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What is a future before it has a value?

Level: 201 · anyone holding a Future and wondering what it is doing

Stub — a question and its expected answers, not yet a lesson. No program behind this page has been written or run, so nothing on it has been through the check that backs every finished page. The table is what the finished lesson is expected to find, and every cell is unverified until a program prints it. (Not machine-checked here.)

One line: A future is a handle to a value that will exist later, and the languages disagree about whether holding one means the work is running: a Java CompletableFuture or a Go channel is fed by work already underway, while a Rust future does nothing at all until something polls it — so dropping a Rust future cancels work that, in every other language, would have continued.

The question

Create a future for a computation that prints as it runs. Do not await it. Wait two seconds. Did it print? In Java, yes: the executor was already running it. In Python's asyncio, a coroutine object prints nothing until awaited, and a Task prints because create_task scheduled it. In Rust, nothing prints, and the compiler warns that the future is unused. The page runs that experiment, then awaits each and shows the value arrive.

What each language is expected to say

Expected answer, to be verified by the program
Rust an async block is inert: #[must_use] warning, and nothing runs until .await or spawn
Go no future type; a goroutine plus a channel is already running when created
C no futures; a thread and a pipe, running
C++ std::async with std::launch::async runs at once; std::launch::deferred runs on get()
Java CompletableFuture.supplyAsync runs at once on the common pool; get waits
Python a coroutine object is inert; asyncio.create_task schedules it; concurrent.futures.Future is fed by a running executor

What the programs have to show

  • the unawaited future in each language: printed, or silent, after two seconds
  • the Rust #[must_use] warning text as a .sh driver key
  • each future awaited: the value, and for C++ the deferred one running only then

See also