The operations we do with sequential data commonly follow the map-filter-reduce paradigm: We apply the same transformation to all elements, filter some of them out, and calculate summary statistics from the remaining ones.
An essential idea in this chapter is that, in many situations, we need not have all the data materialized in memory. Instead, iterators allow us to process sequential data on a one-by-one basis.
Examples for iterators are the map, filter, and generator types.