A variant of the 'Dart decoding using dart:convert' example using extension types

pkginfo_extension_type.dart

import 'package:http/http.dart' as http;
import 'dart:convert';

main() async {
  const pubUrl = "https://pub.dartlang.org/api/packages/protobuf";
  var response = await http.get(Uri.parse(Uri.encodeFull(pubUrl)));
  if (response.statusCode == 200) {
    PkgInfo info = PkgInfo(json.decode(response.body));
    print('Package ${info.name}, v ${info.latest.pubspec.version}');
  } else {
    throw Exception('Failed to load package info');
  }
}

extension type PkgInfo(Map<String, dynamic> json) {
  String get name => json['name']!;
  PkgVersion get latest => PkgVersion(json['latest']!);
  String get version => json['version']!;
}

extension type PkgVersion(Map<String, dynamic> json) {
  String get archiveUrl => json['archive_url']!;
  PkgPubspec get pubspec => PkgPubspec(json['pubspec']!);
}

extension type PkgPubspec(Map<String, dynamic> json) {
  String get version => json['version']!;
  String get name => json['name']!;
}

It might be cool to have nested extension members (similar to Kotlin's):

typedef JsonMap = Map<String, dynamic>;

extension type _Base(JsonMap json) {
  (String s).operator ~() => json[s]!;
}

extension type PkgInfo(JsonMap json) implements _Base {
  String get name => ~'name';
  PkgVersion get latest => PkgVersion(~'latest');
  String get version => ~'version';
}

extension type PkgVersion(JsonMap json) implements _Base {
  String get archiveUrl => ~'archive_url';
  PkgPubspec get pubspec => PkgPubspec(~'pubspec');
}

extension type PkgPubspec(JsonMap json) implements _Base {
  String get version => ~'version';
  String get name => ~'name';
}

They're a bit weird, though, because they can only be invoked in a context where the current instance of the enclosing type declaration is available implicitly (for instance, inside the body of the declaration of _Base), such that there is a way to give meaning to expressions like json.

But I digress. ;-)

dart-lang/language#3240 explores this idea further.