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Context and requirements

Goal: read an Effect³⁵ type and say which services the work still needs.¶

An Effect type has three parts: what the work returns, how it can fail, and which services it needs. The third part is the requirements. A context is the set of services that running work receives.¶

Worked example: inspecting a file

rat-stack’s file inspection reads a file through a FileInspector service³⁰. The actor²⁷ that does the read asks for that service by name:¶

const readStats = createEffectLogic({
  effect: ({ input }) =>
    FileInspector.use((inspector) => inspector.inspect(input.path)),
  schemas: { input: Schema.Struct({ path: Schema.String }) },
});

FileInspector.use returns an Effect whose requirements include FileInspector. Nothing runs yet. The type now records a debt: some caller must supply a FileInspector.¶

The CLI pays that debt once, at startup:¶

  Effect.provide(
    Layer.mergeAll(
      Layer.provideMerge(FileInspector.layer, NodeServices.layer),

Effect.provide removes FileInspector from the requirements. FileInspector.layer itself needs a file system, and NodeServices.layer supplies it.¶

The tests pay the same debt with their own Layer:¶

const TestLayer = Layer.provideMerge(FileInspector.layer, NodeServices.layer);

The diagram above shows the same move on a small Greeter service: provide turns Greeter into never.¶

Why the type carries it

The compiler rejects execution while a required service remains unsupplied. Dependency injection¹⁴ shows that rule. A forgotten service fails the typecheck before the program runs.¶

Sam Goodwin⁸ describes this requirements channel as a way to model permissions before execution, which he calls a static effect. He connects that idea to Alchemy inferring which identity and access management (IAM) policy a program needs. Happy Path 116, 11:54¶

Michael Arnaldi⁷ describes an Effect by its output, predictable failures, and requirements. Happy Path 81, 49:15¶

Next

Read how a service captures its dependencies³⁰ to see what FileInspector keeps inside itself. Then read why dependency injection matters¹⁴.¶

See Effect basics³⁵ for the whole type, and infrastructure as effects¹⁰ for Sam’s infrastructure reading.¶

Last changed 2026-11-08 in 4f96c98. Source on GitHub. Change log.