actionbook/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<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 |