Spill fails on the empty list

[HeinrichApfelmus]

Reid Barton reports:

By the way, I also encountered a simple bug involving spill: trying to spill an empty list causes a "head []" type of exception. I've worked around it by using

spill' = R.spill . R.filterE (not . null)

but I imagine that there might be a better implementation in terms of the internal primitives.