Fp Async

Practical Async Patterns with fp-ts

Stop writing nested try/catch blocks. Stop losing error context. Start building clean async pipelines that handle errors properly.

TaskEither is simply an async operation that tracks success or failure. That's it. No fancy terminology needed.

// TaskEither<Error, User> means:
// "An async operation that either fails with Error or succeeds with User"

When to Use

• You need async error handling in TypeScript with TaskEither.
• The task involves wrapping Promises, composing API calls, or replacing nested try/catch flows.
• You want practical fp-ts async patterns instead of academic explanations.
• You are building API clients, database repositories, or file I/O layers that need typed errors.
• You need parallel vs sequential execution control with proper error propagation.
• You want retry, fallback, or conditional recovery patterns without callback hell.

Prerequisites

• TypeScript project with fp-ts installed.
• Install if missing:

npm install fp-ts

• Familiarity with Either and pipe from fp-ts is helpful but not required — all patterns are self-contained.
• Node.js 18+ recommended for native fetch support in examples.

Procedure

1. Wrapping Promises Safely

The core idea: wrap once, handle cleanly everywhere.

The Problem — Try/Catch Everywhere:

// BEFORE: Try/catch hell
async function getUserData(userId: string) {
  try {
    const response = await fetch(`/api/users/${userId}`)
    if (!response.ok) {
      throw new Error(`HTTP ${response.status}`)
    }
    const user = await response.json()

    try {
      const posts = await fetch(`/api/users/${userId}/posts`)
      if (!posts.ok) {
        throw new Error(`HTTP ${posts.status}`)
      }
      const postsData = await posts.json()
      return { user, posts: postsData }
    } catch (postsError) {
      // Now what? Return partial data? Rethrow? Log?
      console.error('Failed to fetch posts:', postsError)
      return { user, posts: [] }
    }
  } catch (error) {
    // Lost all context about what failed
    console.error('Something failed:', error)
    throw error
  }
}

The Solution — Wrap Once, Handle Cleanly:

import * as TE from 'fp-ts/TaskEither'
import * as E from 'fp-ts/Either'
import { pipe } from 'fp-ts/function'

// One wrapper function - reuse everywhere
const fetchJson = <T>(url: string): TE.TaskEither<Error, T> =>
  TE.tryCatch(
    async () => {
      const response = await fetch(url)
      if (!response.ok) {
        throw new Error(`HTTP ${response.status}: ${response.statusText}`)
      }
      return response.json()
    },
    (error) => error instanceof Error ? error : new Error(String(error))
  )

// AFTER: Clean and composable
const getUser = (userId: string) => fetchJson<User>(`/api/users/${userId}`)
const getPosts = (userId: string) => fetchJson<Post[]>(`/api/users/${userId}/posts`)

tryCatch Explained:

TE.tryCatch takes two things:

1. An async function that might throw
2. A function to convert the thrown value into your error type

TE.tryCatch(
  () => somePromise,           // The async work
  (thrown) => toError(thrown)  // Convert failures to your error type
)

Creating Success and Failure Values:

// Wrap a value as success
const success = TE.right<Error, number>(42)

// Wrap a value as failure
const failure = TE.left<Error, number>(new Error('Nope'))

// From a nullable value (null/undefined becomes error)
const fromNullable = TE.fromNullable(new Error('Value was null'))
const result = fromNullable(maybeUser) // TaskEither<Error, User>

// From a condition
const mustBePositive = TE.fromPredicate(
  (n: number) => n > 0,
  (n) => new Error(`Expected positive, got ${n}`)
)

2. Chaining Async Operations

The Problem — Nested Awaits:

// BEFORE: Deeply nested, hard to follow
async function processOrder(orderId: string) {
  try {
    const order = await fetchOrder(orderId)
    if (!order) throw new Error('Order not found')

    try {
      const user = await fetchUser(order.userId)
      if (!user) throw new Error('User not found')

      try {
        const inventory = await checkInventory(order.items)
        if (!inventory.available) throw new Error('Out of stock')

        try {
          const payment = await chargePayment(user, order.total)
          if (!payment.success) throw new Error('Payment failed')

          try {
            const shipment = await createShipment(order, user)
            return { order, shipment, payment }
          } catch (e) {
            await refundPayment(payment.id)
            throw e
          }
        } catch (e) { throw e }
      } catch (e) { throw e }
    } catch (e) { throw e }
  } catch (e) {
    console.error('Order processing failed', e)
    throw e
  }
}

