What about the Parser monad? Or the Validation Applicative? I think you're being a bit short-sighted here. I fully appreciate that it might be extremely diffi

What about the Parser monad? Or the Validation Applicative? I think you're being a bit short-sighted here. I fully appreciate that it might be extremely difficult to accommodate these HKTs, etc. in a language with the core values of Rust. That's absolutely fine, because that's probably several research-level projects in and of itself. OTOH, we shouldn't discount the value of such things. It's absolutely fine and reasonable to choose e.g. efficiency over other things, but it's not the only value.