While it's bad form to reply to your own post, I might add this is just what a function is in the large, but you're viewing your program this way. a <- readFro

While it's bad form to reply to your own post, I might add this is just what a function is in the large, but you're viewing your program this way. a <- readFromApi ( Input x ) b <- doBusinessLogic(a) ( f(x) ) c <- writeToPersistence(b) ( Output y = f(x) ) You can also imagine that there are more than one lookup from the db or service calls as I/O in different parts of the pipeline (g(f(x) etc.), but it's always possible to have state pulled in explicitly and pushed down explicitly into business logic as an argument. It tends to make programs have flatter call stacks as well.