[agumonkey]

I'm not even sure it's reducers or transducers. It's mostly a type of functions, not the Array.reduce method. These function don't operate directly, they're meant to be recomposed later by the library. You can [].map(f).map(g).filter(h) and the system will "merge" f, g and h as a one step function that iterate once over the original data and without multiple array allocation. That's my partial understanding, there might be other subtleties.

[wbercx]

I think you're talking about shortcut fusion.