fix(app): a path import that fails says why in the app's language

The importer said what was wrong with a file in English sentences built
in the data layer, the last English left in a German app. It now names
the problem, the step it is in and what the problem names (a field, a
type, a number), and the studyset screen puts it into words: "Step 2:
unknown "type": quiz." or "Schritt 2: unbekannter „type“: quiz.". The
format's own field names stay as they are in the file.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

AGENTS.md

@@ -99,7 +99,7 @@ A spaced-repetition learning app: studysets of flashcards, reviewed on an FSRS s
   - **Routes carry ids, never objects.** A screen finds its item in the repository's live list with `FoundById` (`core/widget/`): a spinner until the list arrives, then the item, or a screen saying it no longer exists (deleted on another device, or an old link). A screen that deletes its item closes, and keeps showing the item while it goes. Why: every location can be built from its path alone, and no screen goes on showing something that has gone.
 - **The look is one theme** (`core/theme/`): `appTheme(brightness)` with hand-written light and dark colour schemes around the brand's cobalt and copper; `Theme.of(context).extras` (`ThemeExtras`) for the colours Material has no role for (states, the four FSRS ratings, a starred star); `PaletteColor` for the 20 colours a person can give content. `theme_test.dart` holds text to WCAG AA contrast (4.5:1) on each palette colour and on the dark theme's primary colour and surface. Layouts follow Material 3's window-size classes (`context.windowSize`, `core/layout/`), and running text stays within `readableContentMaxWidth`. Icons are Lucide (`lucide_icons_flutter`), except a starred star, which needs the fill Lucide lacks; the Material icons left in older screens go when those are restyled. Shared widgets are in `core/widget/` (`SectionCard`, `AppLoadingIndicator`, `confirmDestructiveAction`, `FoundById`, `TitleWithKicker`, `ContentCountsLine`, `showUndoSnackBar`, `ScrollHidingFab`, `PaletteColorPicker`, and the sheets below).
 - **Forms and pickers open as sheets**: `showAdaptiveSheet` with a `SheetScaffold` (title and close button on top, the body scrolling, the action pinned below) shows a bottom sheet below expanded windows and a dialog from there on. A sheet that asks before discarding changes (a `PopScope`) passes `swipeable: false`: a bottom sheet swiped away closes without asking its `PopScope` (Flutter issue 152120, open with v3.47). A sheet offering an Undo passes `hasSnackBars: true`, so its snack bars show on it rather than behind its barrier. A picker that changes something outside it reports every change as it happens (`onChanged`) rather than as its result, since a sheet swiped away has none.
-- **Text comes from `AppLocalizations`** (Flutter's gen-l10n, `lib/l10n/`): no string a person reads is written into a widget. The app speaks British English, American English and German (informal "du"; the terms of `app_de.arb`, such as Lernset and Karteikarte, stay consistent). `app_en.arb` is the template and holds British English, also the fallback for any language the app doesn't speak; `app_en_US.arb` overrides only what American English spells differently, and `app_en_GB.arb` is empty, there so that Material and dates format the British way. A new message goes into `app_en.arb` and `app_de.arb` together (`test/l10n/arb_files_test.dart` fails otherwise, and checks the placeholders), and into `app_en_US.arb` if its spelling differs. `resolveAppLocale` (`core/l10n/`) picks the language, never by country alone, and both `MaterialApp`s take `appLocalizationsDelegates`, not gen-l10n's list: that brings flutter_localizations' Material texts, which serve only the SDK's Material, so material_ui's widgets would stay English. The editor's own placeholders are replaced by ours (`buildersWithUnknown`), since the editor's texts follow a process-wide locale of their own. The generated `lib/l10n/app_localizations*.dart` are committed and never edited, analysed or formatted by hand. `flutter pub get` writes them anew, so CI fails when they don't match the `.arb` files.
+- **Text comes from `AppLocalizations`** (Flutter's gen-l10n, `lib/l10n/`): no string a person reads is written into a widget. The app speaks British English, American English and German (informal "du"; the terms of `app_de.arb`, such as Lernset and Karteikarte, stay consistent). `app_en.arb` is the template and holds British English, also the fallback for any language the app doesn't speak; `app_en_US.arb` overrides only what American English spells differently, and `app_en_GB.arb` is empty, there so that Material and dates format the British way. A new message goes into `app_en.arb` and `app_de.arb` together (`test/l10n/arb_files_test.dart` fails otherwise, and checks the placeholders), and into `app_en_US.arb` if its spelling differs. `resolveAppLocale` (`core/l10n/`) picks the language, never by country alone, and both `MaterialApp`s take `appLocalizationsDelegates`, not gen-l10n's list: that brings flutter_localizations' Material texts, which serve only the SDK's Material, so material_ui's widgets would stay English. Errors found in data go up as values the widgets put into words (`PathImportProblem`, `PathImportTexts`), never as English sentences. The editor's own placeholders are replaced by ours (`buildersWithUnknown`), since the editor's texts follow a process-wide locale of their own. The generated `lib/l10n/app_localizations*.dart` are committed and never edited, analysed or formatted by hand. `flutter pub get` writes them anew, so CI fails when they don't match the `.arb` files.
 
