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?

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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"	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
   • 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 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