Lack of null and monads are kinda orthogonal. Yes, you can use monads to model "early return", but you can use them for so many other things. F# already offers

Lack of null and monads are kinda orthogonal. Yes, you can use monads to model "early return", but you can use them for so many other things. F# already offers computation expressions, which you can use for monads (but with no typing). But even Haskell provides no enforcement of the monad laws. Surely you must know about F#, so your comment makes little sense to me. If you want HKT's in F#, why not just say so?