# Layers — Building Your Dependency Graph

You've seen how `R` encodes what an effect needs, and how `Context` holds the values at runtime. But who *builds* the context?

In small programs you can construct context manually with `ctx!` and hand values to `provide`. In real applications, you need something more powerful: a way to declare *how* to build each piece of the environment, with automatic dependency ordering and lifecycle management.

That's what **Layers** are for.

A Layer is a recipe for building part of an environment. It knows what it produces, what it needs to produce it, and (optionally) how to clean up afterward. Wire Layers together, and effectful figures out the right build order automatically.
