Functional purity is the wrong abstraction to ask for, and doesn't protect you as such (consider a Haskell library that, very purely, asks to write to ~/.bashrc).

Sandboxing modules is what you want, and that can be done with impure languages too. Sure, pure functional programming might make it easier, but as others pointed out, Safe Haskell and friends are very very far from mainstream.

Meanwhile, Firefox compiles some C/C++ dependencies into wasm, and runs them in sandboxes.