Single fork at a time

// long-running process here
const doStuff = (input: string) => Promise.resolve()

export const think = (input: string) =>
  Effect.gen(function* () {
    const fiberRef = yield* FiberSingleton

    const maybeFiber = yield* fiberRef.get
    if (Option.isSome(maybeFiber)) {
      yield* Fiber.interrupt(maybeFiber.value)
    }

    const fiber: Fiber.Fiber<void, UnknownException> = yield* Effect.fork(
      Effect.tryPromise(() => doStuff(input)).pipe(
        Effect.andThen(() => Ref.set(fiberRef, Option.none())),
      ),
    )

    yield* Ref.set(fiberRef, Option.some(fiber))
  })

export class FiberSingleton extends Context.Tag('FiberSingleton')<
  FiberSingleton,
  Ref.Ref<Option.Option<Fiber.Fiber<void, UnknownException>>>
>() {
  static Live = Layer.effect(
    FiberSingleton,
    Ref.make<Option.Option<Fiber.Fiber<void, UnknownException>>>(Option.none()),
  )
}