Scope · Effect 中文社区

了解 Effect 如何借助 Scope 简化资源管理，确保在长时间运行的应用程序中高效清理并安全地处理资源。

Scope 数据类型是 Effect 中的核心构件，用于以安全且可组合的方式管理资源。

一个 scope 代表一个或多个资源的生命周期。当 scope 被关闭时，其中的所有资源都会被释放，从而确保不会有资源泄漏。Scope 还允许添加 finalizer，由它来定义如何释放资源。

借助 Scope 数据类型，你可以：

添加 finalizer：finalizer 指定资源的清理逻辑。
关闭 scope：当 scope 被关闭时，所有资源都会被释放，且 finalizer 会被执行。

示例（管理 Scope）

import { Scope, Effect, Console, Exit } from "effect"

const program =
  // create a new scope
  Scope.make().pipe(
    // add finalizer 1
    Effect.tap((scope) =>
      Scope.addFinalizer(scope, Console.log("finalizer 1")),
    ),
    // add finalizer 2
    Effect.tap((scope) =>
      Scope.addFinalizer(scope, Console.log("finalizer 2")),
    ),
    // close the scope
    Effect.andThen((scope) =>
      Scope.close(scope, Exit.succeed("scope closed successfully")),
    ),
  )

Effect.runPromise(program)
/*
Output:
finalizer 2 <-- finalizers are closed in reverse order
finalizer 1
*/

在上面的示例中，finalizer 被添加到 scope 中；当 scope 被关闭时，这些 finalizer 会以相反的顺序执行。

这种相反的顺序很重要，因为它能确保资源按正确的次序释放。

例如，如果你先获取一个网络连接，然后访问远程服务器上的文件，那么必须先关闭文件再关闭网络连接，以避免出错。

addFinalizer

Effect.addFinalizer 函数是一个高层 API，它允许你把 finalizer 添加到某个 effect 的 scope 中。finalizer 是一段保证会在关联 scope 关闭时运行的代码。finalizer 的行为会根据 Exit 值而变化，该值表示 scope 是以何种方式关闭的：是成功还是出错。

示例（在成功时添加 finalizer）

Using Effect.gen

import { Effect, Console } from "effect"

//      ┌─── Effect<string, never, Scope>
//      ▼
const program = Effect.gen(function* () {
  yield* Effect.addFinalizer((exit) =>
    Console.log(`Finalizer executed. Exit status: ${exit._tag}`),
  )
  return "some result"
})

// Wrapping the effect in a scope
//
//      ┌─── Effect<string, never, never>
//      ▼
const runnable = Effect.scoped(program)

Effect.runPromiseExit(runnable).then(console.log)
/*
Output:
Finalizer executed. Exit status: Success
{ _id: 'Exit', _tag: 'Success', value: 'some result' }
*/

Using pipe

import { Effect, Console } from "effect"

//      ┌─── Effect<string, never, Scope>
//      ▼
const program = Effect.addFinalizer((exit) =>
  Console.log(`Finalizer executed. Exit status: ${exit._tag}`),
).pipe(Effect.andThen(Effect.succeed("some result")))

// Wrapping the effect in a scope
//
//      ┌─── Effect<string, never, never>
//      ▼
const runnable = Effect.scoped(program)

Effect.runPromiseExit(runnable).then(console.log)
/*
Output:
Finalizer executed. Exit status: Success
{ _id: 'Exit', _tag: 'Success', value: 'some result' }
*/

在这个示例中，我们使用 Effect.addFinalizer 添加一个 finalizer，它会在 scope 关闭后记录 exit 状态。该 finalizer 会在 effect 结束时执行，并记录 effect 是成功完成还是失败。

类型签名如下：

const program: Effect<string, never, Scope>

这表明该工作流需要 Scope 才能运行。你可以使用 Effect.scoped 函数来提供这个 Scope：它会创建一个新的 scope，在其中运行该 effect，并确保 scope 关闭时执行这些 finalizer。

Finalizer Execution Order

finalizer 会按与添加时相反的顺序执行，确保资源以恰当的次序释放，就像栈展开一样。

示例（在失败时添加 finalizer）

Using Effect.gen

import { Effect, Console } from "effect"

