> but if you're looking to make this something that works the way imperative programmers expect

I think you're misunderstanding me and I regret bringing up the appeal to "imperative programmers." That was simply to illustrate that the semantics of the syntax are immediately accessible to the majority of programmers who have never encountered denotational semantics. But if denotational semantics are what are important to you, let me back up because the whole appeal to "imperative programmers" is not the purpose of `!`, the purpose is rather more meant here in what I wrote:

> having to contort perfectly fine code into a for-comprehension when I introduce some effects simply because I didn't write my pure code with the Identity monad the first time around.

The perspective I'm coming from is based on Moggi's original paper, which is to realize that semantically monadic code is simply a generalization of "pure" (this is kind of an underspecified term here, but I think we both understand what this is meant to refer to) code, and to have the syntax reflect that generalization, rather than draw a sharp divide between monadic code and pure code that `do`-notation introduces (most noticeably by forcing A-normal form). You still get that division at the type level and through various fencing syntax, which is good for making sure you don't lose purity guarantees, but it's important to realize that these things share the same fundamentals (this is why e.g. Haskell can claim to be pure even in the presence of effects, which is confusing to newcomers who view do-notation as a completely different part of Haskell).

Indeed I'll make a perhaps bold claim. This `!` or async-await syntax is a far more faithful representation of the denotational semantics laid out in Moggi's original paper describing the semantics of monads and computations than do-notation is.

So since we're on the subject of denotational semantics, let's start with the denotational semantics of monads. When considering denotational semantics, what is a monad? We can appeal to Moggi's original presentation of this using Kleisli triples, most notably the Kleisli extension operator and the component of the natural transformation on the identity endofunctor (for Haskell programmers this is the categorical version of the reverse bind operator and pure). In particular, Moggi gives a compelling overview of how to achieve a meaningful denotational semantic mapping here here, given a well-defined monad. This doesn't come for free in syntax inspired by the lambda calculus because the categorical representation translate function application to composition (as it does everything see e.g. the categorical representation of monoids), but we can get there by thinking of "expression sections, " by analogy to tuple sections. That is, roughly speaking, you can think of `(_ subExpr0 subExpr1)` as being the function `x -> (x subExpr0 subExpr1)` and more generally of parenthesized expression in general as a series of compositions of expression sections (with atomic values such as literals being the usual morphisms from terminal objects in the category, given that we're in the lambda calculus and therefore in a Cartesian closed category we're guaranteed these exist).

Fundamentally Moggi's insight is that the denotational semantics behind

f x

can be augmented via a monad to be

f* x*

where `*` here is the Kleisli extension operator (what Moggi refers to as `_*`). In Haskell terms this is

(=<< f) (pure x)

The fact that we're using reverse-bind in one case and pure in the other case is a quirk of function application in the lambda calculus; again at the categorical level where everything including function application is just composition, these are both just applications of the Kleisli extension operator.

So what is `!` then? Well `!` is simply the absence of `*`!

f !x = f* x

Denotationally that's all `!` is! Indeed we can choose to think of `!` as syntactic sugar, but that's optional. You can instead give it an immediate denotation instead of having to desugar things first. The reason we use `!` to denote the absence of Kleisli extension rather than its presence is because most of the time you care only about its absence. Otherwise your code would be absolutely filled with `!`s.

So that's the denotational semantics of what's going on here. There was a lot there, but it's all in service of a single point: if you care about denotational semantics `!`/monadless syntax has a much stronger story than do-notation (e.g. what exactly is the denotational semantic of `<-`?). To describe the denotational semantics of do-notation you have to essentially perform the syntax desugaring first before resorting to the non-monadic denotational semantics of the desugared code. This is completely ignoring Moggi's insight!

So from the perspective of denotational semantics, do-notation is absolutely no help! We have to actively ignore our monadic context and "undo" the do-notation and resort to the underlying original denotational semantics instead of getting any semantic help from our monad at all.

But of course most programmers have not heard of denotational semantics and don't care about denotational semantics. So for them, what is the point of `!`? The point is not to force-fit imperative code. Indeed in that case, as you point out, the natural evolution is to throw in all sorts of different kinds of if-statements, while-statements, exceptions and the like, which is not what I'm advocating. The point is to realize that all that is just a reflection of the same thing! You just need to assign a different semantic mapping is all!

[Identity| f x |]

is one mapping where everything is "pure" in the way we normally think of it

[Maybe| f x |]

is one with short-circuiting failure.

[List| f x |]

is non-determinism and so on.

This desire has appeared many times in the FP community and given birth to many proposals (notably I'm basically stealing syntax from McBride's idiom brackets here, the `!` addition here essentially can be thought of as an addition to make idiom brackets work for monads instead of just applicatives, and indeed `!`-notation itself has already been implemented in Idris although in a slightly different fashion than how I've presented it).

The point of `!` is to expose the fact that monadic semantics are exactly what matters and the only thing that matters!*

That we cannot seamlessly express this relationship syntactically by just appending brackets and instead must do so through contortion to an entirely different syntax in the form of do-notation represents to me a failure of denotational reasoning! Indeed while e.g. Idris describes this notation in terms of do-notation (http://docs.idris-lang.org/en/latest/tutorial/interfaces.htm...) (one thing I like about monadless and why I use it as my go-to example is because it doesn't define itself in terms of for-comprehensions), as I've outlined above, I believe it has a far stronger standalone semantic interpretation.