JSON

8.8

JSON

Eli Barzilay
and Dave Herman

package: base

This library provides utilities for parsing and producing data in the JSON data exchange format to/from Racket values. See the JSON web site and the JSON RFC for more information about JSON.

1 JS-Expressions

procedure
(jsexpr? x [#:null jsnull]) → boolean?
x : any/c
jsnull : any/c = (json-null)

Performs a deep check to determine whether x is a jsexpr.

This library defines a subset of Racket values that can be represented as JSON strings, and this predicates checks for such values. A JS-Expression, or jsexpr, is one of:

the value of jsnull, 'null by default, which is recognized using eq?
boolean?
string?
(or/c exact-integer? (and/c inexact-real? rational?))
(listof jsexpr?)
(hash/c symbol? jsexpr?)

Examples:

> (jsexpr? 'null)
#t
> (jsexpr? #t)
#t
> (jsexpr? "cheesecake")
#t
> (jsexpr? 3.5)
#t
> (jsexpr? (list 18 'null #f))
#t
> (jsexpr? #hasheq((turnip . 82)))
#t
> (jsexpr? (vector 1 2 3 4))
#f
> (jsexpr? #hasheq(("turnip" . 82)))
#f
> (jsexpr? +inf.0)
#f

parameter
(json-null) → any/c
(json-null jsnull) → void?
jsnull : any/c

This parameter determines the default Racket value that corresponds to a JSON “null”. By default, it is the 'null symbol. In some cases a different value may better fit your needs, therefore all functions in this library accept a #:null keyword argument for the value that is used to represent a JSON “null”, and this argument defaults to (json-null).

Note that the value of (json-null) (or an explicitly-provided #:null argument) is recognized using eq?.

2 Generating JSON Text from JS-Expressions

procedure
(write-json x
[ out
#:null jsnull
#:encode encode]) → any
  x : jsexpr?
  out : output-port? = (current-output-port)
  jsnull : any/c = (json-null)
  encode : (or/c 'control 'all) = 'control

Writes the x jsexpr, encoded as JSON, to the out output port.

By default, only ASCII control characters are encoded as “\uHHHH”. If encode is given as 'all, then in addition to ASCII control characters, non-ASCII characters are encoded as well. This can be useful if you need to transport the text via channels that might not support UTF-8. Note that characters in the range of U+10000 and above are encoded as two \uHHHH escapes, see Section 2.5 of the JSON RFC.

Examples:

> (with-output-to-string
    (λ () (write-json #hasheq((waffle . (1 2 3))))))
"{\"waffle\":[1,2,3]}"
> (with-output-to-string
    (λ () (write-json #hasheq((와플 . (1 2 3)))
                      #:encode 'all)))
"{\"\\uc640\\ud50c\":[1,2,3]}"

procedure
(jsexpr->string x
[ #:null jsnull
#:encode encode]) → string?
  x : jsexpr?
  jsnull : any/c = (json-null)
  encode : (or/c 'control 'all) = 'control

Generates a JSON source string for the jsexpr x.

Example:

> (jsexpr->string #hasheq((waffle . (1 2 3))))
"{\"waffle\":[1,2,3]}"

procedure
(jsexpr->bytes x
[ #:null jsnull
#:encode encode]) → bytes?
  x : jsexpr?
  jsnull : any/c = (json-null)
  encode : (or/c 'control 'all) = 'control

Generates a JSON source byte string for the jsexpr x. (The byte string is encoded in UTF-8.)

Example:

> (jsexpr->bytes #hasheq((waffle . (1 2 3))))
#"{\"waffle\":[1,2,3]}"

3 Parsing JSON Text into JS-Expressions

procedure
(read-json [in #:null jsnull]) → (or/c jsexpr? eof-object?)
in : input-port? = (current-input-port)
jsnull : any/c = (json-null)

Reads a jsexpr from a single JSON-encoded input port in as a Racket (immutable) value, or produces eof if only whitespace remains. Like read, the function leaves all remaining characters in the port so that a second call can retrieve the remaining JSON input(s). If the JSON inputs aren’t delimited per se (true, false, null), they must be separated by whitespace from the following JSON input.

