"By the way... because they're (mostly) transparent and you can easily have multiple types attached to a single blob of data, you can use the type system to attach any arbitrary metadata you want. I can't help but feel that there's some connection there to monads..."
Yep! There's a pretty close connection to comonads, the opposite of monads. Comonads are for structures that can be unwrapped into a value at any time, but which can also be transformed without unwrapping by... confusingly, sending them a function that would unwrap them.
extract :: MyWrap x -> x
extend :: (MyWrap x -> y) -> MyWrap x -> MyWrap y
The point is that the wrapper that appears around y is generally related to the wrapper x started with. Note that extend's function argument might actually process multiple (MyWrap x) values, all hidden inside the main (MyWrap x).[1]
To illustrate, a pair of a Kernel expression and a Kernel environment can be used as a comonad, as long as we design Kernel expressions so that they can reify arbitrary values using 'quote. The extract method is eval, and the extend method bundles up the result as a literal expression together with the original environment.
[1] Aside from a few laws this has to follow, this kind of variation of behavior is particular to the implementation of each comonad. There will typically be some special-case utilities outside the comonad interface that take advantage of these specific features.
