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This file is part of Logtalk <https://logtalk.org/>  
SPDX-FileCopyrightText: 1998-2026 Paulo Moura <pmoura@logtalk.org> and
Jacinto Dávila <jdavila@optimusprime.ai>  
SPDX-License-Identifier: Apache-2.0

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
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See the License for the specific language governing permissions and
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`json`
======

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

- https://www.rfc-editor.org/rfc/rfc8259
- https://ecma-international.org/publications-and-standards/standards/ecma-404/

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 [../../apis/library_index.html#json](../../apis/library_index.html#json)
link in a web browser.


Loading
-------

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

	| ?- logtalk_load(json(loader)).


Testing
-------

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:

https://www.json.org/


Representation
--------------

The following choices of syntax have 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
		yes

		| ?- json(atom)::parse(codes([34,104,101,108,108,111,34]), Term).
		Term = hello
		yes

		| ?- json(chars)::parse(codes([34,104,101,108,108,111,34]), Term).
		Term = chars([h,e,l,l,o])
		yes

		| ?- json(codes)::parse(codes([34,104,101,108,108,111,34]), Term).
		Term = codes([104,101,108,108,111])
		yes

- 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}            |
|    {}                         |    {}                         |

and:

|    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}            |

and:

|    JSON object                |    term (equal)               |
| :---------------------------: | :---------------------------: |
|    {"a":1, "b":2, "c":3}      |    {a=1, b=2, c=3}            |

and:

|    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}
	yes

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

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


Encoding
--------

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]
	yes

Alternatively:

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

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

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
--------

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]}]
	yes

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.