 ## Testing
 

halkyon_learn_flutter/lib/l10n/app_de.arb

@@ -832,5 +832,32 @@
   "homeWidgetPomodoro": "Pomodoro-Timer",
   "pomodoroAlerts": "Pomodoro-Meldungen",
   "pomodoroUntil": "Bis {time}",
-  "pomodoroLeft": "Noch {left}"
+  "pomodoroLeft": "Noch {left}",
+  "pathImportAtStep": "Schritt {step}: {problem}",
+  "pathImportNotJson": "Das ist kein JSON: {detail}",
+  "pathImportNoPath": "ein „path“-Objekt fehlt.",
+  "pathImportNoTitle": "der Pfad hat keinen „title“.",
+  "pathImportDescriptionNotText": "„description“ des Pfads ist kein Text.",
+  "pathImportNotAnObject": "kein Objekt.",
+  "pathImportUnknownStepType": "unbekannter „type“: {detail}.",
+  "pathImportRightOptionMissing": "die richtige Option {detail} fehlt unter den Optionen.",
+  "pathImportRightOptionsMissing": "eine genannte richtige Option fehlt unter den Optionen.",
+  "pathImportNoPairing": "kein „correct_pairing“.",
+  "pathImportPairingTargetMissing": "Elemente werden einem fehlenden Ziel zugeordnet: {detail}.",
+  "pathImportPairingIncomplete": "jedes Element braucht genau ein Ziel.",
+  "pathImportNoPassage": "keine „passage“.",
+  "pathImportBlanksMismatch": "für jedes ___ in der Passage braucht es ein Wort in „blanks“.",
+  "pathImportBlockWithoutType": "ein Block ohne „type“.",
+  "pathImportMalformedLevel": "eine Überschrift mit fehlerhafter Ebene.",
+  "pathImportMalformedListItem": "ein fehlerhafter Listenpunkt.",
+  "pathImportMalformedMath": "ein fehlerhafter Formelblock.",
+  "pathImportMalformedCode": "ein fehlerhafter Codeblock.",
+  "pathImportUnknownBlock": "ein unbekannter Block: {detail}.",
+  "pathImportTextlessBlock": "ein „{detail}“ ohne „text“ oder „runs“.",
+  "pathImportMalformedRun": "ein fehlerhafter Textabschnitt.",
+  "pathImportListExpected": "eine Liste wurde erwartet.",
+  "pathImportNotText": "„{detail}“ muss Text sein.",
+  "pathImportTooFew": "es braucht mindestens zwei „{detail}“.",
+  "pathImportMalformedNumber": "eine fehlerhafte Zahl.",
+  "pathImportTitleNotText": "ein Titel muss Text sein."
 }

halkyon_learn_flutter/lib/l10n/app_en.arb

@@ -997,5 +997,41 @@
   "pomodoroUntil": "Until {time}",
   "@pomodoroUntil": {"placeholders": {"time": {"type": "String"}}},
   "pomodoroLeft": "{left} left",
-  "@pomodoroLeft": {"placeholders": {"left": {"type": "String"}}}
+  "@pomodoroLeft": {"placeholders": {"left": {"type": "String"}}},
+  "pathImportAtStep": "Step {step}: {problem}",
+  "@pathImportAtStep": {"placeholders": {"step": {"type": "int"}, "problem": {"type": "String"}}},
+  "pathImportNotJson": "This is no JSON: {detail}",
+  "@pathImportNotJson": {"placeholders": {"detail": {"type": "String"}}},
+  "pathImportNoPath": "a \"path\" object is missing.",
+  "pathImportNoTitle": "the path has no \"title\".",
+  "pathImportDescriptionNotText": "the path's \"description\" is no text.",
+  "pathImportNotAnObject": "no object.",
+  "pathImportUnknownStepType": "unknown \"type\": {detail}.",
+  "@pathImportUnknownStepType": {"placeholders": {"detail": {"type": "String"}}},
+  "pathImportRightOptionMissing": "its right option {detail} is not among its options.",
+  "@pathImportRightOptionMissing": {"placeholders": {"detail": {"type": "String"}}},
+  "pathImportRightOptionsMissing": "it names a right option that is not among its options.",
+  "pathImportNoPairing": "no \"correct_pairing\".",
+  "pathImportPairingTargetMissing": "it pairs items with a target that isn't there: {detail}.",
+  "@pathImportPairingTargetMissing": {"placeholders": {"detail": {"type": "String"}}},
+  "pathImportPairingIncomplete": "every item must be paired with one target.",
+  "pathImportNoPassage": "no \"passage\".",
+  "pathImportBlanksMismatch": "it needs one word in \"blanks\" for each ___ in its passage.",
+  "pathImportBlockWithoutType": "a block without \"type\".",
+  "pathImportMalformedLevel": "a heading with a malformed level.",
+  "pathImportMalformedListItem": "a malformed list item.",
+  "pathImportMalformedMath": "a malformed math block.",
+  "pathImportMalformedCode": "a malformed code block.",
+  "pathImportUnknownBlock": "an unknown block: {detail}.",
+  "@pathImportUnknownBlock": {"placeholders": {"detail": {"type": "String"}}},
+  "pathImportTextlessBlock": "a {detail} without \"text\" or \"runs\".",
+  "@pathImportTextlessBlock": {"placeholders": {"detail": {"type": "String"}}},
+  "pathImportMalformedRun": "a malformed run of text.",
+  "pathImportListExpected": "a list was expected.",
+  "pathImportNotText": "\"{detail}\" must be text.",
+  "@pathImportNotText": {"placeholders": {"detail": {"type": "String"}}},
+  "pathImportTooFew": "it needs at least two \"{detail}\".",
+  "@pathImportTooFew": {"placeholders": {"detail": {"type": "String"}}},
+  "pathImportMalformedNumber": "a malformed number.",
+  "pathImportTitleNotText": "a title must be text."
 }

