An old paper (more of an essay really) that extols the benefits of structuring your program as a set of side-effect free composable functions. Some interesting examples in numerical methods and AI-inspired graph searching are provided. It seemed to be a widely referenced classic, so I gave it a read. These same arguments are put forth now, over 25 years later, so either the world is very thick headed, or perhaps something else has stood in the way of adoption of functional programming techniques for the past quarter century.