[zenhack]

HKT would somewhat ease the type signature question; as I understand it in the rust community being able to parametrize over ownership/mutability is the big selling point that gets discussed. You could instead have a single Fn<Foo>, and stuff may require Fn<mut> or Fn<const>, rather than a separate FnMut. There's no keyword now, but owned could be part of that dance too. There's obviously the question of how to deal with backwards compatibility though.

There are more convenient ways of composing monads than monad transformers; extensible effects are a lot easier to work with.

Ulttimately I agree though, monads are not the right abstraction for a systems language. I'm quite happy using them in languages like Haskell and OCaml, where you've not only got a garbage collector solving the tricky ownership qustions for you, it's also faster than it has any right to be, so you can basically ignore the overhead of heap allocating a closure.

They become much less attractive when you're in a problem domain where you actually want to worry about fiddly details around memory allocation. This is much smaller space than people think it is, but it's what Rust is for.