The json library provides predicates for parsing and generating data in the JSON format based on the specification and standard found at:

It includes parametric objects whose parameters allow selecting the representation for parsed JSON objects (curly or list), JSON text strings (atom, chars, or codes) and JSON pairs (dash, equal, or colon).

API documentation

Open the ../../docs/library_index.html#json link in a web browser.


To load all entities in this library, load the loader.lgt file:

| ?- logtalk_load(json(loader)).


To test this library predicates, load the tester.lgt file:

| ?- logtalk_load(json(tester)).

Some of the sample JSON test files are based on examples published at:



The following choices of syntax has been made to represent JSON elements as terms:

  • By default, JSON objects are represented using curly-bracketed terms, {Pairs}, where each pair uses the representation Key-Value (see below for alternative representations).

  • Arrays are represented using lists.

  • Text strings can be represented as atoms, chars(List), or codes(List). The default when decoding is to use atoms when using the json object. To decode text strings into lists of chars or codes, use the json/1 with the parameter bound to chars or codes. For example:

    | ?- json::parse(codes([34,104,101,108,108,111,34]), Term).
    Term = hello
    | ?- json(atom)::parse(codes([34,104,101,108,108,111,34]), Term).
    Term = hello
    | ?- json(chars)::parse(codes([34,104,101,108,108,111,34]), Term).
    Term = chars([h,e,l,l,o])
    | ?- json(codes)::parse(codes([34,104,101,108,108,111,34]), Term).
    Term = codes([104,101,108,108,111])
  • The JSON values false, true and null are represented by, respectively, the @false, @true and @null compound terms.

The following table exemplifies the term equivalents of JSON elements using default representations for objects, pairs, and strings:

JSON term
[1,2] [1,2]
true @true
false @false
null @null
-1 -1
[1.2345] [1.2345]
[] []
[2147483647] [2147483647]
[0] [0]
[1234567890123456789] [1234567890123456789]
[false] [@false]
[-2147483648] [-2147483648]
{“a”:null,”foo”:”bar”} {a-@null, foo-bar}
[2.225073858507201e-308] [2.225073858507201e-308]
[0,1] [0,1]
[2.2250738585072014e-308] [2.2250738585072014e-308]
[1.7976931348623157e+308] [1.7976931348623157e+308]
[0.0] [0.0]
[4294967295] [4294967295]
[-1234567890123456789] [-1234567890123456789]
[“foo”] [foo]
[1] [1]
[null] [@null]
[-1.2345] [-1.2345]
[5.0e-324] [5.0e-324]
[-1] [-1]
[true] [@true]
[9223372036854775807] [9223372036854775807]

For JSON objects that are two possible term representations:

JSON object term (curly)
{“a”:1, “b”:2, “c”:3} {a-1, b-2, c-3}
{} {}


JSON object term (list)
{“a”:1, “b”:2, “c”:3} json([a-1, b-2, c-3])
{} json([])

For JSON pairs that are three possible representations:

JSON object term (dash)
{“a”:1, “b”:2, “c”:3} {a-1, b-2, c-3}


JSON object term (equal)
{“a”:1, “b”:2, “c”:3} {a=1, b=2, c=3}


JSON object term (colon)
{“a”:1, “b”:2, “c”:3} {a:1, b:2, c:3}

By default, the curly-term representation and the dash pair representation are used. The json/3 parametric object allows selecting the desired representation choices. For example:

| ?- json(curly,dash,atom)::parse(atom('{"a":1, "b":2, "c":3}'), JSON).
JSON = {a-1, b-2, c-3}

| ?- json(list,equal,atom)::parse(atom('{"a":1, "b":2, "c":3}'), JSON).
JSON = json([a=1, b=2, c=3])

| ?- json(curly,colon,atom)::parse(atom('{"a":1, "b":2, "c":3}'), JSON).
JSON = {a:1, b:2, c:3}


Encoding is accomplished using the generate/2 predicate. For example:

| ?- json::generate(codes(Encoding), [a,{b-c}]).
Encoding = [91,34,97,34,44,123,34,98,34,58,34,99,34,125,93]


| ?- json::generate(chars(Encoding), [a,{b-c}]).
Encoding = ['[','"',a,'"',',','{','"',b,'"',:,'"',c,'"','}',']']

| ?- json::generate(atom(Encoding), [a,{b-c}]).
Encoding = '["a",{"b":"c"}]'

Notice that generate/2 takes, as second argument, a Prolog term that corresponds to the JSON Syntax in Prolog and produces the corresponding JSON output in the format specified in the first argument: (codes(Variable), stream(Stream), file(File), chars(Variable) or atom(Variable)).


Decoding is accomplished using the parse/2 predicate. For example, to decode a given json file:

| ?- json::parse(file('simple/roundtrip_array_obj_array.json'), Term).
Term = [{a-[b]}]

The parse/2 predicate first argument must indicate the input source (codes(Codes), stream(Stream), line(Stream), file(Path), chars(Chars) or atom(Atom)) containing a JSON payload to be decoded into the Prolog term in the second argument.

Known issues

Some tests, notably parse_simple_valid_files and roundtrip_hexadecimals, fail on backends such as ECLiPSe and GNU Prolog that don’t support Unicode.