json-pointer:   Referring to bits of JSON
1 JSON Pointers and their expressions
2 Parsing and rendering
3 Evaluation of JSON Pointers
4 License

json-pointer: Referring to bits of JSON

Jesse Alama <jesse@lisp.sh>

 (require json-pointer) package: json-pointer

JSON Pointer (RFC 6901) is a straightforward notation for referring to values embedded within a JSON document. Given a JSON object like this:


    "foo": 5,

    "bar": [ 1, 2 ]


the JSON Pointer expression /foo has the value 5, whereas the expressions /bar/0 and /bar/1 have the values 1 and 2, respectively. /baz and /bar/42 do not refer to anything.

Nothing terribly fancy is going on here: JSON Pointers are nothing more than Racket strings, and we work with jsexpr? values.

1 JSON Pointers and their expressions

This library uses two terms: JSON Pointer and JSON Pointer expression. Briefly, the first is a string, and the second is a list. A predicate for the first:


(json-pointer? x)  boolean?

  x : any/c

Returns #t iff x is a string that adheres to the syntax laid out in RFC 6901.

Next, a JSON Pointer expression is a represenation of the data encoded in JSON Pointers. Think of it as the “parse tree” of a JSON Pointer. We represent this data as a list (possibly empty) of strings (which are themselves possibly empty).


(json-pointer-expression? x)  boolean?

  x : any/c

Returns #t iff x is a list of strings.

There are essentially no constraints. The list might be empty. The strings in the list may themselves be empty, too. Duplicates are allowed.

2 Parsing and rendering

How do you parse JSON Pointers to get JSON Pointer expressions?


(parse-json-pointer p)  json-pointer-expression?

  p : json-pointer?

Given a JSON Pointer, produce a JSON Pointer expression.

What about going the other way around? Can you render a “semantic” JSON Pointer expression into a “syntactic” JSON Pointer? Yes, you can:


(expression->pointer expr)  json-pointer?

  expr : json-pointer-expression?

Given a JSON Pointer expression, produce a JSON Pointer.

3 Evaluation of JSON Pointers


(json-pointer-value jp doc)  jsexpr?

  jp : (or/c json-pointer-expression? json-pointer?)
  doc : jsexpr?

Given a JSON Pointer and a JSON document, evaluate the JSON Pointer within the document. The result is a jsexpr?, if all goes well.

(For the first argument both json-pointer?—a string—as well as a json-pointer-expression?—a list of strings—are allowed. When given a json-pointer?, it will be parsed into a json-pointer-expression?, with which the real computation takes place.)

If things don’t go well, json-pointer-value function throws an exception (of type exn:fail?). These are the conditions under which evaluation might go awry:

4 License

This library is offered under the GNU Lesser Public License (LGPL).