skills/ HoangNguyen0403/agent-skills-standard

swift-error-handling

Standards for throwing functions, Result type, and Never. Use when implementing Swift error throwing, designing error hierarchies, using Result types, or adding do-catch blocks.

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Swift Error Handling

Priority: P0 (CRITICAL)

Workflow: Add Error Handling to Swift Function

  1. Define custom error enum conforming to Error
  2. Mark function throws (or async throws)
  3. Use do-catch at call site with specific catch clauses
  4. Map domain errors to user-facing messages at presentation layer

Implementation Guidelines

Throwing Functions

  • Propagate Errors: Use throws for recoverable errors and async throws for modern concurrency.
  • Do-Catch: Handle close to source; specific catch clauses per error type. Catch-all catch: last resort.
  • Error Types: Define custom errors as enums conforming to Error:

See implementation examples for custom error enums, do-catch patterns, and Result type usage.

  • Optional Try: Use try? only for non-critical errors where nil acceptable.

Result Type

  • Async Alternatives: Use throws for synchronous code. Use Result for callbacks and non-async deferred error states.
  • Transformations: Use .map(), .flatMap() for functional composition.
  • Conversion: Use .get() to convert Result to throwing for use in try-catch.

Never Type & Preconditions

  • Fatalisms: Use Never return type only for unrecoverable crash scenarios or to indicate unreachable code. Never for expected errors.
  • Preconditions: Use precondition(), assert(), and fatalError() for programmer errors. Use assertionFailure() for debug-only checks.

Anti-Patterns

  • No try!: Use try? or do-catch.
  • No try? without nil check: Handle or log.
  • No Error(message): Use typed errors.

References

Canonical response anchors

When this skill applies, preserve the following domain terminology or equivalent concrete examples in the answer when relevant:

  • Never for expected

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