The Solution — Clean Pipelines with chain:

// AFTER: Flat, readable pipeline
const processOrder = (orderId: string) =>
  pipe(
    fetchOrder(orderId),
    TE.chain(order => fetchUser(order.userId)),
    TE.chain(user =>
      pipe(
        checkInventory(order.items),
        TE.chain(inventory => chargePayment(user, order.total))
      )
    ),
    TE.chain(payment => createShipment(order, user, payment))
  )

chain vs map:

Use map when your transformation is synchronous and can't fail:

pipe(
  fetchUser(userId),
  TE.map(user => user.name.toUpperCase())  // Just transforms the value
)

Use chain (or flatMap) when your transformation is async or can fail:

pipe(
  fetchUser(userId),
  TE.chain(user => fetchOrders(user.id))  // Returns another TaskEither
)

Building Context with Do Notation:

When you need values from multiple steps:

// BEFORE: Have to thread values through manually
const processOrderManual = (orderId: string) =>
  pipe(
    fetchOrder(orderId),
    TE.chain(order =>
      pipe(
        fetchUser(order.userId),
        TE.chain(user =>
          pipe(
            chargePayment(user, order.total),
            TE.map(payment => ({ order, user, payment }))
          )
        )
      )
    )
  )

// AFTER: Do notation keeps everything accessible
const processOrder = (orderId: string) =>
  pipe(
    TE.Do,
    TE.bind('order', () => fetchOrder(orderId)),
    TE.bind('user', ({ order }) => fetchUser(order.userId)),
    TE.bind('payment', ({ user, order }) => chargePayment(user, order.total)),
    TE.bind('shipment', ({ order, user }) => createShipment(order, user)),
    TE.map(({ order, payment, shipment }) => ({
      orderId: order.id,
      paymentId: payment.id,
      trackingNumber: shipment.tracking
    }))
  )

3. Parallel vs Sequential Execution

When to Use Each:

• Sequential (one after another): each operation depends on the previous result; you need to respect rate limits; order matters.
• Parallel (all at once): operations are independent; you want speed; fetching multiple resources by ID.

Sequential Chaining:

// Operations depend on each other - must be sequential
const getUserWithOrg = (userId: string) =>
  pipe(
    fetchUser(userId),                              // First: get user
    TE.chain(user => fetchTeam(user.teamId)),      // Then: get their team
    TE.chain(team => fetchOrganization(team.orgId)) // Finally: get org
  )

Parallel Execution:

import { sequenceT } from 'fp-ts/Apply'

// Independent operations - run in parallel
const getDashboardData = (userId: string) =>
  sequenceT(TE.ApplyPar)(
    fetchUser(userId),
    fetchNotifications(userId),
    fetchRecentActivity(userId)
  ) // Returns TaskEither<Error, [User, Notification[], Activity[]]>

// With destructuring:
const getDashboard = (userId: string) =>
  pipe(
    sequenceT(TE.ApplyPar)(
      fetchUser(userId),
      fetchNotifications(userId),
      fetchRecentActivity(userId)
    ),
    TE.map(([user, notifications, activities]) => ({
      user,
      notifications,
      activities,
      unreadCount: notifications.filter(n => !n.read).length
    }))
  )

Parallel Array Operations:

// Fetch multiple users in parallel
const userIds = ['1', '2', '3', '4', '5']

// TE.traverseArray runs all fetches in parallel
const fetchAllUsers = pipe(
  userIds,
  TE.traverseArray(fetchUser)
) // TaskEither<Error, readonly User[]>

// Note: Fails fast - if ANY request fails, the whole thing fails
// All errors after the first are lost

Parallel with Batch Control:

When you need to limit concurrent requests:

const chunk = <T>(arr: T[], size: number): T[][] => {
  const chunks: T[][] = []
  for (let i = 0; i < arr.length; i += size) {
    chunks.push(arr.slice(i, i + size))
  }
  return chunks
}