//      ┌─── Effect<never, string, Scope>
//      ▼
const program = Effect.gen(function* () {
  yield* Effect.addFinalizer((exit) =>
    Console.log(`Finalizer executed. Exit status: ${exit._tag}`),
  )
  return yield* Effect.fail("Uh oh!")
})

// Wrapping the effect in a scope
//
//      ┌─── Effect<never, string, never>
//      ▼
const runnable = Effect.scoped(program)

Effect.runPromiseExit(runnable).then(console.log)
/*
Output:
Finalizer executed. Exit status: Failure
{
  _id: 'Exit',
  _tag: 'Failure',
  cause: { _id: 'Cause', _tag: 'Fail', failure: 'Uh oh!' }
}
*/

Using pipe

import { Effect, Console } from "effect"

//      ┌─── Effect<never, string, Scope>
//      ▼
const program = Effect.addFinalizer((exit) =>
  Console.log(`Finalizer executed. Exit status: ${exit._tag}`),
).pipe(Effect.andThen(Effect.fail("Uh oh!")))

// Wrapping the effect in a scope
//
//      ┌─── Effect<never, string, never>
//      ▼
const runnable = Effect.scoped(program)

Effect.runPromiseExit(runnable).then(console.log)
/*
Output:
Finalizer executed. Exit status: Failure
{
  _id: 'Exit',
  _tag: 'Failure',
  cause: { _id: 'Cause', _tag: 'Fail', failure: 'Uh oh!' }
}
*/

在这种情况下，即使 effect 失败，finalizer 也会执行。日志输出表明 finalizer 在失败之后运行，并记录了失败的详细信息。

示例（在中断时添加 finalizer）

Using Effect.gen

import { Effect, Console } from "effect"

//      ┌─── Effect<never, never, Scope>
//      ▼
const program = Effect.gen(function* () {
  yield* Effect.addFinalizer((exit) =>
    Console.log(`Finalizer executed. Exit status: ${exit._tag}`),
  )
  return yield* Effect.interrupt
})

// Wrapping the effect in a scope
//
//      ┌─── Effect<never, never, never>
//      ▼
const runnable = Effect.scoped(program)

Effect.runPromiseExit(runnable).then(console.log)
/*
Output:
Finalizer executed. Exit status: Failure
{
  _id: 'Exit',
  _tag: 'Failure',
  cause: {
    _id: 'Cause',
    _tag: 'Interrupt',
    fiberId: {
      _id: 'FiberId',
      _tag: 'Runtime',
      id: 0,
      startTimeMillis: ...
    }
  }
}
*/

Using pipe

import { Effect, Console } from "effect"

//      ┌─── Effect<never, never, Scope>
//      ▼
const program = Effect.addFinalizer((exit) =>
  Console.log(`Finalizer executed. Exit status: ${exit._tag}`),
).pipe(Effect.andThen(Effect.interrupt))

// Wrapping the effect in a scope
//
//      ┌─── Effect<never, never, never>
//      ▼
const runnable = Effect.scoped(program)

Effect.runPromiseExit(runnable).then(console.log)
/*
Output:
Finalizer executed. Exit status: Failure
{
  _id: 'Exit',
  _tag: 'Failure',
  cause: {
    _id: 'Cause',
    _tag: 'Interrupt',
    fiberId: {
      _id: 'FiberId',
      _tag: 'Runtime',
      id: 0,
      startTimeMillis: ...
    }
  }
}
*/

这个示例展示了 effect 被中断时 finalizer 的行为。finalizer 会在中断之后运行，而 exit 状态也反映出该 effect 是在执行途中被停止的。

手动创建和关闭 Scope

当你在单个操作中处理多个受 scope 管理的资源时，理解这些 scope 之间如何交互很重要。默认情况下，这些 scope 会合并成一个，但你可以手动创建和关闭 scope，从而更精细地控制每个 scope 的关闭时机。

