m06-error-handling

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" Is error granularity right? Thinking Prompt

Before handling an error:

What kind of failure is this?

Expected → Result Absence normal → Option 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)

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? 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

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

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

Quick Reference Pattern When Example Result Recoverable error fn read() -> Result Option 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 │ └─ No → Result │ ├─ 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 everywhere Lost type info thiserror 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