#lang racket

(define mh (make-hash))
(hash-set! mh 1 "one")
(hash-set! mh 2 "two")

(define (fail-hash val)
  (printf "no such key: ~a\n " val))

(hash-ref mh 1)
(hash-ref mh 10 (lambda () (fail-hash 10)))

;; output:
;; "one"
;; no such key: 10