halkyon_learn_flutter/lib/l10n/app_localizations.dart

@@ -4988,6 +4988,168 @@ abstract class AppLocalizations {
   /// In en, this message translates to:
   /// **'{left} left'**
   String pomodoroLeft(String left);
+
+  /// No description provided for @pathImportAtStep.
+  ///
+  /// In en, this message translates to:
+  /// **'Step {step}: {problem}'**
+  String pathImportAtStep(int step, String problem);
+
+  /// No description provided for @pathImportNotJson.
+  ///
+  /// In en, this message translates to:
+  /// **'This is no JSON: {detail}'**
+  String pathImportNotJson(String detail);
+
+  /// No description provided for @pathImportNoPath.
+  ///
+  /// In en, this message translates to:
+  /// **'a \"path\" object is missing.'**
+  String get pathImportNoPath;
+
+  /// No description provided for @pathImportNoTitle.
+  ///
+  /// In en, this message translates to:
+  /// **'the path has no \"title\".'**
+  String get pathImportNoTitle;
+
+  /// No description provided for @pathImportDescriptionNotText.
+  ///
+  /// In en, this message translates to:
+  /// **'the path\'s \"description\" is no text.'**
+  String get pathImportDescriptionNotText;
+
+  /// No description provided for @pathImportNotAnObject.
+  ///
+  /// In en, this message translates to:
+  /// **'no object.'**
+  String get pathImportNotAnObject;
+
+  /// No description provided for @pathImportUnknownStepType.
+  ///
+  /// In en, this message translates to:
+  /// **'unknown \"type\": {detail}.'**
+  String pathImportUnknownStepType(String detail);
+
+  /// No description provided for @pathImportRightOptionMissing.
+  ///
+  /// In en, this message translates to:
+  /// **'its right option {detail} is not among its options.'**
+  String pathImportRightOptionMissing(String detail);
+
+  /// No description provided for @pathImportRightOptionsMissing.
+  ///
+  /// In en, this message translates to:
+  /// **'it names a right option that is not among its options.'**
+  String get pathImportRightOptionsMissing;
+
+  /// No description provided for @pathImportNoPairing.
+  ///
+  /// In en, this message translates to:
+  /// **'no \"correct_pairing\".'**
+  String get pathImportNoPairing;
+
+  /// No description provided for @pathImportPairingTargetMissing.
+  ///
+  /// In en, this message translates to:
+  /// **'it pairs items with a target that isn\'t there: {detail}.'**
+  String pathImportPairingTargetMissing(String detail);
+
+  /// No description provided for @pathImportPairingIncomplete.
+  ///
+  /// In en, this message translates to:
+  /// **'every item must be paired with one target.'**
+  String get pathImportPairingIncomplete;
+
+  /// No description provided for @pathImportNoPassage.
+  ///
+  /// In en, this message translates to:
+  /// **'no \"passage\".'**
+  String get pathImportNoPassage;
+
+  /// No description provided for @pathImportBlanksMismatch.
+  ///
+  /// In en, this message translates to:
+  /// **'it needs one word in \"blanks\" for each ___ in its passage.'**
+  String get pathImportBlanksMismatch;
+
+  /// No description provided for @pathImportBlockWithoutType.
+  ///
+  /// In en, this message translates to:
+  /// **'a block without \"type\".'**
+  String get pathImportBlockWithoutType;
+
+  /// No description provided for @pathImportMalformedLevel.
+  ///
+  /// In en, this message translates to:
+  /// **'a heading with a malformed level.'**
+  String get pathImportMalformedLevel;
+
+  /// No description provided for @pathImportMalformedListItem.
+  ///
+  /// In en, this message translates to:
+  /// **'a malformed list item.'**
+  String get pathImportMalformedListItem;
+
+  /// No description provided for @pathImportMalformedMath.
+  ///
+  /// In en, this message translates to:
+  /// **'a malformed math block.'**
+  String get pathImportMalformedMath;
+
+  /// No description provided for @pathImportMalformedCode.
+  ///
+  /// In en, this message translates to:
+  /// **'a malformed code block.'**
+  String get pathImportMalformedCode;
+
+  /// No description provided for @pathImportUnknownBlock.
+  ///
+  /// In en, this message translates to:
+  /// **'an unknown block: {detail}.'**
+  String pathImportUnknownBlock(String detail);
+
+  /// No description provided for @pathImportTextlessBlock.
+  ///
+  /// In en, this message translates to:
+  /// **'a {detail} without \"text\" or \"runs\".'**
+  String pathImportTextlessBlock(String detail);
+
+  /// No description provided for @pathImportMalformedRun.
+  ///
+  /// In en, this message translates to:
+  /// **'a malformed run of text.'**
+  String get pathImportMalformedRun;
+
+  /// No description provided for @pathImportListExpected.
+  ///
+  /// In en, this message translates to:
+  /// **'a list was expected.'**
+  String get pathImportListExpected;
+
+  /// No description provided for @pathImportNotText.
+  ///
+  /// In en, this message translates to:
+  /// **'\"{detail}\" must be text.'**
+  String pathImportNotText(String detail);
+
+  /// No description provided for @pathImportTooFew.
+  ///
+  /// In en, this message translates to:
+  /// **'it needs at least two \"{detail}\".'**
+  String pathImportTooFew(String detail);
+
+  /// No description provided for @pathImportMalformedNumber.
+  ///
+  /// In en, this message translates to:
+  /// **'a malformed number.'**
+  String get pathImportMalformedNumber;
+
+  /// No description provided for @pathImportTitleNotText.
+  ///
+  /// In en, this message translates to:
+  /// **'a title must be text.'**
+  String get pathImportTitleNotText;
 }
 
