Async

async def, await, and the Task[T] / Future[T] types parse and type-check.

Basic shape

async def number() -> i32:
    return 42

async def combine() -> i32:
    a = await number()
    b = await number()
    return a + b

• Calling an async def does not return its declared type — it returns a Task[T] where T is the declared return type.
• await unwraps a Task[T] back to T.
• await may only appear inside an async def; using it in a plain function is a compile error.
• An async def may itself be awaited, or run from an async main.

async def main(argv: list[str]) -> i32:
    print(await combine())
    return 0

The result type propagates through await, so the usual typing rules apply:

async def fetch_name() -> str:
    return "sere"

async def greet() -> str:
    name: str = await fetch_name()    # typed str, not Any
    return "hello " + name

Argument typing

Arguments are checked against the declared parameter types, not the task:

async def scale(value: i32, factor: i32 = 2) -> i32:
    return value * factor

async def use() -> i32:
    return await scale(3, factor=4)    # 12

An async def main is the entry point for an async program. Error handling is unchanged — try / except / finally work across await, and defer bodies still run while an exception unwinds.

Not implemented

Some constructs parse but are rejected rather than silently miscompiled:

Incomplete lowering reports NotImplementedError with the source location instead of generating wrong code.

See also

• functions.md — def, defaults, callable types.
• exceptions.md — error handling.
• ../language.md — reference summary.