swift-error-handling

name	swift-error-handling
description	Standards for Throwing Functions, Result Type, and Never. Use when implementing Swift error throwing, Result<T,E>, or designing error hierarchies. (triggers: **/*.swift, throws, try, catch, Result, Error)

Swift Error Handling

Priority: P0

Implementation Guidelines

Throwing Functions

• Propagate Errors: Use throws for recoverable errors and async throws for modern concurrency.
• Do-Catch: Handle errors close to source with specific catch clauses for each error type. Catch-all catch should be the last resort.
• Error Types: Define custom errors as enum conforming to Error (e.g., enum NetworkError: Error { case connectionLost }) for user-facing descriptions.
• Optional Try: Use try? only for non-critical errors.

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

• Error Types & Result