[Haskell-cafe] Higher-kinded Quantification

I have a type constructor (Iterator i o m a) of kind (* -> * -> (* -> *) -> *), which is a monad transformer, and I'd like to use the type system to express the fact that some computations must be "pure", by writing the impredicative type (Iterator i o (forall m. m) a). However I've run into a bit of difficulty expressing this, due to the kind of m. I've attached a minimal-ish example. Is there a way to express this in GHC?