我们先来看看默认情况下 scope 是如何合并的：

示例（合并 scope）

import { Effect, Console } from "effect"

const task1 = Effect.gen(function* () {
  console.log("task 1")
  yield* Effect.addFinalizer(() => Console.log("finalizer after task 1"))
})

const task2 = Effect.gen(function* () {
  console.log("task 2")
  yield* Effect.addFinalizer(() => Console.log("finalizer after task 2"))
})

const program = Effect.gen(function* () {
  // The scopes of both tasks are merged into one
  yield* task1
  yield* task2
})

Effect.runPromise(Effect.scoped(program))
/*
Output:
task 1
task 2
finalizer after task 2
finalizer after task 1
*/

在这里，task1 和 task2 的 scope 被合并成单个 scope；运行该程序时，它会以特定顺序输出这些任务及其 finalizer。

如果你想更精细地控制每个 scope 的关闭时机，可以手动创建和关闭它们：

示例（手动创建和关闭 scope）

import { Console, Effect, Exit, Scope } from "effect"

const task1 = Effect.gen(function* () {
  console.log("task 1")
  yield* Effect.addFinalizer(() => Console.log("finalizer after task 1"))
})

const task2 = Effect.gen(function* () {
  console.log("task 2")
  yield* Effect.addFinalizer(() => Console.log("finalizer after task 2"))
})

const program = Effect.gen(function* () {
  const scope1 = yield* Scope.make()
  const scope2 = yield* Scope.make()

  // Extend the scope of task1 into scope1
  yield* task1.pipe(Scope.extend(scope1))

  // Extend the scope of task2 into scope2
  yield* task2.pipe(Scope.extend(scope2))

  // Manually close scope1 and scope2
  yield* Scope.close(scope1, Exit.void)
  yield* Console.log("doing something else")
  yield* Scope.close(scope2, Exit.void)
})

Effect.runPromise(program)
/*
Output:
task 1
task 2
finalizer after task 1
doing something else
finalizer after task 2
*/

在这个示例中，我们创建了两个独立的 scope：scope1 和 scope2，并把每个任务的 scope 扩展进各自的 scope。运行该程序时，它输出的任务及其 finalizer 顺序有所不同。

Extending a Scope

Scope.extend 函数允许你把一个需要 scope 的 effect 工作流，扩展进另一个 scope，并且在该工作流执行完毕时不会关闭这个 scope。这样，你就可以把一个带 scope 的值扩展进更大的 scope 中。

你可能会好奇：如果 scope 已经关闭，但该 scope 中的某个任务尚未完成，会发生什么？关键在于，关闭 scope 并不会强制中断该任务。

示例（在存在未完成任务时关闭 scope）

import { Console, Effect, Exit, Scope } from "effect"

const task = Effect.gen(function* () {
  yield* Effect.sleep("1 second")
  console.log("Executed")
  yield* Effect.addFinalizer(() => Console.log("Task Finalizer"))
})

const program = Effect.gen(function* () {
  const scope = yield* Scope.make()

  // Close the scope immediately
  yield* Scope.close(scope, Exit.void)
  console.log("Scope closed")

  // This task will be executed even if the scope is closed
  yield* task.pipe(Scope.extend(scope))
})

Effect.runPromise(program)
/*
Output:
Scope closed
Executed <-- after 1 second
Task Finalizer
*/

定义资源

acquireRelease

Effect.acquireRelease(acquire, release) 函数允许你定义这样的资源：它们会被获取，并在不再需要时被安全地释放。这对于管理文件句柄、数据库连接或网络套接字之类的资源很有用。

要使用 Effect.acquireRelease，你需要定义两个动作：

获取资源：描述获取该资源的 effect，例如打开文件或建立数据库连接。
释放资源：确保资源被正确释放的清理 effect，例如关闭文件或连接。

获取过程是不可中断的，以确保部分获取资源不会让系统处于不一致的状态。

Effect.acquireRelease 函数保证：一旦资源被成功获取，当 Scope 关闭时，它的释放步骤总是会被执行。

示例（定义一个简单资源）

import { Effect } from "effect"

// Define an interface for a resource
interface MyResource {
  readonly contents: string
  readonly close: () => Promise<void>
}

// Simulate resource acquisition
const getMyResource = (): Promise<MyResource> =>
  Promise.resolve({
    contents: "lorem ipsum",
    close: () =>
      new Promise((resolve) => {
        console.log("Resource released")
        resolve()
      }),
  })

