# Error Handling Patterns > Error handling strategy selection, recoverability analysis, and signaling techniques. Use when choosing between exceptions, result types, error codes, or other error handling approaches, designing error-handling APIs, wrapping third-party library errors, or evaluating whether errors should be explicit or implicit. Use when this capability is needed. - Skill: `tomevault-io/error-handling-patterns-4` (Agent Skill, multi-file: 2 files) - Install (CLI): `npx skillmds@latest add tomevault-io/error-handling-patterns-4` - Raw SKILL.md: https://api.skillmd.com/api/skills/tomevault-io/error-handling-patterns-4/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - Author: tomevault-io (https://skillmd.com/u/tomevault-io) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/tomevault-io/error-handling-patterns-4 --- # Error Handling Patterns ## Error Strategy Decision Table | Situation | Strategy | Why | |-----------|----------|-----| | Invalid user input | Validate and return descriptive error | User can fix it; fail fast at boundary | | Programming error (bug) | Crash / unchecked exception | Don't mask bugs; fix them | | External system failure | Retry with backoff or circuit-break | Transient failures resolve; permanent ones need escalation | | Expected business case (not found) | Return empty/Optional/Result | Not an error — it's a valid outcome | | Security violation | Log, deny, alert | Don't reveal details to caller | | Resource exhaustion (OOM, disk) | Fail fast with clear message | Can't recover; make the failure diagnosable | ## Recoverability Framework ``` Can the caller realistically recover? ├── Yes → Use explicit signaling (Result type, checked exception) │ ├── Expected failure (not found, validation) → Return type encodes the failure │ └── Transient failure (timeout, rate limit) → Retry with backoff ├── No → Let it propagate (unchecked exception, panic) │ ├── Bug → Crash and fix the code │ └── Infrastructure failure → Propagate to top-level handler └── Caller determines → Provide both options (Result + throw helper) ``` ## Signaling Comparison Table | Technique | Explicit? | Composable? | Performance | Best For | |-----------|-----------|-------------|-------------|----------| | Checked exceptions | Yes (compiler-enforced) | No | Moderate | Public APIs, Java | | Unchecked exceptions | No (caller may miss) | No | Moderate | Bugs, unrecoverable | | Result/Either types | Yes (type-enforced) | Yes (map/flatMap) | Low overhead | Functional style, Rust/Kotlin | | Optional/Nullable | Partial | Partial | Low overhead | "Not found" cases only | | Error codes | No (easy to ignore) | No | Minimal | Low-level, C interop, hot paths | ## Error-Hiding Antipatterns | Antipattern | Symptom | Severity | Fix | |-------------|---------|----------|-----| | **Empty catch block** | `catch (e) {}` — error vanishes | Critical | Handle, log, or propagate | | **Swallow and return null** | `catch → return null` hides root cause | Critical | Distinguish "not found" from "system error" | | **Log and continue** | Error logged but execution continues in broken state | Warning | Decide: recover meaningfully or propagate | | **Generic catch-all** | `catch (Exception e)` masks different failure modes | Warning | Catch specific exceptions; let unexpected ones propagate | | **Error as magic value** | Return `-1` or `""` to signal failure | Warning | Use typed error channels (Result, Optional, exceptions) | | **Exception wrapping without context** | `throw new RuntimeException(e)` | Note | Add domain context: `throw new PaymentFailedException("charge declined", e)` | ## Strategy Selection Flowchart ``` What kind of code are you writing? ├── Public API / library │ ├── Caller must handle → Checked exception or Result type │ └── Caller can't handle → Unchecked exception (document it) ├── Internal service code │ ├── Expected failure → Result type │ └── Bug / unexpected → Let it propagate ├── Hot path (measured) │ └── Error codes or Result types (avoid exception overhead) └── Script / CLI └── Crash with clear message (fail fast) ``` ## Checklist Before shipping error handling code: - [ ] Every catch block either recovers, propagates, or fails explicitly - [ ] No empty catch blocks - [ ] "Not found" and "system failure" use different error channels - [ ] Error messages are actionable (what happened, what to do) - [ ] Third-party exceptions wrapped in domain types at boundaries - [ ] Error paths have test coverage --- > Converted and distributed by [TomeVault](https://tomevault.io/claim/smileynet) — claim your Tome and manage your conversions.