checkerboard.rkt

#lang slideshow

(define (empty-square color [sidelen 70])
  (let ([sq (filled-rectangle sidelen sidelen)])
    (colorize sq color)))

(let ([colors #("black" "red")])
  (for/fold ([rows (blank)]) ([r 8])
    (vc-append rows (for/fold ([row (blank)]) ([c 8])
                      (let* ([idx (remainder (+ c r) 2)]
                             [clr (vector-ref colors idx)]
                             [sq (empty-square clr)])
                        (hc-append row sq))))))