[vjerancrnjak]

It's more about how the code ends up evolving.

Async-await in JS is sometimes used to swallow exceptions. It's very often used to do 1 thing at a time when N things could be done instead. It serializes the execution a lot when it could be concurrent.

    if (await is_something_true()) {
      // here is_something_true() can be false
    }

And above, the most common mistake.

Similar side-effects happen in other languages that have async-await sugar.

It smells as bad as the Zig file interface with intermediate buffers reading/writing to OS buffers until everything is a buffer 10 steps below.

It's fun for small programs but you really have to be very strict to not have it go wrong (performance, correctness).

[tomck]

I think you replied to the wrong person.

That being said, I don't understand your `is_something_true` example.

> It's very often used to do 1 thing at a time when N things could be done instead

That's true, but I don't think e.g. fibres fare any better here. I would say that expressing that type of parallel execution is much more convenient with async/await and Promise.all() or whatever alternative, compared to e.g. raw promises or fibres.

[vjerancrnjak]

I replied directly to you. There are valid arguments as to why async-await sucks, even after you're deeply familiar with how it works. Even though it's just generators/coroutines beneath, async-await pollutes the code completely if you're not strict about its usage.

`is_something_true` is very simple, if condition is true, and then inside the block, if you were to check again it can be false, something that can't happen in synchronous code, yet now, with async-await, it's very easy to get yourself into situations like these, even though the code seems to yell at you that you're in the true branch. the solution is adding a lock, but with such ease to write async-await, it's rarely caught

[tomck]

> I replied directly to you

My comment was responding only to the person who equated threads and async. My comment only said that async and threading are completely orthogonal, even though they are often conflated

> `is_something_true` is very simple, if condition is true, and then inside the block, if you were to check again it can be false, something that can't happen in synchronous code

It can happen in synchronous code, but even if it couldn't - why is async/await the problem here? what is your alternative to async/await to express concurrency?

Here are the ways it can happen:

1. it can happen with fibers, coroutines, threads, callbacks, promises, any other expression of concurrency (parallel or not!). I don't understand why async/await specifically is to blame here.

2. Even without concurrency, you can mutate state to make the value of is_something_true() change.

3. is_something_true might be a blocking call to some OS resource, file, etc - e.g. the classic `if (file_exists(f)) open(f)` bug.

I am neutral on async/await, but your example isn't a very good argument against it

Seemingly nobody ever has any good arguments against it

> async-await pollutes the code completely if you're not strict about its usage

This is a good thing, if a function is async then it does something that won't complete after the function call. I don't understand this argument about 'coloured functions' polluting code. if a function at the bottom of your callstack needs to do something and wait on it, then you need to wait on it for all functions above.

If the alternative is just 'spin up an OS thread' or 'spin up a fiber' so that the function at the bottom of the callstack can block - that's exactly the same as before, you're just lying to yourself about your code. Guess what - you can achieve the same thing by putting 'await' before every function call

Perhaps you have convinced me that async/await is great after all!

[vjerancrnjak]

Async and threading are applied to similar problems, that's how I understood OP.

`if (file_exists(f))` is misuse of the interface, a lesson in interface design, and a faulty pattern that's easy to repeat with async-await.

> I don't understand this argument about 'coloured functions' polluting code.

Let's say you have some state that needs to be available for the program. What happens in the end game is that you've completely unrolled the loads and computation because any previous interface sucks (loading the whole state at the beginning which serializes your program and makes it slower, or defining `async loadPartial` that causes massive async-await pollution at places where you want to read a part of the state, even if it is already in cache).

Think about it, you can't `await` on part of the state only once and then know it's available in other parts of code to avoid async pollution. When you solve this problem, you realize `await` was just in the way and is completely useless and code looks exactly like a callback or any other more primitive mechanism.

A different example is writing a handler of 1 HTTP request. People do it all the time with async-await, but what to do when you receive N HTTP requests? The way to make code perform well is impossible with the serial code that deals with 1 request, so async-await just allowed you to make something very simple and in an easy way, but then it falls apart completely when you go from 1 to N. Pipelining system won't really care for async-await at all, even though it pipelines IO in addition to compute.

I think "how to express concurrency" is a question I'm not even trying to answer, although I could point to approaches that completely eliminate pollution and force you to write code in that "unrolled" way from start, something like Rx or FRP where time is exactly the unit they're dealing with.

[tomck]

> `if (file_exists(f))` is misuse of the interface, a lesson in interface design, and a faulty pattern that's easy to repeat with async-await.

It's even easier to repeat without async-await, where you don't need to tag the function call with `await`!

> Think about it, you can't `await` on part of the state only once and then know it's available in other parts of code to avoid async pollution. When you solve this problem, you realize `await` was just in the way and is completely useless and code looks exactly like a callback or any other more primitive mechanism.

I don't understand why you can't do this by just bypassing the async/await mechanism when you're sure that the data is already loaded

```

data = null

async function getDataOrWait() { await data_is_non_null(); // however you do this return data }

function getData() { if (data == null) { throw new Error('data not available yet'); } return data; }

```

You aren't forced into using async/await everywhere all the time. this sounds like 'a misuse of the interface, a lesson in interface design', etc

> I think "how to express concurrency" is a question I'm not even trying to answer

You can't criticise async/await, which is explicitly a way to express concurrency, if you don't even care to answer the question - you're just complaining about a solution that solves a problem that you clearly don't have (if you don't need to express concurrency, then you don't need async/await, correct!)

> point to approaches that completely eliminate pollution and force you to write code in that "unrolled" way from start, something like Rx or FRP where time is exactly the unit they're dealing with.

So they don't 'eliminate pollution', they just pollute everything by default all the time (???)

[vjerancrnjak]

> You aren't forced into using async/await everywhere all the time. this sounds like 'a misuse of the interface, a lesson in interface design', etc

Exactly, async-await does not allow you to create correct interfaces. You cannot write code that partially loads and then has sync access, without silly error raises sprinkled all over the place when you're 100% sure there's no way the error will raise or when you want to write code that is 100% correct and will access the part of the state when it is available.

Your example is obvious code pollution. For "correctness" sake I need to handle your raise even though it should not ever happen, or at least the null, to satisfy a type checker.

> So they don't 'eliminate pollution', they just pollute everything by default all the time (???)

That's not the case at all. They just push you immediately in direction where you'll land when you stop using async-await to enforce correctness and performance. edit: you stop modelling control flow and start thinking of your data dependency/computation graph where it's very easy and correct to just change computation/loads to batch mode. `is_something_true` example is immediately reframed to be correct all of the time (as a `file_exists` is now a signal and will fire on true and false)

> You can't criticise async/await, which is explicitly a way to express concurrency, if you don't even care to answer the question - you're just complaining about a solution that solves a problem that you clearly don't have (if you don't need to express concurrency, then you don't need async/await, correct!)

I'm critical of async-await, I'm not comparing it to something else, I can do that but I don't think it is necessary. I've pointed to FRP as a complete solution to the problem, where you're forced to deal with the dataflow from the start, for correctness sake, and can immediately batch, for pipelining/performance sake.

IMO, just like file_exists is a gimmick, or read_bytes(buffer), or write_bytes(buffer) is a leaky abstraction where now your program is required to manage unnecessary wasteful buffers, async-await pushes you into completely useless coding rituals, and your throw is a great example. The way you achieved middleground is with code pollution, because either full async load at the beginning is not performing well, or async-await interleaved with computation pollutes everything to be async and makes timeline difficult to reason about.

This late in the game, any concurrency solution should avoid pointless rituals.