You could (usually, but not always) compose the functions you're passing into filter and map and use reduce instead to do both operations in a single pass.

(Edit: This is a big point in the article -- apologies, I hadn't finished reading it yet.)

fair point, in js it will, in other languages that idiom gets deforested into a single pass though

Cool, which languages do this?

This ought to work with any lazy language (or lazy data structure), which is one of the huge benefits of laziness for FP.