Tags and Context — Compile-Time Service Lookup

Chapter 4 showed how R encodes dependencies as types. We used simple types like Database and Logger. That works for small programs, but breaks down as the dependency graph grows.

This chapter introduces the solution: Tags. Tags give values compile-time identities, and Context assembles them into a heterogeneous list that the compiler can query by name, not by position.

By the end you'll understand why id_effect uses this structure instead of tuples, and how to extract exactly the service you need from any environment.