IV.3 Errors

• Return concrete @error types, not strings. An error carries context in fields (NotFound{ what, id }).
• ? is the default; match — only when different errors are handled differently.
• _ = f() — only with a comment saying why the error can be ignored.
• Do not turn errors into a panic (!) outside tests and initialisation.

Source: docs/spec/oxl-standard.en.md