rontolisp:json-parse

(rontolisp:json-parse string)

Parses a JSON document string into Lisp values, following the defaults of the com.inuoe.jzon library: a JSON object becomes a hash table with string keys, an array a vector, and true/false/null become t, nil and the symbol null. rontolisp:json-parse is a lightweight subset of jzon, so a program can start here and later switch to jzon without changing shape — with a single deliberate exception, the wide-integer rule noted below.

Switch to com.inuoe.jzon when you outgrow the subset: for its richer features (pretty-printing, a streaming writer, a :replacer, custom serialization), or to make the JSON code portable to other Common Lisp implementations — com.inuoe.jzon is a standard library, while rontolisp:json-* runs only on rontolisp.

Value mapping

JSON	Lisp
object	hash table with string keys (`equal` test)
array	vector
string	string (`\uXXXX` escapes and surrogate pairs are decoded)
number	integer, or float when it has a fraction, an exponent or more than 18 digits
`true`	`t`
`false`	`nil`
`null`	the symbol `null`

The one incompatibility with jzon

Integers wider than 18 digits become floats on every backend -- a shared library rule that keeps the parse identical across all backends. jzon instead keeps them as exact integers of any width, so this is the single point where rontolisp:json-parse and jzon:parse disagree — a 13-digit millisecond timestamp parses exactly on both, but a 19-digit integer parses as a float here and as an exact integer under jzon. Everything else round-trips identically.

Errors

Invalid JSON and trailing characters after the value signal an error when json-parse is called:

CL-USER> (rontolisp:json-parse "{\"a\": ")
Error: json-parse: unexpected end of input
CL-USER> (rontolisp:json-parse "1 2")
Error: json-parse: unexpected trailing characters

Limitations

A JSON object always parses to a hash table, so {} (an empty hash table) is distinct from false/nil, from an empty array #(), and from the null symbol — unlike JavaScript, the four are never conflated.
On the WASM backends a float with magnitude 2³¹ or larger parses correctly but cannot be printed (print/princ-to-string trap); see the WASM guide.

Backend support

Works on every backend and in every WASM mode (Preview 1 included): the parser is written in rontolisp itself and is compiled into the program when used. The typical use is parsing a rontolisp:fetch response body:

(print (gethash "url"
                (rontolisp:json-parse
                 (getf (rontolisp:await (rontolisp:fetch "https://httpbin.ik.am/get")) :body))))   ; "https://httpbin.ik.am/get"

The inverse operation is rontolisp:json-stringify.