Marcin wrote (snipped):

I think global variables are a lot less evil if they behave as if they were dynamically scoped, like Lisp special variables.

That is, there is a construct which gives the variable a new mutable binding visible in the given IO action. It's used more often than assignment. Assignment is still available though.

I agree entirely. The fact that declaring global variables using unsafePerformIO introduces an artificial notion of "global" and forces it on every part of the program is a major disadvantage which we haven't heard enough about in this discussion. It prevents you doing all sorts of things. It's bad for parallelism. It prevents you running two independent copies of a (main) action. It prevents you writing a Haskell controller which runs over Haskell actions as subprograms.