You want libraries to be able to offer implementations that are polymorphically async-or-not (perhaps using higher-kinded types). That's how I'm used to working in Scala.
https://github.com/urllib3/urllib3/issues/1323 is a discussion of this. "Solution: we maintain one copy of the code – the version with async/await annotations – and then a little script maintains the synchronous copy by automatically stripping them out again. It's not beautiful, but as far as I can tell all the alternatives are worse.
https://github.com/python-trio/unasync is the current production implementation of this approach.
Colored functions are a problem in JavaScript because it has to share an event loop with the rest of the browser. For Node.js and Python, that is less of an issue, but those languages are also either practically or actually single-threaded, which also means async behavior is contagious. Rust does not have this problem, at least not until you're writing WASM programs with it (in which case, you're restricted to sharing one thread with the rest of the browser again).
I do somewhat agree with that desire - I don't think that this is a problem for performance, but since most of my use of Rust is dropping it into existing C code, I think that spawning a thread is likely to have annoying visible side effects on the calling code, which might not be expecting me to do that.
Using a proper, single-threaded executor and running it on the current thread seems like it would work, yes. (To be fair, I also feel like just having the sync version of a Python API call "trio.run(self.equivalent_async_api)" would probably also work, and I don't totally follow why that's insufficient....)
Why do I need all this other bloat just to run a function? Why can't I just run the function?
Maybe we should start trying to think about async as being something can use if they want and ignore if they want. Code being async compatible rather than async required.
Let's say I have code like this, in Python asyncio:
class ShardedDBClient:
async def query(self, key):
tasks = [self.query_shard(key) for shard in self.shards]
results = await asyncio.gather(*tasks)
for partial_result in results:
if key in partial_result:
return partial_result[key]
return None
The obvious way to make it not be "async required" is to say, we get rid of the async/await keywords - but what do you do with that "await asyncio.gather" instruction? Do you call each of those callbacks serially?
Generally, even in Rust (perhaps especially in Rust), I would expect this to use some OS facility for waiting on multiple sockets (possibly even just boring select(), but preferably epoll/kqueue) to send a bunch of database requests out in parallel and then wait on all their sockets to handle responses as they arrive. I would expect that even if my own code doesn't involve async/await at all.
The easy way to implement that is
def sync_query(self, key):
return asyncio.run(self.query(key))
This is going to be a lot faster than querying those shards one at a time! And it also can semantically change how the library behaves - imagine that there's a timeout parameter, and I set a 100ms timeout. I probably mean that to be 100ms for the entire operation, not 100ms per request, but I probably also don't expect my calls to always fail if each query takes 10ms and there are more than 10 shards.
The downside is that this library is quietly using asyncio without you knowing. But how exactly is that a downside? I already expect the library to be using select/epoll/kqueue without me knowing. And in a language like Rust, the executor should basically compile out - it should be a "zero-cost abstraction" compared to writing the event-handling code by hand.
And it'd be great to check and optimize away all this at compile time.
The Rust standard library's block_on uses a global ThreadPool, and the docs recommend using a LocalPool if you need finer grained control.
So, to answer your question it depends on the executor (the thing that implements block_on).
So for anyone reading for the correct details: the block_on I was thinking of is part of the futures crate, which is not in std, it's not an "official library".