# Design > UForm for Swift - __A thin layer over the C ABI.__ One Swift file per header - `Types`, `Model`, `Encode`, `Train` - and a C verb `uform__` is a camel-cased method here, such as `imageEncodePixels`. - __The receiver chooses, the verb stays.__ The model runs a verb on the calling thread, `on(executor)` blocks on an executor, and `deferred(executor)` submits it, all with the same arguments. - __Deferred calls use the language's own idioms.__ `status` polls through `uform_outcome_status`, `wait()` blocks through `uform_outcome_wait`, and `try await call.value` runs that wait on a Dispatch thread, so an `async` caller suspends. - __No life of its own.__ The binding starts no threads, keeps no queues and caches nothing; releasing a `Deferred` rides its `deinit`, which waits for the call first. - __Arenas are lent and reused, never shared.__ Every output verb takes an arena and answers views borrowing it; a view remembers which call wrote it and traps once a later one did. - __Statuses and modes are enums.__ `Status`, `Scalar`, `Mood`, `CPUClass`, `GPUClass`, `Modality`, `Objective`, `Negatives` and `MultimemTier` carry their C enumerators and spell themselves through the C name functions, never strings. - __Nothing costs anything until it is used.__ Blocking calls take no deferred-table entry, a surface `outputs` leaves out is never computed, and an arena allocates only when a call writes into it. - __What this binding does not expose.__ The constructors address no model or trainer yet, so no receiver offers them; each blocks, the model loads on an optional `executor` and the trainer and mesh on a required one. A trainer is bound to the executor it was made on, so its verbs block there and take no receiver.