clojure.core/for examples

for is lazy and can be terminated

I'm iterating an integer vector twice checking for specific sum, but would like to stop once I find a first match. How would one do that?

(defn find-all-sums-to-2020 [expenses]
  (for [x expenses, y (rest expenses)
        :let [sum (+ x y)]
        :when (= sum 2020)]
    [x y]))(defn find-sum-to-2020 [expenses]
  (first (find-all-sums-to-2020 expenses)))

for is not a loop, but it is lazy, you can just stop consuming it
there's also the :while arg (edited)
what you are doing there is the normal way to stop it early

So (first) function will evaluate only the first result and therefore stop the inner for from continuing, is that what you are saying?

right, more precisely it never asks the inner for another value, so the value isn't calculated
that's how laziness works - you don't stop it from the outside, you just never force it to execute, so it doesn't

the distinction becomes important when you have chunking (so asking for one item might make it decide to calculate 32 items) - you aren't forcing it to stop, you're asking it to calculate

Start with the clojure.core/for example and the discussion of first and lazily consuming the first result. Clarify the example so it explains how taking the first match affects evaluation and notes the chunking caveat. Done means the example and explanation clearly answer how to stop after the first match.