> For AI agents: the complete documentation index is available at /llms.txt, the full documentation bundle is available at /llms-full.txt. # Types Model alternatives with enums and validate external values before treating them as domain data. ## Valid states ### Why An enum prevents combinations of optional fields that no operation can handle meaningfully. ### Rules - MUST use enums for closed alternatives and Option for meaningful absence. \[review] - MUST use Result for a failure with an explanation rather than overload Option. \[review] - MUST keep external deserialization separate from domain validation. \[review] - MUST use private fields or validated constructors when arbitrary construction would violate an invariant. \[review] - MUST use generics or `impl Trait` when static capability dispatch is sufficient. \[review] - MUST justify dynamic dispatch and heap allocation rather than select them by habit. \[review] - MAY use newtypes to distinguish values with the same underlying representation. \[review] \[review] The compiler checks enum and capability types. Validation tests establish invariants that types alone cannot express. ## Enforcement The compiler checks enum and capability types. Validation tests establish invariants that types alone cannot express.