Capability services and Env — Compile-Time Service Lookup

Chapter 4 showed how R encodes dependencies. For small programs a single service in caps!(T) is enough. As the graph grows you need named capability services so the compiler can distinguish dependencies — even when they share the same Rust type.

This chapter covers:

• Why positional/tuple R breaks down
• `` — declaring key types
• Env — the order-independent runtime container
• Needs<K> and ~Key — accessing services inside effects

By the end you'll know how capability lookup works and why insertion order in Env never matters.