I have a hard time imagining how such a thing would actually work. How would the compiler decide that mutable state somewhere is encapsulatable and somewhere else it is not? A compiler can (very easily in fact) monadify all code for you, but what you get is an inefficient imperative language.

It seems to me that a much better approach is to allow mutable state everywhere, but to inform the programmer of the consequences via the editor/IDE by showing the result of a static analysis that analyzes which functions are pure and which are not.