In most languages, including Rust, async functions can call synchronous functions, so this isn't entirely true.

> unless they handle the future manually without await.

At some point, the futures are always handled manually, e.g., via the run-time, or in Rust case, by scheduling on an Executor, which is nothing but a fancy name for a library that manually handles a lot of futures.

> If you look at typical code in languages with async/await such as JavaScript and C#, awaiting is the common case.

Do you have any example of code that uses await more than synchronous functions ?

> So if async causes your code to be littered with awaits anyway, it makes more sense to make awaiting the default (just like it's the default for the rest of the language) and "deferring" the exception. Call it "defer" or something.

If the purpose is just to change the defaults, then you'd need `defer vec.len()` to access the length of a vector, or `defer array[i]` to index into an array, or `defer obj.foo` to access the field `foo` of an object, ... unless you want to make all of those operations `async`.

If you make await the default and add a `defer` operation that doesn't awaits, then it's hard to imagine how the resulting code could contain less `defer` annotations than `await` ones.

At that point you might choose one of the many non-zero-cost solutions to removing the async distinction, and call it a day like Go does.

First, async infects the call graph, that is, going up the stack. Not down.

Secondly, by "manually" I mean using the future API -- and_then() and so forth.

Thirdly, "defer vec.len()" doesn't make sense unless you want to execute it asynchronously and get a future. And that doesn't make sense unless you know it to be an expensive operation you want to run concurrently. I'm proposing that the hypothetical "defer" would act like Go's "go", except it would return a future.

Lastly, my point was that most code is serial and relies on awaits. For example, if you're doing a name lookup, then opening a socket, then writing to it -- three async calls that must be serialized with await because the actions depend on each other. Not awaiting is needed when you intend to use the future for some purpose, such as adding to a queue or putting in a map, or perhaps you're doing N connections that you put in a vector to ask the runtime to wait for all of them (does Rust's await syntax support awaiting multiple futures?). But most high-level "user" code doesn't juggle futures, it just awaits whenever it needs a result. From a language-ergonomics perspective, it makes more sense to optimize for the common case.

I've never written any async/await code in Rust, but I've written plenty in JavaScript, and it's the same story.

No, but you use a join combinator, which itself returns a future, that you can then await.

But that was not what I was arguing. Rather, I was arguing that await is more common than capturing the raw future. Case in point: The article, where all of the "real code" snippets rely purely on await.

What I proposed was that async functions would always be automatically awaited unless invoked with "defer", because most code is expressed as a series of invocations that use the return value of the function call.

There are times you want to store the raw future as some kind of state (for example, in a GraphQL server you might "resolve" the GraphQL tree as futures), or wait for multiple futures, or similar, but those are, in my experience, the exceptions, not the norm. Most code is:

  let v1 = do1().await;
  let v2 = do2(v1).await;
  // etc.

You can "handle the future manually without await" very easily, it's just one function call. The same sort of thing is available in most other languages with async/await- you can prevent async from going up the call stack by synchronously blocking on a future instead of awaiting it.

The caller is absolutely in charge of whether something runs asynchronously. The only difference is a syntactic default- and in fact some languages even make async functions calls default to synchronous execution. For example take a look at Kotlin's `suspend fun`s.

Not in Rust. Or C#. Or any language except JavaScript.

In Rust, a synchronous function can call and block on an async function with executor::block_on(). C# has Task.Wait() and Task.Result(). Python 3 has asyncio.run(). Scala has Await.ready() and Await.result(). Kotlin has runBlocking {}.