Crosscut

If functions have one input and one output, that means there is always one "current value". Let's call this the "active value". I don't know if that nomenclature is going to stick, but it'll do for now.

Expressions transform the active value. If you're going to concatenate multiple expressions into a larger one, they might not cleanly fit together. Especially if composite values are involved, that a given expression might only need parts of. So we'll want combinators to fit in between and convert as necessary.

This is something I have thought about quite a few times, but never very deeply. There's definitely design work left to do. I find optics to be an interesting direction, although I'm certain there's going to be more to it. So combinators will surely become a topic here at some point.