To add to what everyone else is saying, higher kinded types (HKTs) are what make monadic code really reusable. It lets you write code that is polymorphic over a variable kind of monad, rather than code that works for a specific monad like a list. Without HKTs, you might have a polymorphic type signature List[<T>] of a list of some variable kind of elements, or a Set[<T>], or a Maybe[<T>]. In haskell syntax it's List a, Set a, Maybe a.

When you write code that operates on something like a monad, without caring about what the monad is, you want code that has a type like <T1>[<T2>]. In haskell syntax, it's m a. <T1> (or m) in this case is a type that takes another type as an argument to complete a concrete type. AFAIK, rust doesn't have type variables that take type variables (or types) as arguments like that.