M06 Error Handling

by actionbook5c40d3ad7851No license1.5K starsListed Oct 8, 2026Updated Oct 8, 2026Repository updated 6 weeks ago

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

Instructions onlySoftware Development

AI-generated overview

Guides Rust error-handling choices between Result, Option, panic, thiserror and anyhow.

What it does: This instruction-only skill provides a decision framework for Rust error handling. It helps an agent decide whether a failure is expected or a bug, then choose between Result, Option, panic, thiserror and anyhow. It includes quick-reference tables, a decision flowchart, common errors, anti-patterns and pointers to related skills.
When to use it: Use it when writing or reviewing Rust code that involves Result, Option, ?, unwrap, expect, panic, anyhow or thiserror. It is meant for questions about when to panic versus return a Result, how to propagate errors with context, and how to design custom error types.
Requirements: No scripts or runtime dependencies; it is instructions only. It references Rust crates thiserror and anyhow as topics, but does not require them to be installed.

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?

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?

Thinking Prompt

Before handling an error:

What kind of failure is this?
- Expected → Result<T, E>
- Absence normal → Option<T>
- Bug/invariant → panic!
- Unrecoverable → panic!
Who handles this?
- Caller → propagate with ?
- Current function → match/if-let
- User → friendly error message
- Programmer → panic with message
What context is needed?
- Type of error → thiserror variants
- Call chain → anyhow::Context
- Debug info → anyhow or tracing

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)

"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)

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")

"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")

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

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 ?

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 ?

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>`

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

Related Skills

Source and attribution

Source:actionbook/rust-skillsinskills/m06-error-handlingat commit5c40d3a

License: No license

Content belongs to its original authors. SourceWeft indexes it from a public repository.

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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?

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

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

M06 Error Handling

Error Handling

Core Question

Error → Design Question

Thinking Prompt

Trace Up ↑

Trace Down ↓

Quick Reference

Library vs Application

Decision Flowchart

Common Errors

Anti-Patterns

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