Why all the MVars?

[treeowl]

It's not at all obvious to me why we need MVars here. Why not just use StateT to handle the state of the computation? Is there some way for mapDeRef to exploit this through concurrency or lazy IO? If so, that should be documented. If not, let's simplify them away.

[treeowl]

Even if we really do need concurrency variables here for some reason, we can at least generate the uniques in a more performant way. Atomically reading and incrementing a counter should be cheaper than mucking about with an MVar.

[andygill]

This is code written 10 years ago. I can't remember. However, if it can be rewritten using StateT, then all power to you. I'm happy reviewing changes, and there are already tests that can be used.