/m06-error-handling

name: m06-error-handling
description: "CRITICAL: Use for error handling. Triggers: Result, Option, Error, ?, unwrap, expect, panic, anyhow, thiserror, when to panic vs return Result, custom error, error propagation, 错误处理, Result 用法, 什么时候用 panic"
user-invocable: false

Error Handling

> **Layer 1: Language Mechanics**

Core Question

**Is this failure expected or a bug?**

Before choosing error handling strategy:

• Can this fail in normal operation?
• Who should handle this failure?
• What context does the caller need?

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Error → Design Question

| Pattern | Don't Just Say | Ask Instead | |---------|----------------|-------------| | unwrap panics | "Use ?" | Is None/Err actually possible here? | | Type mismatch on ? | "Use anyhow" | Are error types designed correctly? | | Lost error context | "Add .context()" | What does the caller need to know? | | Too many error variants | "Use Box<dyn Error>" | Is error granularity right? |

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Thinking Prompt

Before handling an error:

1. **What kind of failure is this?**

• Expected → Result<T, E>
• Absence normal → Option<T>
• Bug/invariant → panic!
• Unrecoverable → panic!

2. **Who handles this?**

• Caller → propagate with ?
• Current function → match/if-let
• User → friendly error message
• Programmer → panic with message

3. **What context is needed?**

• Type of error → thiserror variants
• Call chain → anyhow::Context
• Debug info → anyhow or tracing

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Trace Up ↑

When error strategy is unclear:

"Should I return Result or Option?"
    ↑ Ask: Is absence/failure normal or exceptional?
    ↑ Check: m09-domain (what does domain say?)
    ↑ Check: domain-* (error handling requirements)

| Situation | Trace To | Question | |-----------|----------|----------| | Too many unwraps | m09-domain | Is the data model right? | | Error context design | m13-domain-error | What recovery is needed? | | Library vs app errors | m11-ecosystem | Who are the consumers? |

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Trace Down ↓

From design to implementation:

"Expected failure, library code"
    ↓ Use: thiserror for typed errors

"Expected failure, application code"
    ↓ Use: anyhow for ergonomic errors

"Absence is normal (find, get, lookup)"
    ↓ Use: Option<T>

"Bug or invariant violation"
    ↓ Use: panic!, assert!, unreachable!

"Need to propagate with context"
    ↓ Use: .context("what was happening")

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Quick Reference

| Pattern | When | Example | |---------|------|---------| | `Result<T, E>` | Recoverable error | `fn read() -> Result<String, io::Error>` | | `Option<T>` | Absence is normal | `fn find() -> Option<&Item>` | | `?` | Propagate error | `let data = file.read()?;` | | `unwrap()` | Dev/test only | `config.get("key").unwrap()` | | `expect()` | Invariant holds | `env.get("HOME").expect("HOME set")` | | `panic!` | Unrecoverable | `panic!("critical failure")` |

Library vs Application

| Context | Error Crate | Why | |---------|-------------|-----| | Library | `thiserror` | Typed errors for consumers | | Application | `anyhow` | Ergonomic error handling | | Mixed | Both | thiserror at boundaries, anyhow internally |

Decision Flowchart

Is failure expected?
├─ Yes → Is absence the only "failure"?
│        ├─ Yes → Option<T>
│        └─ No → Result<T, E>
│                 ├─ Library → thiserror
│                 └─ Application → anyhow
└─ No → Is it a bug?
        ├─ Yes → panic!, assert!
        └─ No → Consider if really unrecoverable

Use ? → Need context?
├─ Yes → .context("message")
└─ No → Plain ?

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Common Errors

| Error | Cause | Fix | |-------|-------|-----| | `unwrap()` panic | Unhandled None/Err | Use `?` or match | | Type mismatch | Different error types | Use `anyhow` or `From` | | Lost context | `?` without context | Add `.context()` | | `cannot use ?` | Missing Result return | Return `Result<(), E>` |

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Anti-Patterns

| Anti-Pattern | Why Bad | Better | |--------------|---------|--------| | `.unwrap()` everywhere | Panics in production | `.expect("reason")` or `?` | | Ignore errors silently | Bugs hidden | Handle or propagate | | `panic!` for expected errors | Bad UX, no recovery | Result | | Box<dyn Error> everywhere | Lost type info | thiserror |

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Related Skills

| When | See | |------|-----| | Domain error strategy | m13-domain-error | | Crate boundaries | m11-ecosystem | | Type-safe errors | m05-type-driven | | Mental models | m14-mental-model |