Outside the package that declares it, though if you have it as an interface value you'd have to do work to penetrate it.

Since it's not like I'm trying to sell this as a solution to all immutable data requirements, I didn't worry too much about that detail. In the end it's all about effort, anyhow; with the right invocations in Haskell I can mutate "immutable" data just fine. So even an interface in the package that declared it is still nontrivial protection against accidentally mutating it.