// Define how the resource is acquired
const acquire = Effect.tryPromise({
  try: () =>
    getMyResource().then((res) => {
      console.log("Resource acquired")
      return res
    }),
  catch: () => new Error("getMyResourceError"),
})

// Define how the resource is released
const release = (res: MyResource) => Effect.promise(() => res.close())

// Create the resource management workflow
//
//      ┌─── Effect<MyResource, Error, Scope>
//      ▼
const resource = Effect.acquireRelease(acquire, release)

在上面的代码中，Effect.acquireRelease 函数创建了一个需要 Scope 的资源工作流：

const resource: Effect<MyResource, Error, Scope>

这意味着该工作流需要一个 Scope 才能运行，并且当 scope 关闭时，资源会被自动释放。

现在，你可以使用 Effect.andThen 或类似函数来串联操作，从而使用这个资源。

借助 Effect.andThen 或其他 Effect 操作符，我们可以按自己的需要长时间继续使用该资源。例如，下面展示了如何读取其中的内容：

示例（使用该资源）

import { Effect } from "effect"

// Define an interface for a resource
interface MyResource {
  readonly contents: string
  readonly close: () => Promise<void>
}

// Simulate resource acquisition
const getMyResource = (): Promise<MyResource> =>
  Promise.resolve({
    contents: "lorem ipsum",
    close: () =>
      new Promise((resolve) => {
        console.log("Resource released")
        resolve()
      }),
  })

// Define how the resource is acquired
const acquire = Effect.tryPromise({
  try: () =>
    getMyResource().then((res) => {
      console.log("Resource acquired")
      return res
    }),
  catch: () => new Error("getMyResourceError"),
})

// Define how the resource is released
const release = (res: MyResource) => Effect.promise(() => res.close())

// Create the resource management workflow
const resource = Effect.acquireRelease(acquire, release)

//      ┌─── Effect<void, Error, Scope>
//      ▼
const program = Effect.gen(function* () {
  const res = yield* resource
  console.log(`content is ${res.contents}`)
})

为确保资源得到妥善管理，当你用完资源后应当关闭 Scope。Effect.scoped 函数会替你处理这件事：它创建一个 Scope，运行该 effect，并在 effect 结束时关闭这个 Scope。

示例（使用 Effect.scoped 提供 Scope）

import { Effect } from "effect"

// Define an interface for a resource
interface MyResource {
  readonly contents: string
  readonly close: () => Promise<void>
}

// Simulate resource acquisition
const getMyResource = (): Promise<MyResource> =>
  Promise.resolve({
    contents: "lorem ipsum",
    close: () =>
      new Promise((resolve) => {
        console.log("Resource released")
        resolve()
      }),
  })

// Define how the resource is acquired
const acquire = Effect.tryPromise({
  try: () =>
    getMyResource().then((res) => {
      console.log("Resource acquired")
      return res
    }),
  catch: () => new Error("getMyResourceError"),
})

// Define how the resource is released
const release = (res: MyResource) => Effect.promise(() => res.close())

// Create the resource management workflow
const resource = Effect.acquireRelease(acquire, release)

//      ┌─── Effect<void, Error, never>
//      ▼
const program = Effect.scoped(
  Effect.gen(function* () {
    const res = yield* resource
    console.log(`content is ${res.contents}`)
  }),
)

// We now have a workflow that is ready to run
Effect.runPromise(program)
/*
Resource acquired
content is lorem ipsum
Resource released
*/

示例模式：顺序执行操作

在某些场景中，你可能需要执行一连串链式操作，其中每个操作能否成功都取决于前一个操作。但是，只要其中有任何操作失败，你就希望撤销此前所有成功操作产生的影响。当你需要确保要么所有操作都成功、要么它们都不产生任何影响时，这个模式就很有价值。

让我们看一个实现该模式的示例。假设我们要在应用中创建一个“Workspace”，这涉及创建一个 S3 存储桶、一个 ElasticSearch 索引，以及一条依赖前两者的 Database 记录。

