attac-t Skill: Guard vs Exception
"Guards return. Exceptions signal failure."
The Decision
Return early when:
- Nothing to do is acceptable (empty collection, already processed)
- Caller can continue normally with the return value
- It's a business rule, not an error
Throw exception when:
- Caller must handle the failure
- Invalid state that shouldn't exist
- Programmer error (null where null shouldn't be)
The Heuristic
Ask: "Can the caller proceed normally with a return value?"
- Yes → Return (Result object, empty collection, early exit)
- No → Throw (caller must handle or propagate)
The Quick Test
| Scenario | Answer | Use |
|---|---|---|
| Empty input, nothing to do? | Ok | Return |
| Already processed? | Ok | Return |
| Rate limited, retry later? | Maybe | Return Result |
| Missing required data? | Not ok | Throw |
| Unauthorized access? | Not ok | Throw |
| Invalid state (bug)? | Not ok | Throw |
Real-World Examples
See examples.md.