time.rkt

#lang racket

(define N 100000000)
(define K 50000000)

(time 
 (for/fold ([c 0]) ([i (in-range N)] #:break (not c))
   (if (= i K)
       #f
       (add1 c))))

(time 
 (let/ec exit
   (for/fold ([c 0]) ([i (in-range N)] #:break (not c))
     (if (= i K)
         (exit #f)
         (add1 c)))))