effect-dependency-injection

Dependency injection in Effect uses Context.Tag to define type-safe service identifiers and Layer to compose services with their dependencies. Define a Context.Tag for each service, build Layers that describe how to construct or acquire services, and provide those Layers to your program for deterministic initialization and error handling across multi-service architectures.

Layer construction methods differ by lifecycle: succeed creates a Layer from a pure value with no setup; effect wraps an Effect to perform async initialization; scoped handles resource acquisition and release, ensuring cleanup runs when the Layer is torn down. Choose based on whether your service is stateless, needs async wiring, or manages resources.

Yes. Effect Layers compose into dependency graphs automatically. Define each service as a Layer with explicit Context.Tag dependencies, then combine them using Layer composition operators. Effect resolves the graph, initializes services in dependency order, and handles errors deterministically across the entire service tree.

Use Layer.provide to inject Layers into an Effect. Pass the Layer (or layered composition) to the program; Effect handles provision at runtime, setting up all required services before the program runs and tearing down scoped resources afterward with predictable sequencing.

Context.Tag enables type-safe service composition without threading arguments through deep call stacks. Services are stored in an implicit Context, reducing boilerplate and making dependency graphs explicit and testable. Layers then compose these tags into reusable, deterministic initialization logic.

Layer construction effects propagate errors through the dependency graph. If any service fails to initialize, the entire Layer provision fails with that error, preventing partial setup and ensuring consistency. Your program receives the error for handling or recovery before any code runs.