[zhong-j-yu]

There is no problem to modify UI state from any thread; just put up some synchronizations.

The hard part is, if the modifications do not form a single, predictable, serialized chain, how can the programmer reason about them? This problem is independent of async/sync. If you use C# async for a UI action, you still need to worry about it.

[MaulingMonkey]

> There is no problem to modify UI state from any thread;

Some UI APIs aren't thread safe and can't reasonably be made thread safe. They may even be kind enough to check the calling thread and intentionally throw exceptions (or otherwise abort) if invoked from a non-UI thread.

Hilariously, I've had to work around race conditions in such an API. Just because my access was forced to be on one thread, doesn't mean it's implementation was!

> The hard part is, if the modifications do not form a single, predictable, serialized chain, how can the programmer reason about them?"

Reusable async patterns can help you knock off the "single" and "serialized" bits of the microcosm you care about, leaving you to grapple with only the "predictable" one without the pointless "easy" boilerplate distracting you with additional complexity, line count, and bugs.

> If you use C# async for a UI action, you still need to worry about it.

Agreed. If you just spam the async and await keywords without understanding what you're doing, you're not magically going to get the benefits of a single, serialized chain of events. And they're not going to turn the inherently unpredictable response timings and contents of a series of web queries into a predictable one.

[kaoD]

> There is no problem to modify UI state from any thread; just put up some synchronizations.

It reminded me of the "I know, I'll use regular expressions" joke.

[zhong-j-yu]

The single-event-thread design is not without its own problem; programmers have difficulty in understanding and abiding to it too.

Here we have an inherently concurrent problem - user actions and some IO actions occur concurrently. That problem cannot be reduced by some API or language trick.

[kaoD]

> The single-event-thread design is not without its own problem; programmers have difficulty in understanding and abiding to it too.

Definitely not my experience. jQuery is very popular among unskilled programmers because its async model is very easy to understand. I use it to teach async!

> That problem cannot be reduced by some API or language trick.

Why? Reducing problem by API or language tricks is exactly what abstraction is for.

[fleitz]

Given the Universal Turning Machine and the Church-Turing thesis in actuality all of programming except for assembler is pretty much some API or language trick.

[dllthomas]

Arguably, even machine language is "some API" and assembler "some language trick".

[fleitz]

Good point, I forgot for a second that the CPU turns assembler into microcode before executing it.

[dllthomas]

The CPU often turns machine code into microcode, true, and that's a good point but it wasn't the point I was trying to make. Even in simpler processors that actually do directly execute the machine code, I think you can view that machine code as an API for controlling the processor system.