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json: readjson, writejson, JValue - read, write and represent values in JavaScript Object Notation


include "json.m";
json := load JSON JSON->PATH;

JValue: adt {
    Object =>
        mem:   cyclic list of (string, ref JValue);
    Array =>
        a:     cyclic array of ref JValue;
    String =>
        s:     string;
    Int =>
        value: big;
    Real =>
        value: real;
    True or
    False or
    Null =>
    isarray:   fn(v: self ref JValue): int;
    isfalse:   fn(v: self ref JValue): int;
    isint:     fn(v: self ref JValue): int;
    isnull:    fn(v: self ref JValue): int;
    isnumber:  fn(v: self ref JValue): int;
    isobject:  fn(v: self ref JValue): int;
    isreal:    fn(v: self ref JValue): int;
    isstring:  fn(v: self ref JValue): int;
    istrue:    fn(v: self ref JValue): int;
    copy:      fn(v: self ref JValue): ref Jvalue;
    eq:        fn(v: self ref JValue, v: ref JValue): int;
    get:       fn(v: self ref JValue, mem: string): ref JValue;
    set:       fn(v: self ref JValue, mem: string, value: ref JValue);
    text:      fn(v: self ref JValue): string;

init:      fn(bufio: Bufio);
readjson:  fn(input: ref Bufio->Iobuf): (ref JValue, string);
writejson: fn(output: ref Bufio->Iobuf, val: ref JValue): int;

jvarray:   fn(a: array of ref JValue): ref JValue.Array;
jvbig:     fn(b: big): ref JValue.Int;
jvfalse:   fn(): ref JValue.False;
jvint:     fn(i: int): ref JValue.Int;
jvnull:    fn(): ref JValue.Null;
jvobject:  fn(m: list of (string, ref JValue)): ref JValue.Object;
jvreal:    fn(r: real): ref JValue.Real;
jvstring:  fn(s: string): ref JValue.String;
jvtrue:    fn(): ref JValue.True;


JSON provides value representations, and encoding and decoding operations for the JavaScript Object Notation (JSON) described by Internet RFC4627 and summarised in json(6).

Init must be called before invoking any other operation of the module. The bufio parameter must refer to the instance of bufio(2) that provides the Iobuf parameters used for input and output.

JValue is the internal representation of values transmitted by the JSON encoding. They are distinguished in a pick adt:

Represents the value false.
Represents a null value of any type.
Represents the value true.
Represents as a Limbo big a JavaScript number with no fractional part.
Represents as a Limbo real a JavaScript number with a fractional part.
Represents a JavaScript string of Unicode characters from +U'0000' to +U'FFFF'.
Represents a JavaScript array of values of any type.
Represents a tuple of (member/property name, value) pairs corresponding to the value of a JavaScript object, or an associative array or table, indexed by strings. The member names must be unique within a value.

Readjson reads a single value in JSON format from the input stream and returns a tuple (val, err). On success, val is a JValue that represents the value successfully read. If an error occurs, val is nil and err contains a diagnostic.

Writejson writes to the output stream in JSON format a representation of the value v. It returns 0 on success and -1 on error (setting the system error string).

The easiest way to create a new JValue for subsequent output is with one of the module-level functions jvarray, jvint, jvobject, jvstring, and so on. As values of a pick adt, a JValue can be inspected using Limbo's tagof operator and the appropriate variant accessed using a pick statement. JValue also supports several groups of common operations, for smaller, tidier code. First, the set of enquiry functions v.isX() return true if the value v is an instance of the JavaScript type X (array, int, object, real, string, etc). A numeric value is either int or real. The other operations are:

Return a (reference to) a copy of value v. The copying is not recursive.
Return true if the values of v and w are equal, including the values of corresponding subcomponents, recursively
Return the value associated with the member named mem in v; return null if there is none.
v.set(mem, value)
Adds or changes the value associated with the member named mem in v, which must be a JValue.Object( raises exception json: set non-object otherwise).
Return a string containing the JSON representation of v.




sexprs(2), ubfa(2), json(6), sexprs(6), ubfa(6)
D Crockford, ``The application/json Media Type for JavaScript Object Notation (JSON)'', RFC4627.

JSON(2 ) Rev:  Thu Feb 15 14:43:26 GMT 2007