> I'm pretty sure that it is possible to make a function in Haskell which, while it mutates variables, these variables are all local, so you are still able to o

> I'm pretty sure that it is possible to make a function in Haskell which, while it mutates variables, these variables are all local, so you are still able to offer up a purely functional interface to the world outside of that function. Indeed, it's called the ST monad. The internals aren't that magical, behind the scenes GHC does some state passing like IO but nothing too fancy. The real trick is using -XRankNTypes for the runST function[1] to track "state threads" at the type-level. example1 :: Int example1 = runST $ do x <- newSTRef 0 forM_ [1..1000] $ \j -> do writeSTRef x j readSTRef x [1]: http://research.microsoft.com/en-us/um/people/simonpj/Papers/lazy-functional-state-threads.ps.Z http://research.microsoft.com/en-us/um/people/simonpj/Papers...