There's a number of issues with that:
- I'd be missing all the optimizations that can be performed due to purity.
- There's more to Haskell's type system than just vanilla Hindley-Milner, and the implementation of it isn't particularly trivial. https://github.com/stylewarning/coalton is the closest thing and it's still missing a large amount of the type system.
- Doing the implementation would be a significant amount of work to get it to integrate well with the language, and it would be a layer tightly glued on top instead of integrated with the language. I've seen many good DSLs embedded in lisp, but a type system is hard to embed in any language because it changes fundamental semantics of the language. Typed Racket is a massive project and it's lacking things like ADTs.
- A major part of Haskell is the standard library, a good chunk of the semantics of Haskell people use on a day to day basis, like monads and etc, are a part of the standard library.