首先，我们为所需的服务定义领域模型：

S3
ElasticSearch
Database

import { Effect, Context, Data } from "effect"

class S3Error extends Data.TaggedError("S3Error")<{}> {}

interface Bucket {
  readonly name: string
}

class S3 extends Context.Tag("S3")<
  S3,
  {
    readonly createBucket: Effect.Effect<Bucket, S3Error>
    readonly deleteBucket: (bucket: Bucket) => Effect.Effect<void>
  }
>() {}

class ElasticSearchError extends Data.TaggedError("ElasticSearchError")<{}> {}

interface Index {
  readonly id: string
}

class ElasticSearch extends Context.Tag("ElasticSearch")<
  ElasticSearch,
  {
    readonly createIndex: Effect.Effect<Index, ElasticSearchError>
    readonly deleteIndex: (index: Index) => Effect.Effect<void>
  }
>() {}

class DatabaseError extends Data.TaggedError("DatabaseError")<{}> {}

interface Entry {
  readonly id: string
}

class Database extends Context.Tag("Database")<
  Database,
  {
    readonly createEntry: (
      bucket: Bucket,
      index: Index,
    ) => Effect.Effect<Entry, DatabaseError>
    readonly deleteEntry: (entry: Entry) => Effect.Effect<void>
  }
>() {}

接下来，我们定义三个 create 操作，以及 Workspace 的总体事务（make）。

import { Effect, Context, Exit, Data } from "effect"

class S3Error extends Data.TaggedError("S3Error")<{}> {}

interface Bucket {
  readonly name: string
}

class S3 extends Context.Tag("S3")<
  S3,
  {
    readonly createBucket: Effect.Effect<Bucket, S3Error>
    readonly deleteBucket: (bucket: Bucket) => Effect.Effect<void>
  }
>() {}

class ElasticSearchError extends Data.TaggedError("ElasticSearchError")<{}> {}

interface Index {
  readonly id: string
}

class ElasticSearch extends Context.Tag("ElasticSearch")<
  ElasticSearch,
  {
    readonly createIndex: Effect.Effect<Index, ElasticSearchError>
    readonly deleteIndex: (index: Index) => Effect.Effect<void>
  }
>() {}

class DatabaseError extends Data.TaggedError("DatabaseError")<{}> {}

interface Entry {
  readonly id: string
}

class Database extends Context.Tag("Database")<
  Database,
  {
    readonly createEntry: (
      bucket: Bucket,
      index: Index,
    ) => Effect.Effect<Entry, DatabaseError>
    readonly deleteEntry: (entry: Entry) => Effect.Effect<void>
  }
>() {}

// Create a bucket, and define the release function that deletes the
// bucket if the operation fails.
const createBucket = Effect.gen(function* () {
  const { createBucket, deleteBucket } = yield* S3
  return yield* Effect.acquireRelease(createBucket, (bucket, exit) =>
    // The release function for the Effect.acquireRelease operation is
    // responsible for handling the acquired resource (bucket) after the
    // main effect has completed. It is called regardless of whether the
    // main effect succeeded or failed. If the main effect failed,
    // Exit.isFailure(exit) will be true, and the function will perform
    // a rollback by calling deleteBucket(bucket). If the main effect
    // succeeded, Exit.isFailure(exit) will be false, and the function
    // will return Effect.void, representing a successful, but
    // do-nothing effect.
    Exit.isFailure(exit) ? deleteBucket(bucket) : Effect.void,
  )
})

// Create an index, and define the release function that deletes the
// index if the operation fails.
const createIndex = Effect.gen(function* () {
  const { createIndex, deleteIndex } = yield* ElasticSearch
  return yield* Effect.acquireRelease(createIndex, (index, exit) =>
    Exit.isFailure(exit) ? deleteIndex(index) : Effect.void,
  )
})

// Create an entry in the database, and define the release function that
// deletes the entry if the operation fails.
const createEntry = (bucket: Bucket, index: Index) =>
  Effect.gen(function* () {
    const { createEntry, deleteEntry } = yield* Database
    return yield* Effect.acquireRelease(
      createEntry(bucket, index),
      (entry, exit) =>
        Exit.isFailure(exit) ? deleteEntry(entry) : Effect.void,
    )
  })

const make = Effect.scoped(
  Effect.gen(function* () {
    const bucket = yield* createBucket
    const index = yield* createIndex
    return yield* createEntry(bucket, index)
  }),
)