 class _AppLocalizationsDelegate

halkyon_learn_flutter/lib/l10n/app_localizations_de.dart

@@ -3017,4 +3017,108 @@ class AppLocalizationsDe extends AppLocalizations {
   String pomodoroLeft(String left) {
     return 'Noch $left';
   }
+
+  @override
+  String pathImportAtStep(int step, String problem) {
+    return 'Schritt $step: $problem';
+  }
+
+  @override
+  String pathImportNotJson(String detail) {
+    return 'Das ist kein JSON: $detail';
+  }
+
+  @override
+  String get pathImportNoPath => 'ein „path“-Objekt fehlt.';
+
+  @override
+  String get pathImportNoTitle => 'der Pfad hat keinen „title“.';
+
+  @override
+  String get pathImportDescriptionNotText =>
+      '„description“ des Pfads ist kein Text.';
+
+  @override
+  String get pathImportNotAnObject => 'kein Objekt.';
+
+  @override
+  String pathImportUnknownStepType(String detail) {
+    return 'unbekannter „type“: $detail.';
+  }
+
+  @override
+  String pathImportRightOptionMissing(String detail) {
+    return 'die richtige Option $detail fehlt unter den Optionen.';
+  }
+
+  @override
+  String get pathImportRightOptionsMissing =>
+      'eine genannte richtige Option fehlt unter den Optionen.';
+
+  @override
+  String get pathImportNoPairing => 'kein „correct_pairing“.';
+
+  @override
+  String pathImportPairingTargetMissing(String detail) {
+    return 'Elemente werden einem fehlenden Ziel zugeordnet: $detail.';
+  }
+
+  @override
+  String get pathImportPairingIncomplete =>
+      'jedes Element braucht genau ein Ziel.';
+
+  @override
+  String get pathImportNoPassage => 'keine „passage“.';
+
+  @override
+  String get pathImportBlanksMismatch =>
+      'für jedes ___ in der Passage braucht es ein Wort in „blanks“.';
+
+  @override
+  String get pathImportBlockWithoutType => 'ein Block ohne „type“.';
+
+  @override
+  String get pathImportMalformedLevel =>
+      'eine Überschrift mit fehlerhafter Ebene.';
+
+  @override
+  String get pathImportMalformedListItem => 'ein fehlerhafter Listenpunkt.';
+
+  @override
+  String get pathImportMalformedMath => 'ein fehlerhafter Formelblock.';
+
+  @override
+  String get pathImportMalformedCode => 'ein fehlerhafter Codeblock.';
+
+  @override
+  String pathImportUnknownBlock(String detail) {
+    return 'ein unbekannter Block: $detail.';
+  }
+
+  @override
+  String pathImportTextlessBlock(String detail) {
+    return 'ein „$detail“ ohne „text“ oder „runs“.';
+  }
+
+  @override
+  String get pathImportMalformedRun => 'ein fehlerhafter Textabschnitt.';
+
+  @override
+  String get pathImportListExpected => 'eine Liste wurde erwartet.';
+
+  @override
+  String pathImportNotText(String detail) {
+    return '„$detail“ muss Text sein.';
+  }
+
+  @override
+  String pathImportTooFew(String detail) {
+    return 'es braucht mindestens zwei „$detail“.';
+  }
+
+  @override
+  String get pathImportMalformedNumber => 'eine fehlerhafte Zahl.';
+
+  @override
+  String get pathImportTitleNotText => 'ein Titel muss Text sein.';
 }

halkyon_learn_flutter/lib/l10n/app_localizations_en.dart

@@ -2987,6 +2987,109 @@ class AppLocalizationsEn extends AppLocalizations {
   String pomodoroLeft(String left) {
     return '$left left';
   }
+
+  @override
+  String pathImportAtStep(int step, String problem) {
+    return 'Step $step: $problem';
+  }
+
+  @override
+  String pathImportNotJson(String detail) {
+    return 'This is no JSON: $detail';
+  }
+
+  @override
+  String get pathImportNoPath => 'a \"path\" object is missing.';
+
+  @override
+  String get pathImportNoTitle => 'the path has no \"title\".';
+
+  @override
+  String get pathImportDescriptionNotText =>
+      'the path\'s \"description\" is no text.';
+
+  @override
+  String get pathImportNotAnObject => 'no object.';
+
+  @override
+  String pathImportUnknownStepType(String detail) {
+    return 'unknown \"type\": $detail.';
+  }
+
+  @override
+  String pathImportRightOptionMissing(String detail) {
+    return 'its right option $detail is not among its options.';
+  }
+
+  @override
+  String get pathImportRightOptionsMissing =>
+      'it names a right option that is not among its options.';
+
+  @override
+  String get pathImportNoPairing => 'no \"correct_pairing\".';
+
+  @override
+  String pathImportPairingTargetMissing(String detail) {
+    return 'it pairs items with a target that isn\'t there: $detail.';
+  }
+
+  @override
+  String get pathImportPairingIncomplete =>
+      'every item must be paired with one target.';
+
+  @override
+  String get pathImportNoPassage => 'no \"passage\".';
+
+  @override
+  String get pathImportBlanksMismatch =>
+      'it needs one word in \"blanks\" for each ___ in its passage.';
+
+  @override
+  String get pathImportBlockWithoutType => 'a block without \"type\".';
+
+  @override
+  String get pathImportMalformedLevel => 'a heading with a malformed level.';
+
+  @override
+  String get pathImportMalformedListItem => 'a malformed list item.';
+
+  @override
+  String get pathImportMalformedMath => 'a malformed math block.';
+
+  @override
+  String get pathImportMalformedCode => 'a malformed code block.';
+
+  @override
+  String pathImportUnknownBlock(String detail) {
+    return 'an unknown block: $detail.';
+  }
+
+  @override
+  String pathImportTextlessBlock(String detail) {
+    return 'a $detail without \"text\" or \"runs\".';
+  }
+
+  @override
+  String get pathImportMalformedRun => 'a malformed run of text.';
+
+  @override
+  String get pathImportListExpected => 'a list was expected.';
+
+  @override
+  String pathImportNotText(String detail) {
+    return '\"$detail\" must be text.';
+  }
+
+  @override
+  String pathImportTooFew(String detail) {
+    return 'it needs at least two \"$detail\".';
+  }
+
+  @override
+  String get pathImportMalformedNumber => 'a malformed number.';
+
+  @override
+  String get pathImportTitleNotText => 'a title must be text.';
 }
 
