (++ a))] (print i))

Do not forget it is a macro, so expression is like body for do.

[uvtc]

Thanks, @pepe . As for a use case, suppose I want to generate a list of numbers that keep doubling, like 2, 4, 8, 16, ... up until some max-val.

I think I understand that you can use :iterate in a loop as a binding instead of :range.

So, to get that doubling-number list, I tried this:

(var n 2)

(defn foo
  [x]
  (* x 2))

(loop [n :iterate (foo n)
       :while (< n 1000)]
  (pp n))

but it's just sticking at n = 4 and endlessly looping. How do I get the variable in the loop to update to the new value of (foo n) each time through?

[sogaiu]

I think there may be an issue with at least one of the uses of n in the loop "head" (see the docstring of loop for meaning of "head").

Perhaps the following is closer to what you want?

(var n 2)

(defn foo [x]
  (* x 2))

(loop [i :iterate (foo n) 
       :while (< n 10)
       :after (set n i)]
  (pp i))

[uvtc]

Hm. So, the :iterate sets i (local to the loop) for each iteration of the loop, whereas we update n ourselves with :after each time through.

[uvtc]

I think I was confused because I was expecting :iterate to work like Clojure's iterate function. But Janet's loop :iterate works like, "tell me how to compute the next value for this loop".

I see that, for this use case, I might instead write:

(var n 2)

(defn foo
  [x]
  (* x 2))

(while (< n 1000)
  (pp n)
  (set n (foo n)))

# or

(for _ 1 10
  (pp n)
  (set n (foo n)))