added fpdart demo

docs/presentation/fp_dart-demo/lib/app_error.dart

@@ -0,0 +1,46 @@
+import 'package:fpdart/fpdart.dart';
+
+// Fehlerkategorien
+enum AppErrorType {
+  ValidationError,
+  DatabaseError,
+  NetworkError,
+  UnknownError,
+}
+
+class AppError {
+  final String message; // Benutzerfreundliche Fehlermeldung
+  final AppErrorType type; // Typisierte Fehlerkategorie
+  final Exception? exception; // Originale Exception (falls vorhanden)
+
+  AppError({
+    required this.message,
+    this.type = AppErrorType.UnknownError,
+    this.exception,
+  });
+
+  @override
+  String toString() {
+    return 'AppError(message: $message, type: $type, exception: $exception)';
+  }
+
+  /// Hilfsmethode zur einfachen Erstellung eines `Left`-Werts für `Either`
+  static Either<AppError, T> left<T>(
+    String message, {
+    AppErrorType? type,
+    Exception? exception,
+  }) {
+    return Either.left(AppError(
+      message: message,
+      type: type ?? AppErrorType.UnknownError,
+      exception: exception,
+    ));
+  }
+
+  @override
+  bool operator ==(Object other) {
+    if (identical(this, other)) return true;
+
+    return other is AppError && other.message == message && other.type == type;
+  }
+}

docs/presentation/fp_dart-demo/lib/demo.dart

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+import 'package:demo/app_error.dart';
+import 'package:fpdart/fpdart.dart';
+
+// Mock-Datenbank
+final existingEmails = ["existing@example.com"];
+
+// Validierungsfunktionen
+Either<AppError, String> validateName(String name) {
+  return name.isNotEmpty
+      ? Either.right(name)
+      : AppError.left(
+          "Name cannot be empty",
+          type: AppErrorType.ValidationError,
+        );
+}
+
+Either<AppError, String> validateEmail(String email) {
+  final emailRegex = RegExp(r"^[a-zA-Z0-9.]+@[a-zA-Z0-9]+\.[a-zA-Z]+");
+  return emailRegex.hasMatch(email)
+      ? Either.right(email)
+      : AppError.left(
+          "Invalid email format",
+          type: AppErrorType.ValidationError,
+        );
+}
+
+// Asynchrone Überprüfung, ob die E-Mail bereits existiert
+TaskEither<AppError, bool> checkEmailExists(String email) {
+  return TaskEither.tryCatch(
+    () async => existingEmails.contains(email),
+    (error, _) => AppError(
+      message: "Error checking email existence",
+      type: AppErrorType.DatabaseError,
+      exception: error as Exception?,
+    ),
+  );
+}
+
+// Benutzer in der Datenbank speichern (Dummy-Implementierung)
+TaskEither<AppError, bool> saveUser(String name, String email) {
+  return TaskEither.tryCatch(
+    () async {
+      await Future.delayed(Duration(seconds: 1)); // Simulierte Latenz
+      print("User saved: $name, $email");
+      return true;
+    },
+    (error, _) => AppError(
+      message: "Error saving user",
+      type: AppErrorType.DatabaseError,
+      exception: error as Exception?,
+    ),
+  );
+}
+
+// Optional: Begrüßungsnachricht senden
+TaskEither<AppError, bool> sendWelcomeEmail(String email) {
+  return TaskEither.tryCatch(
+    () async {
+      print("Welcome email sent to $email");
+      return true;
+    },
+    (error, _) => AppError(
+      message: "Error sending welcome email",
+      type: AppErrorType.NetworkError,
+      exception: error as Exception?,
+    ),
+  );
+}