 /// The translations for English, as used in the United Kingdom (`en_GB`).

halkyon_learn_flutter/lib/src/feature/learning_path/bloc/path_list_bloc.dart

@@ -50,7 +50,7 @@ class PathListBloc(
               title: '',
               steps: 0,
               skippedBlocks: 0,
-              failure: error.message,
+              failure: error,
             ),
           ),
         );
@@ -154,7 +154,7 @@ typedef ImportResult = ({
   String title,
   int steps,
   int skippedBlocks,
-  String? failure,
+  PathImportException? failure,
 });
 
 /// A studyset's paths, as the list shows them.

halkyon_learn_flutter/lib/src/feature/learning_path/data/path_import.dart

@@ -35,13 +35,52 @@ import 'package:halkyon_learn_flutter/src/feature/learning_path/model/step_conte
 // "image" and "lottie" are left out, keeping an "alt" caption: their files
 // would have to be fetched from the web, which the app never does.
 
-/// A file in the import format that couldn't be read: [message] says why.
+/// What is wrong with a file in the import format.
+enum PathImportProblem() {
+  notJson,
+  noPath,
+  noTitle,
+  descriptionNotText,
+  notAnObject,
+  unknownStepType,
+  rightOptionMissing,
+  rightOptionsMissing,
+  noPairing,
+  pairingTargetMissing,
+  pairingIncomplete,
+  noPassage,
+  blanksMismatch,
+  blockWithoutType,
+  malformedLevel,
+  malformedListItem,
+  malformedMath,
+  malformedCode,
+  unknownBlock,
+  textlessBlock,
+  malformedRun,
+  listExpected,
+  notText,
+  tooFew,
+  malformedNumber,
+  titleNotText,
+}
+
+/// A file in the import format that couldn't be read: the [problem], the
+/// [step] it is in, and what it names ([detail]: a field, a type, a
+/// number); the app says it in its language.
 final class const PathImportException(
-  /// What is wrong, and where.
+  /// What is wrong.
-  final String message,
+  final PathImportProblem problem, {
-) implements Exception {
+
+  /// The step it is in, counting from 1; null for the path itself.
+  final int? step,
+
+  /// What the problem names, as written in the file.
+  final String detail = '',
+}) implements Exception {
   @override
-  String toString() => 'PathImportException: $message';
+  String toString() =>
+      'PathImportException: ${problem.name} in ${step ?? 'the path'}: $detail';
 }
 
 /// A path read from the import format, not stored yet.
@@ -81,33 +120,33 @@ ParsedPath parsePathImport(String source) {
   try {
     json = jsonDecode(source);
   } on FormatException catch (error) {
-    throw PathImportException('This is no JSON: ${error.message}.');
+    throw PathImportException(PathImportProblem.notJson, detail: error.message);
   }
   final path = switch (json) {
     {'path': final Map<String, Object?> path} => path,
-    _ => throw const PathImportException('A "path" object is missing.'),
+    _ => throw const PathImportException(PathImportProblem.noPath),
   };
   final title = switch (path['title']) {
     final String title when title.trim().isNotEmpty => title,
-    _ => throw const PathImportException('The path has no "title".'),
+    _ => throw const PathImportException(PathImportProblem.noTitle),
   };
   final blocks = _Blocks();
   final steps = [
     for (final (index, step) in _list(
       path['steps'],
-      'the path\'s "steps"',
+      null,
     ).indexed)
       _step(step, index + 1, blocks),
   ];
   return (
     content: LearningPathContent(
       title: title,
-      secondaryTitle: _optionalString(path['secondary_title'], 'the path'),
+      secondaryTitle: _optionalString(path['secondary_title'], null),
       description: switch (path['description']) {
         null => RichContent.empty(),
         final String text => RichContent.plain(text),
         _ => throw const PathImportException(
-          'The path\'s "description" is no text.',
+          PathImportProblem.descriptionNotText,
         ),
       },
     ).normalized,
@@ -121,10 +160,10 @@ ParsedPath parsePathImport(String source) {
   int number,
   _Blocks blocks,
 ) {
-  final where = 'step $number';
+  final where = number;
   final step = switch (json) {
     final Map<String, Object?> step => step,
-    _ => throw PathImportException('The $where is no object.'),
+    _ => throw PathImportException(PathImportProblem.notAnObject, step: where),
   };
   final kind = switch (step['type']) {
     'info' => StepKind.info,
@@ -133,7 +172,9 @@ ParsedPath parsePathImport(String source) {
     'drag_and_drop' => StepKind.dragAndDrop,
     'fill_in_the_blank' => StepKind.fillInTheBlank,
     final type => throw PathImportException(
-      'The $where has an unknown "type": $type.',
+      PathImportProblem.unknownStepType,
+      step: where,
+      detail: '$type',
     ),
   };
   final exercise = switch (kind) {
@@ -143,7 +184,9 @@ ParsedPath parsePathImport(String source) {
       final correct = _int(step['correct_option_index'], where);
       if (correct >= options.length) {
         throw PathImportException(
-          "The $where's right option $correct is not among its options.",
+          PathImportProblem.rightOptionMissing,
+          step: where,
+          detail: '$correct',
         );
       }
       return SingleChoice(options: options, correct: correct);
@@ -156,7 +199,8 @@ ParsedPath parsePathImport(String source) {
       };
       if (correct.any((index) => index >= options.length)) {
         throw PathImportException(
-          'The $where names a right option that is not among its options.',
+          PathImportProblem.rightOptionsMissing,
+          step: where,
         );
       }
       return MultipleChoice(options: options, correct: correct);
@@ -168,14 +212,17 @@ ParsedPath parsePathImport(String source) {
       final pairing = switch (step['correct_pairing']) {
         final Map<String, Object?> pairing => pairing,
         _ => throw PathImportException(
-          'The $where has no "correct_pairing".',
+          PathImportProblem.noPairing,
+          step: where,
         ),
       };
       for (final MapEntry(key: target, value: placed) in pairing.entries) {
         final at = int.tryParse(target);
         if (at == null || at >= targets.length) {
           throw PathImportException(
-            "The $where pairs items with a target that isn't there: $target.",
+            PathImportProblem.pairingTargetMissing,
+            step: where,
+            detail: target,
           );
         }
         for (final item in _list(placed, where)) {
@@ -185,7 +232,8 @@ ParsedPath parsePathImport(String source) {
       if (targetOf.length != items.length ||
           targetOf.keys.any((item) => item >= items.length)) {
         throw PathImportException(
-          'The $where must pair every item with one target.',
+          PathImportProblem.pairingIncomplete,
+          step: where,
         );
       }
       return DragAndDrop(
@@ -199,7 +247,10 @@ ParsedPath parsePathImport(String source) {
     StepKind.fillInTheBlank => () {
       final passage = switch (step['passage']) {
         final String passage => passage,
-        _ => throw PathImportException('The $where has no "passage".'),
+        _ => throw PathImportException(
+          PathImportProblem.noPassage,
+          step: where,
+        ),
       };
       final exercise = FillInTheBlank(
         passage: passage,
@@ -207,7 +258,8 @@ ParsedPath parsePathImport(String source) {
       );
       if (!exercise.isComplete) {
         throw PathImportException(
-          'The $where needs one word in "blanks" for each ___ in its passage.',
+          PathImportProblem.blanksMismatch,
+          step: where,
         );
       }
       return exercise;
@@ -230,7 +282,7 @@ class _Blocks() {
   /// How many blocks were left out.
   int skipped = 0;
 