// Process in batches of 5 concurrent requests
const fetchUsersWithLimit = (userIds: string[]) => {
  const batches = chunk(userIds, 5)

  return pipe(
    batches,
    // Process batches sequentially
    TE.traverseArray(batch =>
      // But within each batch, run in parallel
      pipe(batch, TE.traverseArray(fetchUser))
    ),
    TE.map(results => results.flat())
  )
}

Sequential When Parallel Looks Tempting:

// WRONG: This looks parallel but order might matter for DB operations
const createUserAndProfile = (userData: UserData) =>
  sequenceT(TE.ApplyPar)(
    createUser(userData),           // Creates user with ID
    createProfile(userData.profile) // Needs user ID - race condition!
  )

// RIGHT: Sequential when there's a dependency
const createUserAndProfile = (userData: UserData) =>
  pipe(
    createUser(userData),
    TE.chain(user =>
      pipe(
        createProfile(user.id, userData.profile),
        TE.map(profile => ({ user, profile }))
      )
    )
  )

4. Error Recovery Patterns

Fallback to Alternative:

// Try primary API, fall back to cache
const getUserWithFallback = (userId: string) =>
  pipe(
    fetchUserFromApi(userId),
    TE.orElse(() => fetchUserFromCache(userId))
  )

// Chain multiple fallbacks
const getConfigRobust = () =>
  pipe(
    fetchRemoteConfig(),
    TE.orElse(() => loadLocalConfig()),
    TE.orElse(() => TE.right(defaultConfig))
  )

Conditional Recovery:

// Only recover from specific errors
const fetchUserOrCreate = (userId: string) =>
  pipe(
    fetchUser(userId),
    TE.orElse(error =>
      error.message.includes('404') || error.message.includes('not found')
        ? createDefaultUser(userId)
        : TE.left(error)  // Re-throw other errors
    )
  )

Typed Error Recovery:

type ApiError =
  | { _tag: 'NotFound'; id: string }
  | { _tag: 'NetworkError'; cause: Error }
  | { _tag: 'Unauthorized' }

const fetchUser = (id: string): TE.TaskEither<ApiError, User> =>
  TE.tryCatch(
    async () => {
      const res = await fetch(`/api/users/${id}`)
      if (res.status === 404) throw { _tag: 'NotFound', id }
      if (res.status === 401) throw { _tag: 'Unauthorized' }
      if (!res.ok) throw { _tag: 'NetworkError', cause: new Error(`HTTP ${res.status}`) }
      return res.json()
    },
    (e): ApiError =>
      typeof e === 'object' && e !== null && '_tag' in e
        ? e as ApiError
        : { _tag: 'NetworkError', cause: e instanceof Error ? e : new Error(String(e)) }
  )

// Handle specific errors differently
const getUserOrGuest = (userId: string) =>
  pipe(
    fetchUser(userId),
    TE.orElse(error => {
      switch (error._tag) {
        case 'NotFound':
          return TE.right(createGuestUser())
        case 'Unauthorized':
          return TE.left(error) // Propagate auth errors
        case 'NetworkError':
          return fetchUserFromCache(userId) // Try cache on network issues
      }
    })
  )

Retry with Exponential Backoff:

import * as T from 'fp-ts/Task'

const wait = (ms: number): T.Task<void> =>
  () => new Promise(resolve => setTimeout(resolve, ms))

const retry = <E, A>(
  operation: TE.TaskEither<E, A>,
  maxAttempts: number,
  baseDelayMs: number = 1000
): TE.TaskEither<E, A> => {
  const attempt = (remaining: number, delay: number): TE.TaskEither<E, A> =>
    pipe(
      operation,
      TE.orElse(error =>
        remaining <= 1
          ? TE.left(error)
          : pipe(
              TE.fromTask(wait(delay)),
              TE.chain(() => attempt(remaining - 1, delay * 2))
            )
      )
    )

  return attempt(maxAttempts, baseDelayMs)
}

// Usage
const fetchUserWithRetry = (userId: string) =>
  retry(fetchUser(userId), 3, 1000)
  // Attempts: immediate, 1s, 2s delays between retries

Default Values:

// Get value or use default (removes the error channel)
const getUsernameOrDefault = (userId: string) =>
  pipe(
    fetchUser(userId),
    TE.map(user => user.name),
    TE.getOrElse(() => T.of('Anonymous'))
  ) // Task<string> - no more error tracking

