0) I want to say that this is good work and that my criticisms are only an attempt to make it even better.  Thanks for doing this.

1) I don't understand what the point of exposing json-tokens is.  It seems to be useful only if you want to be able to read malformed JSON, which I can't see as a use case.  

I think it makes more sense to pass a port-or-generator directly to json-generator-read and keeping json-tokens behind the scenes.  Also, what is the default value of the argument?  If it's a port-or-generator as I suggest, it should be the value of (current-input-port).

2) A generator implies reading, so json-generator-read might as well just be json-generator (and the same for json-accumulator-write).

3) Why not have json-generator[-read] return single datums instead of pairs?  Just return one of the symbols start-array, end-array, start-object, end-object or null, or else a number, string, or boolean.  That eliminates garbage collection pressure.

4) Boolean is left off the list of things returned by json-generator[-read].

5) If you don't do 4, just for readability change the examples of pairs into dotted-pair (datum) notation rather than Scheme expressions that evaluate to the pairs.

6) The point should be made somewhere that leading whitespace is deleted, and that a #\x1e; before top-level value is also ignored to support RFC 7464.

7) An EOF object while a JSON object is being built should explicitly raise a json-error, whereas one between objects (possibly after some whitespace is read) 

8) The explanation of json-fold is not easy to understand.  Exactly how do values get added to the internal representation of an object or array?

9) There is no default for json-read.  The value of (current-input-port) would make sense, but see below.

10) It would be good if json-read could accept a json-generator, but unfortunately there is no way to discriminate between a json-generator and a string-or-char generator.  Given string ports as standard, maybe string-or-char generators aren't so important.

John Cowan          http://vrici.lojban.org/~cowan        xxxxxx@ccil.org
A few times, I did some exuberant stomping about, like a hippo auditioning
for Riverdance, though I stopped when I thought I heard something at
the far side of the room falling over in rhythm with my feet.  --Joseph Zitt