-  RichContent read(Object? json, String where) {
+  RichContent read(Object? json, int? where) {
     if (json == null) {
       return RichContent.empty();
     }
@@ -245,10 +297,13 @@ class _Blocks() {
   }
 
   /// The editor's nodes for [block]; null for a block left out.
-  List<Map<String, Object?>>? _nodes(Object? block, String where) {
+  List<Map<String, Object?>>? _nodes(Object? block, int? where) {
     final type = switch (block) {
       {'type': final String type} => type,
-      _ => throw PathImportException('The $where has a block without "type".'),
+      _ => throw PathImportException(
+        PathImportProblem.blockWithoutType,
+        step: where,
+      ),
     };
     final json = block as Map<String, Object?>;
     switch (type) {
@@ -259,7 +314,10 @@ class _Blocks() {
         final level = switch (json['level']) {
           final int level when level >= 1 && level <= 6 => level,
           null => 2,
-          _ => throw PathImportException('The $where has a malformed level.'),
+          _ => throw PathImportException(
+            PathImportProblem.malformedLevel,
+            step: where,
+          ),
         };
         return [
           {
@@ -280,7 +338,8 @@ class _Blocks() {
                   final String text => _plain(text),
                   {'runs': final Object? runs} => _runs(runs, where),
                   _ => throw PathImportException(
-                    'The $where has a malformed list item.',
+                    PathImportProblem.malformedListItem,
+                    step: where,
                   ),
                 },
               },
@@ -289,7 +348,10 @@ class _Blocks() {
       case 'math':
         final formula = json['formula'];
         if (formula is! String) {
-          throw PathImportException('The $where has a malformed math block.');
+          throw PathImportException(
+            PathImportProblem.malformedMath,
+            step: where,
+          );
         }
         return [
           {
@@ -300,7 +362,10 @@ class _Blocks() {
       case 'code':
         final source = json['code'];
         if (source is! String) {
-          throw PathImportException('The $where has a malformed code block.');
+          throw PathImportException(
+            PathImportProblem.malformedCode,
+            step: where,
+          );
         }
         return [
           {
@@ -334,14 +399,18 @@ class _Blocks() {
           _ => null,
         };
       default:
-        throw PathImportException('The $where has an unknown block: $type.');
+        throw PathImportException(
+          PathImportProblem.unknownBlock,
+          step: where,
+          detail: type,
+        );
     }
   }
 
   Map<String, Object?> _text(
     String type,
     Map<String, Object?> json,
-    String where,
+    int? where,
   ) => {
     'type': type,
     'data': {
@@ -349,7 +418,9 @@ class _Blocks() {
         {'text': final String text} => _plain(text),
         {'runs': final Object? runs} => _runs(runs, where),
         _ => throw PathImportException(
-          'The $where has a $type without "text" or "runs".',
+          PathImportProblem.textlessBlock,
+          step: where,
+          detail: type,
         ),
       },
     },
@@ -359,7 +430,7 @@ class _Blocks() {
     if (text.isNotEmpty) {'insert': text},
   ];
 
-  List<Map<String, Object?>> _runs(Object? json, String where) => [
+  List<Map<String, Object?>> _runs(Object? json, int? where) => [
     for (final run in _list(json, where))
       switch (run) {
         {'math': final String formula} => {
@@ -376,38 +447,52 @@ class _Blocks() {
               case final attributes when attributes.isNotEmpty)
             'attributes': attributes,
         },
-        _ => throw PathImportException('The $where has a malformed run.'),
+        _ => throw PathImportException(
+          PathImportProblem.malformedRun,
+          step: where,
+        ),
       },
   ];
 }
 
