I think you are imagining something very different than what I am describing. Exposure of state simply allows you to decompose your structures -- which allows

I think you are imagining something very different than what I am describing. Exposure of state simply allows you to decompose your structures -- which allows you to compose them again. This is exactly what you do in FP. Testing at branch points means that you write 2 tests. If you have n branch points it means 2n tests. If you test n branch points down, you need n^2 tests. You don't test functionality at every level -- because these are probes not tests. Your external API is irrelevant to its construction, which is the point of encapsulation. I can write internals that can be probed, or I can decide not to. But how you use it from the outside is not related. You can build as many layers of this as you want -- in the normal way. I'm trying to imagine what you are thinking of, but I'm having trouble so it's hard to rebut your points.