Of course trivial toy examples demand simple answers. But as you add more processing (partitioning + further operations on partitions, take every nth, stop when

Of course trivial toy examples demand simple answers. But as you add more processing (partitioning + further operations on partitions, take every nth, stop when some predicate is true...), transducers let you build each in isolation and compose them easily. Your obvious solution might start to get a bit strained, and the code for any single process in the chain can't be reused.