If you have many references to mutable closures I guess indeed it would be simplest to use IORefs or STRefs. However, you can simulate most of the functionalit

If you have many references to mutable closures I guess indeed it would be simplest to use IORefs or STRefs. However, you can simulate most of the functionality of IORefs and STRefs using a state monad so it is possible to avoid them, at least theoretically.