Output login errors to user

lib/api.dart

@@ -5,6 +5,7 @@ import 'package:flutter/foundation.dart';
 import 'package:http/http.dart' as http;
 import 'package:logging/logging.dart';
 import 'package:path_provider/path_provider.dart'; // For cache storage
+import 'package:sheetless/utility.dart';
 
 import 'sheet.dart';
 
@@ -15,7 +16,7 @@ class ApiClient {
 
   ApiClient({required this.baseUrl, this.token});
 
-  Future<bool> login(String username, String password) async {
+  Future<Result<void>> login(String username, String password) async {
     log.info("Logging in...");
     try {
       final url = '$baseUrl/login';
@@ -28,14 +29,19 @@ class ApiClient {
       if (response.statusCode == 200) {
         token = jsonDecode(response.body);
         log.info('Login successful');
-        return true;
+        return Result.ok(null);
       } else {
         log.warning('Login failed: ${response.statusCode}, ${response.body}');
+        return Result.error(
+          Exception(
+            "Response code ${response.statusCode}\nResponse: ${response.body}",
+          ),
+        );
       }
-    } catch (e) {
+    } on Exception catch (e) {
       log.warning('Error during login', e);
+      return Result.error(e);
     }
-    return false;
   }
 
   void logout() {

lib/login_page.dart

@@ -3,6 +3,7 @@ import 'package:jwt_decoder/jwt_decoder.dart';
 import 'package:sheetless/api.dart';
 import 'package:sheetless/home_page.dart';
 import 'package:sheetless/storage_helper.dart';
+import 'package:sheetless/utility.dart';
 
 class LoginPage extends StatefulWidget {
   const LoginPage({super.key});
@@ -62,17 +63,16 @@ class _LoginPageState extends State<LoginPage> {
     });
     serverUrl = "$serverUrl/api";
     final apiClient = ApiClient(baseUrl: serverUrl);
-    final loginSuccessful = await apiClient.login(email, password);
+    final loginResult = await apiClient.login(email, password);
-    if (loginSuccessful) {
+    if (loginResult.isOk()) {
       await _storageHelper.writeSecure(SecureStorageKey.url, serverUrl);
       await _storageHelper.writeSecure(SecureStorageKey.jwt, apiClient.token!);
       await _storageHelper.writeSecure(SecureStorageKey.email, email);
       await _storageHelper.writeSecure(SecureStorageKey.password, password);
       _navigateToMainPage();
     } else {
-      // TODO: give more varied error messages
       setState(() {
-        _error = "Login failed.";
+        _error = "Login failed.\n${loginResult.error()}";
       });
     }
   }

lib/utility.dart

@@ -0,0 +1,70 @@
+/// Utility class that simplifies handling errors.
+///
+/// Return a [Result] from a function to indicate success or failure.
+///
+/// A [Result] is either an [Ok] with a value of type [T]
+/// or an [Err] with an [Exception].
+///
+/// Use [Result.ok] to create a successful result with a value of type [T].
+/// Use [Result.error] to create an error result with an [Exception].
+///
+/// Evaluate the result using a switch statement:
+/// ```dart
+/// switch (result) {
+///   case Ok(): {
+///     print(result.value);
+///   }
+///   case Error(): {
+///     print(result.error);
+///   }
+/// }
+/// ```
+sealed class Result<T> {
+  const Result();
+
+  /// Creates a successful [Result], completed with the specified [value].
+  const factory Result.ok(T value) = Ok._;
+
+  /// Creates an error [Result], completed with the specified [error].
+  const factory Result.error(Exception error) = Err._;
+
+  bool isOk() {
+    return this is Ok;
+  }
+
+  T value() {
+    Ok<T> ok = this as Ok<T>;
+    return ok._value;
+  }
+
+  bool isErr() {
+    return this is Err;
+  }
+
+  Exception error() {
+    Err<T> err = this as Err<T>;
+    return err._error;
+  }
+}
+
+/// A successful [Result] with a returned [value].
+final class Ok<T> extends Result<T> {
+  const Ok._(this._value);
+
+  /// The returned value of this result.
+  final T _value;
+
+  @override
+  String toString() => 'Result<$T>.ok($_value)';
+}
+
+/// An error [Result] with a resulting [error].
+final class Err<T> extends Result<T> {
+  const Err._(this._error);
+
+  /// The resulting error of this result.
+  final Exception _error;
+
+  @override
+  String toString() => 'Result<$T>.error($_error)';
+}