I'm surprised no-one has mentioned Haskell's ST library yet. It hits this exact use-case, and comes with some pretty sweet safeguards from the type system.

https://wiki.haskell.org/Monad/ST

It allows you mutate arrays and other stuff "on the inside" like this article suggests, but it's also able to wrap up the operation so it appears pure from the outside, so the caller doesn't have to guess or rely on naming conventions to figure out the mutability.

It's your choice on whether you want to do the copy on the way in ('thawing'), on the way out ('freezing'), or both, by using the 'in-place' variants of thaw and freeze.