This is great. I think we should include a type for jsexpr? even if it doesn't have the predicate we would want.

@samth Alright, updated to include jsexpr?.

@AlexKnauth commented about a possible problem, but the comment seems to have disappeared. He said:

Won't something like this break it?:
typed.rkt:

#lang typed/racket
(require typed/json)
(require/typed "untyped.rkt" [change-null (-> Void)])
(change-null)
(ann (string->jsexpr "null") JSExpr) ; #<procedure:not-a-jsexpr>

untyped.rkt:

#lang racket
(provide change-null)
(require json)
(define (not-a-jsexpr) "whatever")
(define (change-null)
  (json-null not-a-jsexpr))

However, just to reply to Alex, I think that will produce a contract error in string->jsexpr. So it doesn't seem like a problem to me.

I took it away because I realized it wouldn't be a problem because it would produce a contract error.

@AlexKnauth I think it's useful to have thought about it, so thanks! Next time, feel free to just add another comment.