go/error-handling

Go Error Handling

Errors are values. Use fmt.Errorf with %w for wrapping. Check with errors.Is/errors.As.

Pattern 1: Error Wrapping

// Default: wrap with context as you propagate up
return fmt.Errorf("finding user %s: %w", id, err)

• Use %w to preserve chain for errors.Is/errors.As
• Lowercase, no trailing punctuation
• Describe operation + identifiers: "finding user %s" not "error in FindByID"

Pattern 2: Sentinel Errors

var (
    ErrNotFound      = errors.New("not found")
    ErrAlreadyExists = errors.New("already exists")
    ErrForbidden     = errors.New("forbidden")
)

Check with errors.Is(err, ErrNotFound).

Pattern 3: Custom Error Types

type ValidationError struct {
    Field   string
    Message string
}

func (e *ValidationError) Error() string {
    return fmt.Sprintf("validation: %s: %s", e.Field, e.Message)
}

Extract with errors.As:

var valErr *ValidationError
if errors.As(err, &valErr) {
    writeJSON(w, http.StatusBadRequest, map[string]string{
        "field": valErr.Field, "message": valErr.Message,
    })
    return
}

Pattern 4: Multi-Error Collection

func (c *Config) Validate() error {
    var errs []error
    if c.Addr == "" {
        errs = append(errs, fmt.Errorf("addr is required"))
    }
    if c.Timeout <= 0 {
        errs = append(errs, fmt.Errorf("timeout must be positive"))
    }
    return errors.Join(errs...)
}

errors.Join returns a single error that wraps all non-nil inputs. Callers can check for specific sentinels via errors.Is (which walks the full tree of joined errors). To enumerate all wrapped errors individually, type-assert to the interface{ Unwrap() []error } interface:

err := cfg.Validate()
if err != nil {
    if joined, ok := err.(interface{ Unwrap() []error }); ok {
        for _, e := range joined.Unwrap() {
            logger.Error("config validation failed", "error", e)
        }
    }
    return err
}

Use this when the caller needs to report each failure individually (e.g., writing field-level validation errors to an HTTP response).

Pattern 5: HTTP Error Mapping

Map domain errors to HTTP responses at the boundary:

func (h *Handler) handleError(w http.ResponseWriter, r *http.Request, err error) {
    switch {
    case errors.Is(err, domain.ErrNotFound):
        writeJSON(w, http.StatusNotFound, errorResponse("not found"))
    case errors.Is(err, domain.ErrForbidden):
        writeJSON(w, http.StatusForbidden, errorResponse("forbidden"))
    default:
        var valErr *domain.ValidationError
        if errors.As(err, &valErr) {
            writeJSON(w, http.StatusBadRequest, valErr)
            return
        }
        h.logger.Error("unhandled error", "error", err, "path", r.URL.Path)
        writeJSON(w, http.StatusInternalServerError, errorResponse("internal error"))
    }
}

Package-Level Error Contract

// Package order manages order lifecycle operations.
//
// Errors:
//   - ErrNotFound: the requested order does not exist
//   - ErrAlreadyShipped: cannot modify shipped order
//   - *ValidationError: invalid order data (check Field and Message)
package order

Anti-Patterns

• Don't panic — only for programmer bugs. Must* acceptable only in main() or test setup
• Don't ignore errors — every err return must be checked
• Don't string match — use errors.Is/errors.As, never strings.Contains(err.Error(), ...)
• Don't over-wrap — "querying user %s: %w" not "error in FindByID: failed to query: %w"
• Don't log and return — either log or return with context, never both

Verification

• go vet ./... passes with no diagnostics
• No bare return err — all errors wrapped with fmt.Errorf("context: %w", err)
• errors.Is/errors.As used for error checks (no strings.Contains(err.Error(), ...))
• Errors include operation context and relevant identifiers when wrapped