Dependency injection is an often-used technique in object-oriented programming to easily modify the behaviours of an object by providing it with objects it would have otherwise generated on its own, to increase modularity. Aspect-oriented programming is a related technique which adds additional behaviour ("advice") to existing code, aiming to address cross-cutting concerns which affect wide areas of an application without sacrificing modularity. Dependency injection might not seem a common topic in functional programming, but application modularity is essential to functional programming in a practical setting. A natural analogue to dependency injection and aspect-oriented programming in functional programming comes from a surprising place, and offers superior modularity to both. The free monad (F f) for a type constructor (and domain-specific language instruction set) f provides a syntax tree with internal nodes as domain-specific language instructions, which in combination with coproduct functors, allow domain-specific languages to be composed and combined easily.