Appendix A · Practice Index — Contracts at Work

Practice	Where it's exercised
`require` / `ensure`	Throughout, first as the CLI's own boundary in Ch. 1; a precondition/postcondition pair guarding an evaluator in Ch. 4.
Class `invariant`	Ch. 3 — the entire specification of a generic LRU cache.
`old` in a postcondition	Ch. 2 — `Task_Store.add`'s `ensure grew: tasks.length = old tasks.length + 1`.
Refinement types (`declare type ... where`)	Ch. 2 — `Priority`, checked at a constructor boundary.
`union` + exhaustive `match`	Ch. 2 (`Task_Status`, `Hub_Event` in Ch. 6); Ch. 4 — a full AST (`Expr`).
Detachable types (`?T`)	Ch. 2 (`Task_Store.find`), Ch. 3 (a cache miss), Ch. 6 (end of stream).
Generics (`[K, V]`)	Ch. 3 — `Lru_Cache [K, V]`.
`intern` and the module-naming constraint	Every chapter; the naming rule itself discussed in Ch. 5.
`do ... rescue ... end`	Ch. 4 (division by zero, undefined variables), Ch. 7 and Ch. 9 (turning a caught failure into an HTTP error response).
`Process` and command-line arguments	Ch. 1.
`io/Path`, `io/Directory`	Ch. 1, Ch. 2, Ch. 5 (recursive tree walking), Ch. 9.
`text/Regex`	Ch. 1 (word splitting), Ch. 4 (tokenizing), Ch. 7 and Ch. 9 (a targeted response-field extraction workaround).
`data/Json`	Ch. 2 (persistence + schema migration), Ch. 7, Ch. 9.
`Task`, `spawn`, `await_all`	Ch. 5 — the book's clearest fan-out/fan-in; also Ch. 6, Ch. 9.
`Task.await(ms)`, `Task.cancel()`	Ch. 5 — a worked, separate illustration.
`Channel[T]`	Ch. 6 — the single-owner-no-locks hub design.
`net/Tcp_Socket`, `net/Server_Socket`	Ch. 6.
`net/Http_Server`, `net/Http_Client`	Ch. 7, Ch. 9.
Java interop — `import`, `with "java" do ... end`, `inherit` a Java interface	Ch. 8 — the book's only crossing into the host platform; reused in Ch. 9's dashboard.
Integration testing against real infrastructure (real sockets, a real server, a real HTTP client) rather than mocks	Ch. 6, Ch. 7, Ch. 9.