% Prolog offers built-in support for JSON encoding and decoding,
% including to and from built-in and custom data types.

:- use_module(library(http/json)).

% We'll use these two compound terms to demonstrate encoding and
% decoding of custom types below.
response1(response1(Page, Fruits)) :-
    integer(Page),
    is_list(Fruits).

response2(response2(Page, Fruits)) :-
    integer(Page),
    is_list(Fruits).

main :-
    % First we'll look at encoding basic data types to
    % JSON strings. Here are some examples for atomic
    % values.
    atom_json_term(BolJSON, true, []),
    writeln(BolJSON),

    atom_json_term(IntJSON, 1, []),
    writeln(IntJSON),

    atom_json_term(FltJSON, 2.34, []),
    writeln(FltJSON),

    atom_json_term(StrJSON, "gopher", []),
    writeln(StrJSON),

    % And here are some for lists and dicts, which encode
    % to JSON arrays and objects as you'd expect.
    SlcD = ["apple", "peach", "pear"],
    atom_json_term(SlcJSON, SlcD, []),
    writeln(SlcJSON),

    MapD = json([apple=5, lettuce=7]),
    atom_json_term(MapJSON, MapD, []),
    writeln(MapJSON),

    % The JSON package can automatically encode your
    % custom data types. It will include all arguments
    % in the encoded output.
    Res1D = response1(1, ["apple", "peach", "pear"]),
    atom_json_term(Res1JSON, Res1D, []),
    writeln(Res1JSON),

    % You can use options to customize the encoded JSON.
    % Check the documentation of atom_json_term for more details.
    Res2D = response2(1, ["apple", "peach", "pear"]),
    atom_json_term(Res2JSON, Res2D, []),
    writeln(Res2JSON),

    % Now let's look at decoding JSON data into Prolog
    % terms. Here's an example for a generic data
    % structure.
    Byt = '{"num":6.13,"strs":["a","b"]}',
    atom_json_term(Byt, Dat, []),
    writeln(Dat),

    % In order to use the values in the decoded term,
    % we can access them directly.
    Num = Dat.num,
    writeln(Num),

    % Accessing nested data is straightforward in Prolog.
    [Str1|_] = Dat.strs,
    writeln(Str1),

    % We can also decode JSON into custom data types.
    % This has the advantage of adding additional
    % type-safety to our programs.
    Str = '{"page": 1, "fruits": ["apple", "peach"]}',
    atom_json_term(Str, Res, []),
    writeln(Res),
    [Fruit|_] = Res.fruits,
    writeln(Fruit),

    % In Prolog, we can directly write JSON to the standard output.
    D = json([apple=5, lettuce=7]),
    json_write(current_output, D, []).

% To run this program, save it as json_example.pl and use:
% swipl -q -t main -s json_example.pl