Example of functional style in Dart using features from version 3.1

functional.dart

// sealed classes are abstract (can't be instatiated) and can only be
// implemented in the same library (library in Dart is one file be default)
sealed class Message {
}

class TextMessage implements Message {
  final String content;
  TextMessage(this.content);
}

class ImageMessage implements Message {
  final String content;
  final String imageUrl;
  ImageMessage(this.content, this.imageUrl);
}

class FileMessage implements Message {
  final String content;
  final String fileUrl;
  FileMessage(this.content, this.fileUrl);
}

// Extract fields from a message and return as Record
(String type, String content, String url) getMessage(Message message) {
  // using switch expression, https://dart.dev/language/branches#switch-expressions
  return switch (message) {
    TextMessage(:var content) => ('Text', content, ''),
    ImageMessage(:var content, :var imageUrl) => ('Image', content, imageUrl),
    FileMessage(:var content, :var fileUrl) => ('File', content, fileUrl)
  };
}

void printTexts(List<Message> messages) {
  // using normal switch statement
  for (var m in messages) {
    switch (m) {
      case TextMessage(content: final text): print(text);
      case ImageMessage(): print('<img>');
      default: print('<smth-else>');
    }
  }
}

void main() {
  var m0 = TextMessage('Hello, John!'), m1 = TextMessage('Hi, Jack!');
  var m2 = ImageMessage('Check this out', 'https://example.com/image1.jpg');
  var m3 = FileMessage('Important document', 'https://example.com/document1.pdf');

  var messages = [m0, m1, m2, m3];
  for (var m in messages){ 
    print(getMessage(m)); 
  }
  printTexts(messages);
}

// Output
//
// (Text, Hello, John!, )
// (Text, Hi, Jack!, )
// (Image, Check this out, https://example.com/image1.jpg)
// (File, Important document, https://example.com/document1.pdf)
// Hello, John!
// Hi, Jack!
// <img>
// <smth-else>

functional.hs

-- Define message types
data Message = TextMessage String
             | ImageMessage String String
             | FileMessage String String
             deriving (Show)

-- Extract fields from a message and return as a tuple
getMessage :: Message -> (String, String, String)
getMessage (TextMessage content) = ("Text", content, "")
getMessage (ImageMessage content imageUrl) = ("Image", content, imageUrl)
getMessage (FileMessage content fileUrl) = ("File", content, fileUrl)

-- Print text of messages
printTexts :: [Message] -> IO ()
printTexts [] = return ()
printTexts (TextMessage content : messages) = do
  putStrLn content
  printTexts messages
printTexts (ImageMessage _ _ : messages) = do
  putStrLn "<img>"
  printTexts messages
printTexts (_ : messages) = do
  putStrLn "<smth-else>"
  printTexts messages

main :: IO ()
main = do
  -- Create sample messages
  let m0 = TextMessage "Hello, John!"
      m1 = TextMessage "Hi, Jack!"
      m2 = ImageMessage "Check this out" "https://example.com/image1.jpg"
      m3 = FileMessage "Important document" "https://example.com/document1.pdf"

      messages = [m0, m1, m2, m3]

  -- Print messages using getMessage
  mapM_ (print . getMessage) messages

  -- Print texts of messages using printTexts
  printTexts messages
  
-- Output
--
-- ("Text","Hello, John!","")
-- ("Text","Hi, Jack!","")
-- ("Image","Check this out","https://example.com/image1.jpg")
-- ("File","Important document","https://example.com/document1.pdf")
-- Hello, John!
-- Hi, Jack!
-- <img>
-- <smth-else>

oop.dart

abstract class Message {
  String get content;
  String getMessage();
}

class TextMessage implements Message {
  @override
  final String content;
  TextMessage(this.content);

  String get url => '';
  String get type => 'Text';
  
  @override
  String getMessage() {
    return '($type, $content, $url)';
  }
}

class ImageMessage implements Message {
  @override
  final String content;
  final String imageUrl;
  ImageMessage(this.content, this.imageUrl);

  String get url => imageUrl;
  String get type => 'Image';
  
  @override
  String getMessage() {
    return '($type, $content, $url)';
  }
}

class FileMessage implements Message {
  @override
  final String content;
  final String fileUrl;
  FileMessage(this.content, this.fileUrl);

  String get url => fileUrl;
  String get type => 'File';

  @override
  String getMessage() {
    return '($type, $content, $url)';
  }
}

void printTexts(List<Message> messages) {
  for (var m in messages) {
    if (m is TextMessage) {
      print(m.content);
    } else if (m is ImageMessage) {
      print('<img>');
    } else {
      print('<smth-else>');
    }
  }
}

void main() {
  var m0 = TextMessage('Hello, John!'), m1 = TextMessage('Hi, Jack!');
  var m2 = ImageMessage('Check this out', 'https://example.com/image1.jpg');
  var m3 = FileMessage('Important document', 'https://example.com/document1.pdf');

  var messages = [m0, m1, m2, m3];
  for (var m in messages) {
    print(m.getMessage());
  }
  printTexts(messages);
}

// Output
//
// (Text, Hello, John!, )
// (Text, Hi, Jack!, )
// (Image, Check this out, https://example.com/image1.jpg)
// (File, Important document, https://example.com/document1.pdf)
// Hello, John!
// Hi, Jack!
// <img>
// <smth-else>