The C++ version can (and often will) use mutable fields behind the scenes. The Haskell version can't. So it is thread safe by default, while the C++ version mig

The C++ version can (and often will) use mutable fields behind the scenes. The Haskell version can't. So it is thread safe by default, while the C++ version might break some internal invariant if used from multiple threads. So even on such simple functions, purity can give some advantage.