Flutter error handling

Skill ComeOnOliver/skillshub/skills/HoangNguyen0403/agent-skills-standard/flutter-error-handling

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npx -y skills add ComeOnOliver/skillshub --skill flutter-error-handling

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Functional error handling with Either/Failure. ALWAYS consult when writing repositories, handling exceptions, defining failures, or using Either in any Flutter layer — not just when setting up error handling. (triggers: lib/domain/**, lib/infrastructure/**, Either, fold, Left, Right, Failure, dartz)

SKILL.md

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Error Handling

Priority: P1 (HIGH)

Standardized functional error handling using dartz and freezed failures.

Implementation Guidelines

Either Pattern: Return Either<Failure, T> from repositories. No exceptions in UI/BLoC.
Failures: Define domain-specific failures using @freezed unions (e.g., UnauthorizedFailure, OutOfStockFailure).
Mapping: Infrastructure catches Exception (e.g., DioException) and returns Left(Failure). Never rethrow to UI.
Consumption: Use .fold(failure, success) in BLoC to emit corresponding states. Remove try/catch from BLoC.
Typed Errors: Use left(Failure()) and right(Value()) from Dartz.
Low-Cardinality Logging: Use stable message templates; pass variable data via metadata/context.
Layer Restriction: try/catch only in Infrastructure. UI/Application/BLoC layers should not catch.
Localization: Use failure.failureMessage (returns TRObject or localized string) for UI-safe text.
Right/Left Restriction: Only Infrastructure may construct Right()/Left().
No Silent Catch: Never swallow errors without logging or a documented retry.

Reference & Examples

For Failure definitions and API error mapping: See references/REFERENCE.md.

Anti-Patterns

❌ try { … } catch (e) { emit(ErrorState()); } in BLoC — try/catch belongs only in Infrastructure; BLoC receives Either, then folds
❌ Left(Failure('Something went wrong')) using a plain String — define typed @freezed Failure subclasses for each domain error
❌ catch (e) {} empty catch — always log and propagate; never swallow silently
❌ Throwing Exception from a repository — return Left(Failure) instead; exceptions must not cross the infrastructure boundary