for #2058, issue discovered by Sam

pkgs/racket-test/tests/json/json.rkt

@@ -1,6 +1,9 @@
 #lang at-exp racket/base
 
+;; Mathias, added test for contracts on read-json 
+
 (require json racket/string tests/eli-tester)
+(require racket/port)
 
 (define T string-append)
 
@@ -114,6 +117,9 @@
         (string->jsexpr @T{ " \b\n\r\f\t\\\"\/ " }) => " \b\n\r\f\t\\\"/ "
         (string->jsexpr @T{ "\uD834\uDD1E" }) => "\U1D11E"
         (string->jsexpr @T{ "\ud834\udd1e" }) => "\U1d11e"
+	;; INPUT PORT is optional 
+	(with-input-from-string "[]" read-json)
+	=> (parameterize ((json-null '())) (json-null))
         ;; EOF detection
         (for/list ([je (in-port read-json
                                 (open-input-string

racket/collects/json/main.rkt

@@ -3,7 +3,9 @@
 ;; Roughly based on the PLaneT package by Dave Herman,
 ;;   Originally released under MIT license.
 
-;; edited: Matthias, organization in preparation for pretty-print
+;; edited:
+;; -- Matthias, organization in preparation for pretty-print
+;; -- Matthias, contracts 
 
 ;; -----------------------------------------------------------------------------
 ;; DEPENDENCIES
@@ -30,8 +32,8 @@
          #:encode (or/c 'control 'all)) ;; 'control
         any)]
   [read-json
-   (->* (input-port?)
-        (#:null any/c) ;; (json-null)
+   (->* ()
+        (input-port? #:null any/c) ;; (json-null)
         any)] ;; jsexpr?
   [jsexpr->string
    (->* (any/c) ;; jsexpr? but dependent on #:null arg