>Do you mind explaining the invalid monad instance of Lucid and how it would impact a library user?

To my knowledge lucid uses a monad instance that follow the monad laws (what I meant by "valid") fine.

>Also, I'm assuming one shouldn't call the `compile` function each time a page needs to be rendered, right? Should one put the results of the `compile` function in a top-level Map and use those to get the speed gains?

The idea is to only compile a template once, ever. If you need to compile a template every time a page is rendered (though I can't actually think of a case where you would), you should probably just skip the middle-man, and use lucid (or blaze). It's not horribly slow, but you'd miss out on the benefits of nice-html by doing this.