Expressing such constraints and having the language/compiler enforce them is part of what I want from an expressive language. That way once I've expressed them, I don't have to ensure those things repeatedly for the entire lifecycle of the program. We each have our definitions and values.
In Haskell you can write an entire application using IO/do blocks and it will be completely imperative. In f# you can declare every variable mut.
Functional languages don't prevent you from writing imperative code. They give you the opportunity to choose not to, and enforce it at compile time. The opposite is not true of imperative languages, there the type system cannot help you to enforce that some code is purely functional.