随后我们创建一些简单的 service 实现，用来测试 Workspace 代码的行为。为此，我们会利用 layers 来构造测试。这些 layer 能够处理各种场景（包括错误），我们可以通过 FailureCase 类型来控制它们。

import { Effect, Context, Exit, Data, Layer, Console } from "effect"

class S3Error extends Data.TaggedError("S3Error")<{}> {}

interface Bucket {
  readonly name: string
}

class S3 extends Context.Tag("S3")<
  S3,
  {
    readonly createBucket: Effect.Effect<Bucket, S3Error>
    readonly deleteBucket: (bucket: Bucket) => Effect.Effect<void>
  }
>() {}

class ElasticSearchError extends Data.TaggedError("ElasticSearchError")<{}> {}

interface Index {
  readonly id: string
}

class ElasticSearch extends Context.Tag("ElasticSearch")<
  ElasticSearch,
  {
    readonly createIndex: Effect.Effect<Index, ElasticSearchError>
    readonly deleteIndex: (index: Index) => Effect.Effect<void>
  }
>() {}

class DatabaseError extends Data.TaggedError("DatabaseError")<{}> {}

interface Entry {
  readonly id: string
}

class Database extends Context.Tag("Database")<
  Database,
  {
    readonly createEntry: (
      bucket: Bucket,
      index: Index,
    ) => Effect.Effect<Entry, DatabaseError>
    readonly deleteEntry: (entry: Entry) => Effect.Effect<void>
  }
>() {}

// Create a bucket, and define the release function that deletes the
// bucket if the operation fails.
const createBucket = Effect.gen(function* () {
  const { createBucket, deleteBucket } = yield* S3
  return yield* Effect.acquireRelease(createBucket, (bucket, exit) =>
    // The release function for the Effect.acquireRelease operation is
    // responsible for handling the acquired resource (bucket) after the
    // main effect has completed. It is called regardless of whether the
    // main effect succeeded or failed. If the main effect failed,
    // Exit.isFailure(exit) will be true, and the function will perform
    // a rollback by calling deleteBucket(bucket). If the main effect
    // succeeded, Exit.isFailure(exit) will be false, and the function
    // will return Effect.void, representing a successful, but
    // do-nothing effect.
    Exit.isFailure(exit) ? deleteBucket(bucket) : Effect.void,
  )
})

// Create an index, and define the release function that deletes the
// index if the operation fails.
const createIndex = Effect.gen(function* () {
  const { createIndex, deleteIndex } = yield* ElasticSearch
  return yield* Effect.acquireRelease(createIndex, (index, exit) =>
    Exit.isFailure(exit) ? deleteIndex(index) : Effect.void,
  )
})

// Create an entry in the database, and define the release function that
// deletes the entry if the operation fails.
const createEntry = (bucket: Bucket, index: Index) =>
  Effect.gen(function* () {
    const { createEntry, deleteEntry } = yield* Database
    return yield* Effect.acquireRelease(
      createEntry(bucket, index),
      (entry, exit) =>
        Exit.isFailure(exit) ? deleteEntry(entry) : Effect.void,
    )
  })

const make = Effect.scoped(
  Effect.gen(function* () {
    const bucket = yield* createBucket
    const index = yield* createIndex
    return yield* createEntry(bucket, index)
  }),
)

// The `FailureCaseLiterals` type allows us to provide different error
// scenarios while testing our
//
// For example, by providing the value "S3", we can simulate an error
// scenario specific to the S3 service. This helps us ensure that our
// program handles errors correctly and behaves as expected in various
// situations.
//
// Similarly, we can provide other values like "ElasticSearch" or
// "Database" to simulate error scenarios for those  In cases
// where we want to test the absence of errors, we can provide
// `undefined`. By using this parameter, we can thoroughly test our
// services and verify their behavior under different error conditions.
type FailureCaseLiterals = "S3" | "ElasticSearch" | "Database" | undefined

class FailureCase extends Context.Tag("FailureCase")<
  FailureCase,
  FailureCaseLiterals
>() {}

