Yes, indeed threads could be problematic.

But as long as you're not storing closures in those shared data-structures, these structures will always be acyclic, and could be collected through simple reference counting. Perhaps that's an acceptable compromise.

isn't:

    let x = 1:x

cyclic, but without closure? unless closure is so broad as to capture essentially any haskell value. it's not going to be obvious when there is a cyclic reference (e.g. if you use `let x = repeat 1` instead of the definition above)