Handling dependencies in Effect requires careful management of service requirements. In your case...

const myReadFile = Effect.gen(function* () {
  const fs = yield* FileSystem;
  // ... do something here
  return "file-contents"
})

class MyService extends Context.Tag("MyService")<{
  doSomething: () => Effect.Effect<string, never, never>
}>() {}

// Create a layer so that we can use MyService.
function createMyService() {
  return Layer.effect(
    MyService,
    Effect.gen(function* () {
      const fs = yield* FileSystem
      return {
        doSomething: () => Effect.gen(function* () {
          const contents = yield* myReadFile("my-file.txt")
          return contents
        })
      }
    })
  )
}