// Create a test layer for the S3 service

const S3Test = Layer.effect(
  S3,
  Effect.gen(function* () {
    const failureCase = yield* FailureCase
    return {
      createBucket: Effect.gen(function* () {
        console.log("[S3] creating bucket")
        if (failureCase === "S3") {
          return yield* Effect.fail(new S3Error())
        } else {
          return { name: "<bucket.name>" }
        }
      }),
      deleteBucket: (bucket) =>
        Console.log(`[S3] delete bucket ${bucket.name}`),
    }
  }),
)

// Create a test layer for the ElasticSearch service

const ElasticSearchTest = Layer.effect(
  ElasticSearch,
  Effect.gen(function* () {
    const failureCase = yield* FailureCase
    return {
      createIndex: Effect.gen(function* () {
        console.log("[ElasticSearch] creating index")
        if (failureCase === "ElasticSearch") {
          return yield* Effect.fail(new ElasticSearchError())
        } else {
          return { id: "<index.id>" }
        }
      }),
      deleteIndex: (index) =>
        Console.log(`[ElasticSearch] delete index ${index.id}`),
    }
  }),
)

// Create a test layer for the Database service

const DatabaseTest = Layer.effect(
  Database,
  Effect.gen(function* () {
    const failureCase = yield* FailureCase
    return {
      createEntry: (bucket, index) =>
        Effect.gen(function* () {
          console.log(
            "[Database] creating entry for bucket" +
              `${bucket.name} and index ${index.id}`,
          )
          if (failureCase === "Database") {
            return yield* Effect.fail(new DatabaseError())
          } else {
            return { id: "<entry.id>" }
          }
        }),
      deleteEntry: (entry) =>
        Console.log(`[Database] delete entry ${entry.id}`),
    }
  }),
)

// Merge all the test layers for S3, ElasticSearch, and Database
// services into a single layer
const layer = Layer.mergeAll(S3Test, ElasticSearchTest, DatabaseTest)

// Create a runnable effect to test the Workspace code. The effect is
// provided with the test layer and a FailureCase service with undefined
// value (no failure case).
const runnable = make.pipe(
  Effect.provide(layer),
  Effect.provideService(FailureCase, undefined),
)

Effect.runPromise(Effect.either(runnable)).then(console.log)

我们来看看 FailureCase 被设为 undefined（正常路径）时的测试结果：

[S3] creating bucket
[ElasticSearch] creating index
[Database] creating entry for bucket <bucket.name> and index <index.id>
{ _id: 'Either', _tag: 'Right', right: { id: '<entry.id>' } }

在这个例子中，所有操作都成功，我们看到了一个成功的结果 right({ id: '<entry.id>' })。

现在，让我们模拟一次 Database 失败：

const runnable = make.pipe(
  Effect.provide(layer),
  Effect.provideService(FailureCase, "Database"),
)

控制台输出将是：

[S3] creating bucket
[ElasticSearch] creating index
[Database] creating entry for bucket <bucket.name> and index <index.id>
[ElasticSearch] delete index <index.id>
[S3] delete bucket <bucket.name>
{ _id: 'Either', _tag: 'Left', left: { _tag: 'DatabaseError' } }

你可以看到，一旦发生 Database 错误，就会有一次完整的回滚：先删除 ElasticSearch 索引，再删除关联的 S3 存储桶。结果是一个包含 DatabaseError 的失败，即 left(new DatabaseError())。

现在，让我们改为让索引创建失败：

const runnable = make.pipe(
  Effect.provide(layer),
  Effect.provideService(FailureCase, "ElasticSearch"),
)

在这种情况下，控制台输出将是：

[S3] creating bucket
[ElasticSearch] creating index
[S3] delete bucket <bucket.name>
{ _id: 'Either', _tag: 'Left', left: { _tag: 'ElasticSearchError' } }

如预期的那样，一旦 ElasticSearch 索引创建失败，就会发生一次回滚，删除 S3 存储桶。结果是一个包含 ElasticSearchError 的失败，即 left(new ElasticSearchError())。