GP is (rhetorically, I assume) asking about whether or not the action of incrementing the mutable variable is typed, not that the variable itself is typed. For

GP is (rhetorically, I assume) asking about whether or not the action of incrementing the mutable variable is typed, not that the variable itself is typed. For example in normal Haskell the code that uses mutation and code that does not use mutation have different types. A function may have the type f :: Int -> Int but with mutation it can become f :: Int -> ST s Int or f :: Int -> IO Int or f :: Int -> STM Int depending on the circumstances. That's not the case in OCaml. The kind of mutation in OCaml references is the most similar to Haskell's ST monad (not to be confused with State or StateT), but in Haskell the type tells us a mutation is happening inside the function call.