# Types & Protocols

URL: /docs/reference/types/
Section: reference
Tags: reference, types, protocols

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Milo's type system uses frozen dataclasses for immutability and protocols for structural typing. All types use @dataclass(frozen=True, slots=True) for immutability and memory efficiency. Core types Type Description Action(type, payload) Frozen dataclass. Dispatched to reducers. Key(char, name, ctrl, alt, shift) Frozen dataclass. Represents a keypress. ReducerResult(state, sagas, cmds, view) Returned by reducers to schedule side effects. Quit(state, code, sagas, cmds, view) Signal the app to exit. Return from a reducer to stop the event loop. ViewState(alt_screen, cursor_visible, window_title, mouse_mode) Declarative terminal state. The renderer diffs previous vs. current and applies only changes. FieldSpec(name, label, field_type, choices, default, validator, placeholder) Declarative form field configuration. FieldState(value, cursor, error, focused) Runtime state of a single form field. FormState(fields, specs, current, submitted) Runtime state of an entire form. Screen(name, template, reducer) A flow screen definition. Transition(from_screen, to_screen, on) A flow transition rule. FlowState(current_screen, screen_states) Runtime state of a multi-screen flow. Saga effect types Type Description Call(fn, args, kwargs) Call a function, receive its return value. Put(action) Dispatch an action to the store. Select(selector) Read current state or a slice. Fork(saga) Launch a concurrent child saga. Delay(seconds) Sleep for a duration. Retry(fn, args, kwargs, max_attempts, backoff, base_delay, max_delay) Call with retry and backoff on failure. Command types Lightweight alternatives to sagas for one-shot effects. See Commands for usage. Type Description Cmd(fn) A thunk () -> Action | None. Runs on the thread pool, dispatches the returned action. Batch(cmds) Run commands concurrently with no ordering guarantees. Sequence(cmds) Run commands serially, in order. Each result is dispatched before the next starts. TickCmd(interval) Schedule a single @@TICK after interval seconds. Return another from @@TICK to keep ticking. compact_cmds(*cmds) Helper: strips None entries from a command tuple. Enums SpecialKey FieldType Other UP, DOWN, LEFT, RIGHT, HOME, END, PAGE_UP, PAGE_DOWN, INSERT, DELETE, BACKSPACE, TAB, ENTER, ESCAPE, F1–F12 TEXT, PASSWORD, SELECT, CONFIRM Enum Description AppStatus Application lifecycle states RenderTarget Render output targets ErrorCode All namespaced error codes Protocols Protocol Signature Reducer (state, Action) -> state | ReducerResult | Quit Saga Generator[effect, result, None] DispatchFn (Action) -> None GetStateFn () -> state Middleware (DispatchFn, GetStateFn) -> DispatchFn FieldValidator (str) -> str | None Note Note Protocols use structural typing — your functions and classes don't need to explicitly inherit from them. If the signature matches, it satisfies the protocol.

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Metadata:
- Author: lbliii
- Word Count: 376
- Reading Time: 2 minutes