without butts, dreams dry up

Let's start with borrowing across yield points. We've solved this problem for real yield points with pin, I have no idea how to solve it using a function interface like >>=

So that means borrows across statements in do notation are just out of the picture. That's extremely limiting.

Rust's imperative control flow statements like `return` and `break` inside of do notation also doesn't make sense, because we do not have TCP preserving closures.

So we only know how to have do notation without early return or borrowing. People say this is an open research question, but IMO its just not possible within our runtime constraints.

Getting beyond that, this is assuming that "future" implements "monad," so if we could just add HKT to have a Monad trait, everything would be hunkydory. That's not true!

the signature of >>= is `m a -> (a -> m b) -> m b` the signature of Future::and_then is roughly `m a -> (a -> m b) -> AndThen (m a) b`

That is, in order to reify the state machine of their control flow for optimization, both Future and Iterator return a new type from their >>= op, not "Self<U>"

Also, our functions are not a `->` type constructor; they come in 3 different flavors, and many of our monads use different ones (FnOnce vs FnMut vs Fn).

Okay, so Monad can't abstract over Future, but still let's have Monad. Problem: we don't have higher kinded polymorphism, and probably never will.

The problem is that without currying at the type level, higher kinded polymorphism makes type inference trivially undecidable. We have no currying.

In order to add higher kinded polymorphism, we'd have to restrict the sorts of types you could use in a way that would feel very arbitrary to users.

In contrast, generic associated types don't have this problem, and directly solve the expressiveness problems we do have, like Iterable (abstracting over everything with a `.iter(&self)` method)

(Don't get me wrong, you can write something to abstract over some monads like Option and Result using generic associated types. But its much less ergonomic than Monad in Haskell, even for those).

IN CONCLUSION: a design that works in a pure FP which lazily evaluates and boxes everything by default doesn't necessarily work in an eager imperative language with no runtime.