Mutating collections with shared state isn't what you do in pure Haskell, though. That's the difference between the now classic deterministic parallelism , and

Mutating collections with shared state isn't what you do in pure Haskell, though. That's the difference between the now classic deterministic parallelism , and the newer side-effect oriented concurrency in Haskell using stm or mvars. Without side effects your /parallel/ code cannot but be thread safe. Because the language requires the implementation to ensure side effects under the hood are not observable -- using cas for example on thunk updates, as GHC does.