Eval considered

Using eval is almost always a bad idea. It's a crutch for bad designs, it opens giant security holes in your code, and in practice it's almost never needed to get actual stuff done.

>>18
Yes, I haven't had experience with concurrent programming yet. But in sequential programming global variables are king. And your

state -> state

thing is inferior because ML and Haskell don't allow in-place modifications. There will be a lot of unnecessary allocations and GC going on: at least the modified record field and the tree spine have to be reallocated on every mutation.