-List<Object?> _list(Object? json, String where) => switch (json) {
+List<Object?> _list(Object? json, int? where) => switch (json) {
   final List<Object?> list => list,
   null => const [],
-  _ => throw PathImportException('In $where, a list was expected.'),
+  _ => throw PathImportException(PathImportProblem.listExpected, step: where),
 };
 
-List<String> _strings(Object? json, String where, String field) {
+List<String> _strings(Object? json, int? where, String field) {
   final strings = [
     for (final value in _list(json, where))
       switch (value) {
         final String text => text,
-        _ => throw PathImportException('The $where\'s "$field" must be text.'),
+        _ => throw PathImportException(
+          PathImportProblem.notText,
+          step: where,
+          detail: field,
+        ),
       },
   ];
   if (strings.length < 2 && field != 'blanks' && field != 'items') {
-    throw PathImportException('The $where needs at least two "$field".');
+    throw PathImportException(
+      PathImportProblem.tooFew,
+      step: where,
+      detail: field,
+    );
   }
   return strings;
 }
 
-int _int(Object? json, String where) => switch (json) {
+int _int(Object? json, int? where) => switch (json) {
   final int number when number >= 0 => number,
-  _ => throw PathImportException('The $where has a malformed number.'),
+  _ => throw PathImportException(
+    PathImportProblem.malformedNumber,
+    step: where,
+  ),
 };
 
-String? _optionalString(Object? json, String where) => switch (json) {
+String? _optionalString(Object? json, int? where) => switch (json) {
   null => null,
   final String text => text,
-  _ => throw PathImportException('In $where, a title must be text.'),
+  _ => throw PathImportException(PathImportProblem.titleNotText, step: where),
 };

halkyon_learn_flutter/lib/src/feature/learning_path/widget/path_import_texts.dart

@@ -0,0 +1,51 @@
+import 'package:halkyon_learn_flutter/l10n/app_localizations.dart';
+import 'package:halkyon_learn_flutter/src/feature/learning_path/data/path_import.dart';
+
+/// What is wrong with an import file, in the app's language.
+extension PathImportTexts on AppLocalizations {
+  /// What [error] says is wrong, with the step it is in.
+  String pathImportProblemText(PathImportException error) {
+    final detail = error.detail;
+    final problem = switch (error.problem) {
+      PathImportProblem.notJson => pathImportNotJson(detail),
+      PathImportProblem.noPath => pathImportNoPath,
+      PathImportProblem.noTitle => pathImportNoTitle,
+      PathImportProblem.descriptionNotText => pathImportDescriptionNotText,
+      PathImportProblem.notAnObject => pathImportNotAnObject,
+      PathImportProblem.unknownStepType => pathImportUnknownStepType(detail),
+      PathImportProblem.rightOptionMissing => pathImportRightOptionMissing(
+        detail,
+      ),
+      PathImportProblem.rightOptionsMissing => pathImportRightOptionsMissing,
+      PathImportProblem.noPairing => pathImportNoPairing,
+      PathImportProblem.pairingTargetMissing => pathImportPairingTargetMissing(
+        detail,
+      ),
+      PathImportProblem.pairingIncomplete => pathImportPairingIncomplete,
+      PathImportProblem.noPassage => pathImportNoPassage,
+      PathImportProblem.blanksMismatch => pathImportBlanksMismatch,
+      PathImportProblem.blockWithoutType => pathImportBlockWithoutType,
+      PathImportProblem.malformedLevel => pathImportMalformedLevel,
+      PathImportProblem.malformedListItem => pathImportMalformedListItem,
+      PathImportProblem.malformedMath => pathImportMalformedMath,
+      PathImportProblem.malformedCode => pathImportMalformedCode,
+      PathImportProblem.unknownBlock => pathImportUnknownBlock(detail),
+      PathImportProblem.textlessBlock => pathImportTextlessBlock(detail),
+      PathImportProblem.malformedRun => pathImportMalformedRun,
+      PathImportProblem.listExpected => pathImportListExpected,
+      PathImportProblem.notText => pathImportNotText(detail),
+      PathImportProblem.tooFew => pathImportTooFew(detail),
+      PathImportProblem.malformedNumber => pathImportMalformedNumber,
+      PathImportProblem.titleNotText => pathImportTitleNotText,
+    };
+    final sentence = switch (error.step) {
+      final step? => pathImportAtStep(step, problem),
+      null => problem,
+    };
+    // The problems read as the rest of a sentence; standing alone, the
+    // first letter goes up.
+    return sentence.isEmpty
+        ? sentence
+        : sentence[0].toUpperCase() + sentence.substring(1);
+  }
+}

halkyon_learn_flutter/lib/src/feature/studyset/widget/studyset_screen.dart

@@ -21,6 +21,7 @@ import 'package:halkyon_learn_flutter/src/feature/anki/data/anki_importer.dart';
 import 'package:halkyon_learn_flutter/src/feature/anki/widget/anki_import_sheet.dart';
 import 'package:halkyon_learn_flutter/src/feature/card/widget/card_list.dart';
 import 'package:halkyon_learn_flutter/src/feature/learning_path/bloc/path_list_bloc.dart';
+import 'package:halkyon_learn_flutter/src/feature/learning_path/widget/path_import_texts.dart';
 import 'package:halkyon_learn_flutter/src/feature/learning_path/widget/path_list.dart';
 import 'package:halkyon_learn_flutter/src/feature/note/widget/note_list.dart';
 import 'package:halkyon_learn_flutter/src/feature/pdf_source/bloc/pdf_list_bloc.dart';
@@ -120,7 +121,9 @@ class const StudysetScreen({
                 context.read<ContentTabCubit>().show(StudysetTab.paths);
               }
               final message = switch (result.failure) {
-                final failure? => l10n.pathImportFailed(failure),
+                final failure? => l10n.pathImportFailed(
+                  l10n.pathImportProblemText(failure),
+                ),
                 null => [
                   l10n.pathImported(result.title, result.steps),
                   if (result.skippedBlocks > 0)

