And that's essentially my complaint about this technique: it's nearly impossible to build large, extensible frameworks with. In my experience, functional programming works great at small levels of granularity; for anything larger, OO is far more useful. (Of course, the FP purists will disagree, and I'm not saying they're wrong.)
I certainly wouldn't describe myself as an FP purist, and I spend most of my time with OO languages these days, but I've found both FP and OO equally useful "in the large". Different techniques certainly, but both equally useful.
