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"

1. Wrapping Promises Safely

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: Callback Hell / 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) {
            // Refund payment? Log? What's the state now?
            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):

• When each operation depends on the previous result
• When you need to respect rate limits
• When order matters

Parallel (all at once):

• When operations are independent
• When you want speed
• When 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')
)

Quick Reference Card

Before/After Summary

Fetching Data

// BEFORE
async function getUser(id: string) {
  try {
    const res = await fetch(`/api/users/${id}`)
    if (!res.ok) throw new Error('Not found')
    return await res.json()
  } catch (e) {
    console.error(e)
    return null
  }
}

// AFTER
const getUser = (id: string) =>
  TE.tryCatch(
    async () => {
      const res = await fetch(`/api/users/${id}`)
      if (!res.ok) throw new Error('Not found')
      return res.json()
    },
    E.toError
  )

Chained Operations

// BEFORE
async function processOrder(orderId: string) {
  const order = await fetchOrder(orderId)
  if (!order) throw new Error('No order')
  const user = await fetchUser(order.userId)
  if (!user) throw new Error('No user')
  const result = await chargePayment(user, order.total)
  return result
}

// AFTER
const processOrder = (orderId: string) =>
  pipe(
    TE.Do,
    TE.bind('order', () => fetchOrder(orderId)),
    TE.bind('user', ({ order }) => fetchUser(order.userId)),
    TE.chain(({ user, order }) => chargePayment(user, order.total))
  )

Error Recovery

// BEFORE
async function getData(id: string) {
  try {
    return await fetchFromApi(id)
  } catch {
    try {
      return await fetchFromCache(id)
    } catch {
      return defaultValue
    }
  }
}

// AFTER
const getData = (id: string) =>
  pipe(
    fetchFromApi(id),
    TE.orElse(() => fetchFromCache(id)),
    TE.getOrElse(() => T.of(defaultValue))
  )