# Error Patterns — Derived Rules

> Derives from `domains/errors/first-principles.md`. Common error-handling patterns and when to use them.

## Pattern 1: Result Type (P1 Errors are Data)

```typescript
type Result<T, E> = { ok: true; value: T } | { ok: false; error: E };

function divide(a, b): Result<number, string> {
  if (b === 0) return { ok: false, error: "division by zero" };
  return { ok: true, value: a / b };
}
```

- Errors are values, not exceptions. The caller handles them explicitly.
- Use when errors are expected (parsing, validation, fallible operations).
- Avoid when errors are truly exceptional (out of memory, programmer error) — use exceptions/panics.

## Pattern 2: Sentinel Error (P4 Preserve Context)

```go
var ErrNotFound = errors.New("not found")
if errors.Is(err, ErrNotFound) { ... }
```

- A sentinel is a known error value the caller checks against.
- Use for a small, known set of error conditions.
- Wrap with context: `fmt.Errorf("load user %d: %w", id, ErrNotFound)`.

## Pattern 3: Typed Error (P1, P3 Fail Specifically)

```rust
enum AppError {
  NotFound(String),
  Invalid(String),
  Internal(String),
}
```

- A typed error carries the kind and the detail.
- The caller matches on kind; the detail is for logging/display.
- Use when there are distinct error categories the caller handles differently.

## Pattern 4: Error Wrapping (P4 Preserve Context)

```go
return fmt.Errorf("query users: %w", err)
```

- Wrap errors as they cross boundaries. The outer error says "what was happening"; the inner says "what went wrong."
- The error chain is the stack trace of intent. Read it top-down: "I was doing X, which failed because Y, which was caused by Z."
- Never wrap with a generic message ("operation failed"). Wrap with the specific operation.

## Pattern 5: Fail Fast (P6 Unrecoverable Means Stop)

```typescript
if (config.secret === undefined) throw new Error("config.secret is required");
```

- For unrecoverable conditions, fail immediately. Do not limp on.
- Use at startup: missing required config, missing database, missing secrets.
- Do not use for recoverable conditions (a 404 is recoverable; a missing secret is not).

## Pattern 6: Retry with Backoff (P5 Recoverable When Possible)

```python
for attempt in range(3):
  try:
    return do_thing()
  except TransientError:
    sleep(2 ** attempt)
raise PermanentError()
```

- Retry transient errors (network, 429, 5xx). Do not retry permanent errors (400, 401).
- Exponential backoff with jitter. A retry storm is worse than the original failure.
- Bounded retries. Infinite retry is infinite hang (P8 Timeout Discipline).

## Pattern 7: Circuit Breaker (P3 Defense in Depth via errors P7)

- After N consecutive failures, stop trying. Return a fallback or error immediately.
- Use for external dependencies (a downstream service, an API).
- The breaker resets after a cooldown. Protects the system and the downstream.

## What Violates Error Patterns

| Violation | Pattern |
|-----------|---------|
| `catch (e) { return null }` | (anti-pattern, P2 Fail Loudly) |
| `throw new Error("error")` | P3 Fail Specifically |
| Retry without backoff | P5, P8 |
| `return null` for "not found" | P1 (errors are data, not absence) |
| `throw` in a recovery path | P6 (fail fast in the wrong place) |