libraries provide pure processing machines that pull data in through well defined input functions (and output through return), and then it's up to the callsite (or host code or however we want to call our code) to decide what kind of color to paint the whole thing eventually.

this helps testability and composability too.

(of course this is not new, there was Scala's Iteratee and there's full program interpretation ... https://zio.dev/1.0.18/overview/overview_background/ )