0-output.txt

1
120
==== but the following will fail ====
fact: contract violation
  expected: natural?
  given: -5
  in: the 1st argument of
      (-> natural? natural?)
  contract from: (function fact)
  blaming: /tmp/ex.rkt
   (assuming the contract is correct)
  at: /tmp/ex.rkt:3.18
==== this runs because the above exception is caught ====

ex.rkt

#lang racket

(define/contract (fact n)
  (-> exact-nonnegative-integer? exact-nonnegative-integer?)
  (if (zero? n)
      1
      (* n (fact (sub1 n)))))

(displayln (fact 0))
(displayln (fact 5))
(displayln "==== but the following will fail ====")
(displayln (with-handlers ([exn:fail:contract? exn-message])
             (fact -5)))
(displayln "==== this runs because the above exception is caught ====")