Examples:

> (with-input-from-string
    "{\"arr\" : [1, 2, 3, 4]}"
    (λ () (read-json)))
'#hasheq((arr . (1 2 3 4)))
> (with-input-from-string
    "\"sandwich\""
    (λ () (read-json)))
"sandwich"
> (with-input-from-string
    "true false"
    (λ () (list (read-json) (read-json))))
'(#t #f)
> (with-input-from-string
    "true[1,2,3]"
    (λ () (list (read-json) (read-json))))
'(#t (1 2 3))
> (with-input-from-string
    "true\"hello\""
    (λ () (list (read-json) (read-json))))
'(#t "hello")
> (with-input-from-string
    "\"world\"41"
    (λ () (list (read-json) (read-json))))
'("world" 41)
> (with-input-from-string
    "sandwich sandwich" ; invalid JSON
    (λ () (read-json)))
string::1: read-json: bad input starting #"sandwich
sandwich"
> (with-input-from-string
    "false42" ; invalid JSON text sequence
    (λ () (read-json)))
string::6: read-json: bad input starting #"false42"

Changed in version 8.1.0.2 of package base: Adjusted the whitespace handling to reject whitespace that isn’t either #\space, #\tab, #\newline, or #\return.

procedure
(string->jsexpr str [#:null jsnull]) → jsexpr?
str : string?
jsnull : any/c = (json-null)

Parses a recognizable prefix of the string str as an immutable jsexpr. If the prefix isn’t delimited per se (true, false, null), it must be separated by whitespace from the remaining characters.

Example:

> (string->jsexpr "{\"pancake\" : 5, \"waffle\" : 7}")
'#hasheq((pancake . 5) (waffle . 7))

procedure
(bytes->jsexpr str [#:null jsnull]) → jsexpr?
str : bytes?
jsnull : any/c = (json-null)

Parses a recognizable prefix of the string str as an immutable jsexpr. If the prefix isn’t delimited per se (true, false, null), it must be separated by whitespace from the remaining bytes.

Example:

> (bytes->jsexpr #"{\"pancake\" : 5, \"waffle\" : 7}")
'#hasheq((pancake . 5) (waffle . 7))

4 A Word About Design

4.1 The JS-Expression Data Type

JSON syntactically distinguishes “null”, array literals, and object literals, and therefore there is a question of what Racket value should represent a JSON “null”. This library uses the Racket 'null symbol by default. Note that this is unambiguous, since Racket symbols are used only as object keys, which are required to be strings in JSON.

Several other options have been used by various libraries. For example, Dave Herman’s PLaneT library (which has been the basis for this library) uses the #\nul character, other libraries for Racket and other Lisps use (void), NIL (some use it also for JSON “false”), and more. The approach taken by this library is to use a keyword argument for all functions, with a parameter that determines its default, making it easy to use any value that fits your needs.

The JSON RFC only states that object literal expressions “SHOULD” contain unique keys, but does not proscribe them entirely. Looking at existing practice, it appears that popular JSON libraries parse object literals with duplicate keys by simply picking one of the key-value pairs and discarding the others with the same key. This behavior is naturally paralleled by Racket hash tables, making them a natural analog.

Finally, the JSON RFC is almost completely silent about the order of key-value pairs. While the RFC only specifies the syntax of JSON, which of course always must represent object literals as an ordered collection, the introduction states:

An object is an unordered collection of zero or more name/value pairs, where a name is a string and a value is a string, number, boolean, null, object, or array.

In practice, JSON libraries discard the order of object literals in parsed JSON text and make no guarantees about the order of generated object literals, usually using a hash table of some flavor as a natural choice. We therefore do so as well.

4.2 Naming Conventions

Some names in this library use “jsexpr” and some use “json”. The rationale that the first is used for our representation, and the second is used as information that is received from or sent to the outside world.

1	JS-Expressions
2	Generating JSON Text from JS-Expressions
3	Parsing JSON Text into JS-Expressions
4	A Word About Design