Json in Pascal

program JSONExample;

uses
  SysUtils, Classes, fpjson, jsonparser;

type
  TResponse1 = class
  public
    Page: Integer;
    Fruits: TStringList;
    constructor Create;
    destructor Destroy; override;
  end;

  TResponse2 = class
  public
    Page: Integer;
    Fruits: TStringList;
    constructor Create;
    destructor Destroy; override;
  end;

constructor TResponse1.Create;
begin
  inherited;
  Fruits := TStringList.Create;
end;

destructor TResponse1.Destroy;
begin
  Fruits.Free;
  inherited;
end;

constructor TResponse2.Create;
begin
  inherited;
  Fruits := TStringList.Create;
end;

destructor TResponse2.Destroy;
begin
  Fruits.Free;
  inherited;
end;

procedure EncodeBasicTypes;
var
  JSONData: TJSONData;
begin
  // Encoding basic data types to JSON strings
  JSONData := TJSONBoolean.Create(True);
  WriteLn(JSONData.AsJSON);
  JSONData.Free;

  JSONData := TJSONIntegerNumber.Create(1);
  WriteLn(JSONData.AsJSON);
  JSONData.Free;

  JSONData := TJSONFloatNumber.Create(2.34);
  WriteLn(JSONData.AsJSON);
  JSONData.Free;

  JSONData := TJSONString.Create('gopher');
  WriteLn(JSONData.AsJSON);
  JSONData.Free;
end;

procedure EncodeArraysAndObjects;
var
  JSONArray: TJSONArray;
  JSONObject: TJSONObject;
begin
  // Encoding arrays and objects
  JSONArray := TJSONArray.Create(['apple', 'peach', 'pear']);
  WriteLn(JSONArray.AsJSON);
  JSONArray.Free;

  JSONObject := TJSONObject.Create;
  JSONObject.Add('apple', 5);
  JSONObject.Add('lettuce', 7);
  WriteLn(JSONObject.AsJSON);
  JSONObject.Free;
end;

procedure EncodeCustomTypes;
var
  Res1, Res2: TJSONObject;
  FruitsArray: TJSONArray;
begin
  // Encoding custom types
  Res1 := TJSONObject.Create;
  FruitsArray := TJSONArray.Create(['apple', 'peach', 'pear']);
  Res1.Add('Page', 1);
  Res1.Add('Fruits', FruitsArray);
  WriteLn(Res1.AsJSON);
  Res1.Free;

  Res2 := TJSONObject.Create;
  FruitsArray := TJSONArray.Create(['apple', 'peach', 'pear']);
  Res2.Add('page', 1);
  Res2.Add('fruits', FruitsArray);
  WriteLn(Res2.AsJSON);
  Res2.Free;
end;

procedure DecodeJSON;
var
  JSONData: TJSONData;
  JSONObject: TJSONObject;
  Num: Double;
  Strs: TJSONArray;
begin
  // Decoding JSON data
  JSONData := GetJSON('{"num":6.13,"strs":["a","b"]}');
  if Assigned(JSONData) and (JSONData.JSONType = jtObject) then
  begin
    JSONObject := TJSONObject(JSONData);
    WriteLn(JSONObject.AsJSON);

    Num := JSONObject.Get('num', 0.0);
    WriteLn(Num);

    Strs := JSONObject.Get('strs', TJSONArray(nil));
    if Assigned(Strs) and (Strs.Count > 0) then
      WriteLn(Strs.Strings[0]);
  end;
  JSONData.Free;
end;

procedure DecodeIntoCustomType;
var
  JSONData: TJSONData;
  JSONObject: TJSONObject;
  Response: TResponse2;
begin
  JSONData := GetJSON('{"page": 1, "fruits": ["apple", "peach"]}');
  if Assigned(JSONData) and (JSONData.JSONType = jtObject) then
  begin
    JSONObject := TJSONObject(JSONData);
    Response := TResponse2.Create;
    try
      Response.Page := JSONObject.Get('page', 0);
      Response.Fruits.AddStrings(TJSONArray(JSONObject.Find('fruits')));
      WriteLn(Format('Page: %d, First Fruit: %s', [Response.Page, Response.Fruits[0]]));
    finally
      Response.Free;
    end;
  end;
  JSONData.Free;
end;

begin
  EncodeBasicTypes;
  EncodeArraysAndObjects;
  EncodeCustomTypes;
  DecodeJSON;
  DecodeIntoCustomType;
end.

This Pascal program demonstrates JSON encoding and decoding using the fpjson and jsonparser units, which are part of the Free Pascal Compiler (FPC) standard library. Here’s a breakdown of the example:

We define two custom types, TResponse1 and TResponse2, similar to the structs in the original Go example.
The EncodeBasicTypes procedure shows how to encode basic data types (boolean, integer, float, and string) to JSON.
EncodeArraysAndObjects demonstrates encoding arrays and objects (maps in Go) to JSON.
EncodeCustomTypes shows how to create and encode custom types as JSON objects.
DecodeJSON illustrates how to decode a JSON string into Pascal data structures.
DecodeIntoCustomType demonstrates decoding JSON data into a custom type (TResponse2).

To run this program, you’ll need to have the Free Pascal Compiler installed. Save the code in a file with a .pas extension (e.g., json_example.pas), and compile it using:

fpc json_example.pas

Then run the compiled executable:

./json_example

This example covers the basics of JSON handling in Pascal. For more advanced usage, refer to the FPC documentation for the fpjson and jsonparser units.

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