crosscue

ADR-0004 — Typed load errors are plain sealed classes

Status: Accepted · Date: 2026 (Sprint 4)

Context

Solve-screen load failures needed exhaustively-switchable error types. Freezed unions work but add codegen weight for types that carry little data.

Decision

SolveLoadError and its subtypes (PuzzleNotFoundError, SolveSessionLoadError) are hand-written plain sealed classes in solve/domain/models/solve_errors.dart. Presentation switches exhaustively: switch (e) { PuzzleNotFoundError() => ..., ... }. The same pattern was later reused for SyncState.

Consequences

• Exhaustiveness checking without build_runner involvement.
• Convention: sealed hierarchies that are mostly tags stay plain Dart; Freezed is for data-carrying value objects (see CONVENTIONS.md).