// Keep error channel but provide fallback value
const getUserWithDefault = (userId: string) =>
  pipe(
    fetchUser(userId),
    TE.orElse(() => TE.right(defaultUser))
  ) // TaskEither<Error, User> - error channel still exists but always succeeds

5. Real API Examples

Complete Fetch Wrapper:

// types.ts
interface ApiError {
  code: string
  message: string
  status: number
  details?: unknown
}

// api.ts
const createApiError = (
  code: string,
  message: string,
  status: number,
  details?: unknown
): ApiError => ({ code, message, status, details })

const request = <T>(
  url: string,
  options: RequestInit = {}
): TE.TaskEither<ApiError, T> =>
  TE.tryCatch(
    async () => {
      const response = await fetch(url, {
        headers: {
          'Content-Type': 'application/json',
          ...options.headers,
        },
        ...options,
      })

      if (!response.ok) {
        const body = await response.json().catch(() => ({}))
        throw createApiError(
          body.code || 'HTTP_ERROR',
          body.message || response.statusText,
          response.status,
          body
        )
      }

      // Handle 204 No Content
      if (response.status === 204) {
        return undefined as T
      }

      return response.json()
    },
    (error): ApiError => {
      if (typeof error === 'object' && error !== null && 'code' in error) {
        return error as ApiError
      }
      return createApiError(
        'NETWORK_ERROR',
        error instanceof Error ? error.message : 'Request failed',
        0
      )
    }
  )

// API client
const api = {
  get: <T>(url: string) => request<T>(url),

  post: <T>(url: string, body: unknown) =>
    request<T>(url, {
      method: 'POST',
      body: JSON.stringify(body)
    }),

  put: <T>(url: string, body: unknown) =>
    request<T>(url, {
      method: 'PUT',
      body: JSON.stringify(body)
    }),

  delete: (url: string) =>
    request<void>(url, { method: 'DELETE' }),
}

// Usage
const getUser = (id: string) => api.get<User>(`/api/users/${id}`)
const createUser = (data: CreateUserDto) => api.post<User>('/api/users', data)
const updateUser = (id: string, data: UpdateUserDto) => api.put<User>(`/api/users/${id}`, data)
const deleteUser = (id: string) => api.delete(`/api/users/${id}`)

Database Operations (Prisma Example):

import { PrismaClient, Prisma } from '@prisma/client'

type DbError =
  | { _tag: 'NotFound'; entity: string; id: string }
  | { _tag: 'UniqueViolation'; field: string }
  | { _tag: 'ConnectionError'; cause: unknown }

const prisma = new PrismaClient()

const wrapPrisma = <T>(
  operation: () => Promise<T>
): TE.TaskEither<DbError, T> =>
  TE.tryCatch(
    operation,
    (error): DbError => {
      if (error instanceof Prisma.PrismaClientKnownRequestError) {
        if (error.code === 'P2002') {
          const field = (error.meta?.target as string[])?.join(', ') || 'unknown'
          return { _tag: 'UniqueViolation', field }
        }
        if (error.code === 'P2025') {
          return { _tag: 'NotFound', entity: 'Record', id: 'unknown' }
        }
      }
      return { _tag: 'ConnectionError', cause: error }
    }
  )

// Repository pattern
const userRepository = {
  findById: (id: string): TE.TaskEither<DbError, User> =>
    pipe(
      wrapPrisma(() => prisma.user.findUnique({ where: { id } })),
      TE.chain(user =>
        user
          ? TE.right(user)
          : TE.left({ _tag: 'NotFound', entity: 'User', id })
      )
    ),

  findByEmail: (email: string): TE.TaskEither<DbError, User | null> =>
    wrapPrisma(() => prisma.user.findUnique({ where: { email } })),

  create: (data: CreateUserInput): TE.TaskEither<DbError, User> =>
    wrapPrisma(() => prisma.user.create({ data })),

  update: (id: string, data: UpdateUserInput): TE.TaskEither<DbError, User> =>
    wrapPrisma(() => prisma.user.update({ where: { id }, data })),

  delete: (id: string): TE.TaskEither<DbError, void> =>
    pipe(
      wrapPrisma(() => prisma.user.delete({ where: { id } })),
      TE.map(() => undefined)
    ),
}