halkyon_learn_flutter/test/feature/learning_path/bloc/path_list_bloc_test.dart

@@ -2,6 +2,7 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:halkyon_learn_flutter/src/feature/learning_path/bloc/path_list_bloc.dart';
 import 'package:halkyon_learn_flutter/src/feature/learning_path/data/learning_path_deletions.dart';
 import 'package:halkyon_learn_flutter/src/feature/learning_path/data/learning_path_repository.dart';
+import 'package:halkyon_learn_flutter/src/feature/learning_path/data/path_import.dart';
 import 'package:halkyon_learn_flutter/src/feature/learning_path/data/path_progress_repository.dart';
 import 'package:halkyon_learn_flutter/src/feature/learning_path/model/exercise.dart';
 import 'package:halkyon_learn_flutter/src/feature/learning_path/model/learning_path.dart';
@@ -116,7 +117,7 @@ void main() {
 
     bloc.add(const PathListEventImportRequested('{'));
     final failed = await until((s) => s.importResult?.failure != null);
-    expect(failed.importResult!.failure, contains('JSON'));
+    expect(failed.importResult!.failure?.problem, PathImportProblem.notJson);
   });
 
   test('creating a path hands back its id, and lists it', () async {

halkyon_learn_flutter/test/feature/learning_path/data/path_import_test.dart

@@ -1,9 +1,12 @@
 import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_learn_flutter/l10n/app_localizations.dart';
 import 'package:halkyon_learn_flutter/src/core/content/content_blocks.dart';
 import 'package:halkyon_learn_flutter/src/core/content/rich_content.dart';
 import 'package:halkyon_learn_flutter/src/feature/learning_path/data/learning_path_repository.dart';
 import 'package:halkyon_learn_flutter/src/feature/learning_path/data/path_import.dart';
 import 'package:halkyon_learn_flutter/src/feature/learning_path/model/exercise.dart';
+import 'package:halkyon_learn_flutter/src/feature/learning_path/widget/path_import_texts.dart';
+import 'package:material_ui/material_ui.dart';
 
 import '../../../support/fake_learning_path_gateway.dart';
 
@@ -158,49 +161,76 @@ void main() {
   });
 
   group('refuses, saying why,', () {
-    void refuses(String source, String reason) => expect(
+    void refuses(String source, PathImportProblem problem, {int? step}) =>
-      () => parsePathImport(source),
+        expect(
-      throwsA(
+          () => parsePathImport(source),
-        isA<PathImportException>().having(
+          throwsA(
-          (e) => e.message,
+            isA<PathImportException>()
-          'message',
+                .having((e) => e.problem, 'problem', problem)
-          contains(reason),
+                .having((e) => e.step, 'step', step),
-        ),
+          ),
-      ),
+        );
-    );
 
-    test('what is no JSON', () => refuses('{', 'JSON'));
+    test('what is no JSON', () => refuses('{', PathImportProblem.notJson));
 
     test('a path without a title', () {
-      refuses('{"path": {"steps": []}}', 'title');
+      refuses('{"path": {"steps": []}}', PathImportProblem.noTitle);
     });
 
     test('a step of an unknown type', () {
       refuses(
         '{"path": {"title": "x", "steps": [{"title": "a", "type": "quiz"}]}}',
-        'step 1',
+        PathImportProblem.unknownStepType,
+        step: 1,
       );
     });
 
     test('a right option that is not there', () {
       refuses(
         '{"path": {"title": "x", "steps": [{"title": "a", '
-            '"type": "single_choice", "options": ["a", "b"], '
+        '"type": "single_choice", "options": ["a", "b"], '
-            '"correct_option_index": 5}]}}',
+        '"correct_option_index": 5}]}}',
-        'step 1',
+        PathImportProblem.rightOptionMissing,
+        step: 1,
       );
     });
 
     test('blanks that do not fit the gaps', () {
       refuses(
         '{"path": {"title": "x", "steps": [{"title": "a", '
-            '"type": "fill_in_the_blank", "passage": "a ___ b ___", '
+        '"type": "fill_in_the_blank", "passage": "a ___ b ___", '
-            '"blanks": ["x"]}]}}',
+        '"blanks": ["x"]}]}}',
-        'step 1',
+        PathImportProblem.blanksMismatch,
+        step: 1,
       );
     });
   });
 
+  test("says what is wrong in the app's language", () {
+    const atStep = PathImportException(
+      PathImportProblem.unknownStepType,
+      step: 2,
+      detail: 'quiz',
+    );
+    const ofPath = PathImportException(PathImportProblem.noTitle);
+
+    final english = lookupAppLocalizations(const Locale('en', 'GB'));
+    final german = lookupAppLocalizations(const Locale('de'));
+    expect(
+      english.pathImportProblemText(atStep),
+      'Step 2: unknown "type": quiz.',
+    );
+    expect(english.pathImportProblemText(ofPath), 'The path has no "title".');
+    expect(
+      german.pathImportProblemText(atStep),
+      'Schritt 2: unbekannter „type“: quiz.',
+    );
+    expect(
+      german.pathImportProblemText(ofPath),
+      'Der Pfad hat keinen „title“.',
+    );
+  });
+
   test('imports a path with its steps into a studyset', () async {
     final paths = LearningPathRepository(FakeLearningPathGateway());
     addTearDown(paths.dispose);