[pka]

For all my love of Clojure, having used its STM implementation I can wholeheartedly agree with this.

Having side effects in a transaction occur more than once because you didn't pay attention is a real pain to debug, mostly because the transaction won't be retried until the program is exposed to heavy load leading to serious memory contention.

At that point, debugging concurrent designs turns into a nightmare, not being any better than having to deal with deadlocks etc, the very thing STM is supposed to magically make go away. In some cases I even had to resort to locks because it was easier to handle IO that way.

Now, Haskell's type system would just not allow having any sort of IO side effects inside a STM transaction. Yes, having to explicitly handle IO may sound as a lot of work, but in my experience it makes everything easier.