Admittedly, Haskell is less preemptive than Erlang, but definitely not at the same level as Go.
A mutating variable is not what you commonly use in Haskell, so the idiomatic equivalent of your code would have memory accesses anyway, and it would be possible to interrupt it.
When your CPU intensive code does not allocate, it will not switch to another green thread.
So point in the example above is absence of memory allocation. And, having decent codegenerator, Haskell could produce the code above where registers are reused for new values (effectively, mutation).
