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I should be clear that I personally agree with you 100%—I prefer to outsource as much as I can to the compiler. It's just that I also recognize that that means being unable to express some things that are otherwise possible, and that most developers don't like that trade-off.

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I don't know what you mean. Just because C++ has "const", you can still make a method that doesn't mutate without the keyword. Seems like a "No True Scotsman" definition of expressiveness.

I feel like this doesn't make sense because mainstream functional languages all provide a way to express imperative code.

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.

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