// Service using repository
const createUserService = (input: CreateUserInput) =>
  pipe(
    // Check email doesn't exist
    userRepository.findByEmail(input.email),
    TE.chain(existing =>
      existing
        ? TE.left({ _tag: 'UniqueViolation' as const, field: 'email' })
        : TE.right(undefined)
    ),
    // Create user
    TE.chain(() => userRepository.create(input))
  )

File Operations (Node.js):

import * as fs from 'fs/promises'
import * as path from 'path'

type FileError =
  | { _tag: 'NotFound'; path: string }
  | { _tag: 'PermissionDenied'; path: string }
  | { _tag: 'IoError'; cause: unknown }

const toFileError = (error: unknown, filePath: string): FileError => {
  if (error instanceof Error) {
    if ('code' in error) {
      if (error.code === 'ENOENT') return { _tag: 'NotFound', path: filePath }
      if (error.code === 'EACCES') return { _tag: 'PermissionDenied', path: filePath }
    }
  }
  return { _tag: 'IoError', cause: error }
}

const readFile = (filePath: string): TE.TaskEither<FileError, string> =>
  TE.tryCatch(
    () => fs.readFile(filePath, 'utf-8'),
    (e) => toFileError(e, filePath)
  )

const writeFile = (filePath: string, content: string): TE.TaskEither<FileError, void> =>
  TE.tryCatch(
    () => fs.writeFile(filePath, content, 'utf-8'),
    (e) => toFileError(e, filePath)
  )

const readJson = <T>(filePath: string): TE.TaskEither<FileError | { _tag: 'ParseError'; cause: unknown }, T> =>
  pipe(
    readFile(filePath),
    TE.chain(content =>
      TE.tryCatch(
        () => Promise.resolve(JSON.parse(content)),
        (e): { _tag: 'ParseError'; cause: unknown } => ({ _tag: 'ParseError', cause: e })
      )
    )
  )

// Usage: Load config with fallback
const loadConfig = () =>
  pipe(
    readJson<Config>('./config.json'),
    TE.orElse(() => readJson<Config>('./config.default.json')),
    TE.getOrElse(() => T.of(defaultConfig))
  )

6. Handling Results

Pattern Matching with fold/match:

// fold: Handle both success and failure, returns a Task (no more error channel)
const displayResult = pipe(
  fetchUser(userId),
  TE.fold(
    (error) => T.of(`Error: ${error.message}`),
    (user) => T.of(`Welcome, ${user.name}!`)
  )
) // Task<string>

// Execute and get the string
const message = await displayResult()

Getting the Raw Either:

// Sometimes you need to work with the Either directly
const result = await fetchUser(userId)() // Either<Error, User>

if (E.isLeft(result)) {
  console.error('Failed:', result.left)
} else {
  console.log('User:', result.right)
}

In Express/Hono Handlers:

// Express
app.get('/users/:id', async (req, res) => {
  const result = await pipe(
    fetchUser(req.params.id),
    TE.fold(
      (error) => T.of({ status: 500, body: { error: error.message } }),
      (user) => T.of({ status: 200, body: user })
    )
  )()

  res.status(result.status).json(result.body)
})

// Cleaner with a helper
const sendResult = <E, A>(
  res: Response,
  te: TE.TaskEither<E, A>,
  errorStatus: number = 500
) =>
  pipe(
    te,
    TE.fold(
      (error) => T.of(res.status(errorStatus).json({ error })),
      (data) => T.of(res.json(data))
    )
  )()

app.get('/users/:id', async (req, res) => {
  await sendResult(res, fetchUser(req.params.id), 404)
})

7. Common Patterns Reference

Quick Transformations:

// Transform success value
TE.map(user => user.name)

// Transform error
TE.mapLeft(error => ({ ...error, timestamp: Date.now() }))

// Transform both at once
TE.bimap(
  error => enhanceError(error),
  user => user.profile
)

Filtering:

// Fail if condition not met
pipe(
  fetchUser(userId),
  TE.filterOrElse(
    user => user.isActive,
    user => new Error(`User ${user.id} is not active`)
  )
)

Side Effects Without Changing Value:

// Log on success, keep the value unchanged
pipe(
  fetchUser(userId),
  TE.tap(user => TE.fromIO(() => console.log(`Fetched user: ${user.id}`)))
)

// Log on error, keep the error unchanged
pipe(
  fetchUser(userId),
  TE.tapError(error => TE.fromIO(() => console.error(`Failed: ${error.message}`)))
)

// chainFirst is like tap but for operations that return TaskEither
pipe(
  createUser(userData),
  TE.chainFirst(user => sendWelcomeEmail(user.email))
) // Returns the created user, not the email result

Converting From Other Types:

// From Either
const fromEither = TE.fromEither(E.right(42))

// From Option
import * as O from 'fp-ts/Option'
const fromOption = TE.fromOption(() => new Error('Value was None'))
const result = fromOption(O.some(42))

// From boolean
const fromBoolean = TE.fromPredicate(
  (x: number) => x > 0,
  () => new Error('Must be positive')
)

Pitfalls

• Fails fast on parallel array operations: TE.traverseArray fails on the first error — all subsequent errors are lost. If you need all errors collected, use a different strategy (e.g., TE.traverseArray with getApplicativeComposition or collect errors manually).
• Race conditions with sequenceT(TE.ApplyPar): Running dependent operations in parallel causes race conditions. If operation B needs a value from operation A, use TE.chain (sequential), not sequenceT.
• map vs chain confusion: TE.map is for synchronous transformations that cannot fail. TE.chain is for async operations or transformations that can fail. Using map with a function that returns TaskEither produces TaskEither<E, TaskEither<E, A>> — a nested type you almost never want.
• getOrElse removes the error channel: After TE.getOrElse, you get a Task<A>, not a TaskEither<E, A>. You can no longer handle errors downstream. Use orElse if you want to keep the error channel.
• fold returns a Task, not TaskEither: After fold, the error channel is consumed. You must provide both success and failure handlers.
• Error type widening: If you mix different error types in a pipeline (e.g., FileError and ApiError), TypeScript will infer a union. This can make error handling harder downstream. Consider a unified error type or use mapLeft to normalize.
• tryCatch error conversion is untyped: The second argument to TE.tryCatch receives unknown. Always narrow and convert to a proper error type — never assume it's an Error instance.
• Do notation requires fp-ts extensions: TE.Do and TE.bind are available in fp-ts 2.x. Ensure your version supports them.
• traverseArray is on the TE module directly: In some fp-ts versions, you may need TE.traverseArray (pipeable) vs A.traverse(TE.ApplicativePar). Check your version's API.

Verification

Verify your setup compiles and patterns work correctly:

1. Check fp-ts is installed:

npm ls fp-ts

Expected output should show an installed version, e.g., fp-ts@2.16.0.

2. Type-check your pipeline:

npx tsc --noEmit

No errors expected if types are correct.

3. Quick smoke test — wrap and execute a TaskEither:

import * as TE from 'fp-ts/TaskEither'
import * as E from 'fp-ts/Either'

const test = TE.tryCatch(
  async () => 42,
  (e) => e instanceof Error ? e : new Error(String(e))
)

const result = await test()

// Verify: result should be { _tag: 'Right', right: 42 }
console.log(E.isRight(result)) // true
console.log(E.isLeft(result))  // false

4. Verify error path:

const fail = TE.tryCatch(
  async () => { throw new Error('boom') },
  (e) => e instanceof Error ? e : new Error(String(e))
)

const failResult = await fail()

// Verify: failResult should be { _tag: 'Left', left: Error('boom') }
console.log(E.isLeft(failResult)) // true
console.log(E.isRight(failResult)) // false

5. Verify parallel execution returns tuple:

import { sequenceT } from 'fp-ts/Apply'

const parallel = sequenceT(TE.ApplyPar)(
  TE.right(1),
  TE.right('hello'),
  TE.right(true)
)

const parResult = await parallel()
// Verify: parResult should be { _tag: 'Right', right: [1, 'hello', true] }
console.log(E.isRight(parResult)) // true

Quick Reference Card

Related Skills

• fp-either — Synchronous Either patterns, the foundation of TaskEither.
• fp-option — Option type for nullable values without exceptions.
• fp-pipe — pipe and flow composition for building readable pipelines.

Limitations

• Use this skill only when the task clearly matches the scope described above.
• Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.