fp-ts-task-either

fp-ts TaskEither Async Patterns

TaskEither combines the laziness of Task with the error handling of Either, providing a powerful abstraction for async operations that can fail.

Core Concepts

import * as TE from 'fp-ts/TaskEither'
import * as T from 'fp-ts/Task'
import * as E from 'fp-ts/Either'
import { pipe, flow } from 'fp-ts/function'

TaskEither<E, A> is equivalent to () => Promise<Either<E, A>>

• E = Error type (left)
• A = Success type (right)
• Lazy: nothing executes until you call the function
• Composable: chain operations without try/catch

---

1. Converting Promises to TaskEither

Using tryCatch

The primary way to lift Promises into TaskEither:

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

// Basic tryCatch pattern
const fetchUser = (id: string): TE.TaskEither<Error, User> =>
  TE.tryCatch(
    () => fetch(`/api/users/${id}`).then(res => res.json()),
    (reason) => new Error(String(reason))
  )

// With typed errors
interface ApiError {
  code: string
  message: string
  status: number
}

const fetchUserTyped = (id: string): TE.TaskEither<ApiError, User> =>
  TE.tryCatch(
    () => fetch(`/api/users/${id}`).then(res => res.json()),
    (reason): ApiError => ({
      code: 'FETCH_ERROR',
      message: reason instanceof Error ? reason.message : 'Unknown error',
      status: 500
    })
  )

From existing Either

// Lift an Either into TaskEither
const fromEither: TE.TaskEither<Error, number> = TE.fromEither(E.right(42))

// From a nullable value
const fromNullable = TE.fromNullable(new Error('Value was null'))
const result = fromNullable(maybeValue) // TaskEither<Error, NonNullable<T>>

// From an Option
import * as O from 'fp-ts/Option'
const fromOption = TE.fromOption(() => new Error('None value'))
const optionResult = fromOption(O.some(42)) // TaskEither<Error, number>

Creating TaskEither values directly

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

// Error value
const failure = TE.left<Error, number>(new Error('Something failed'))

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

---

2. Handling Async Errors Functionally

Mapping over errors

// Transform the error type
const withMappedError = pipe(
  fetchUser('123'),
  TE.mapLeft((error) => ({
    type: 'USER_FETCH_ERROR' as const,
    originalError: error,
    timestamp: Date.now()
  }))
)

// Bifunctor: map both sides
const mapped = pipe(
  fetchUser('123'),
  TE.bimap(
    (error) => new DetailedError(error),  // map error
    (user) => user.profile                 // map success
  )
)

Error filtering

// Filter with error on false
const validateAge = pipe(
  fetchUser('123'),
  TE.filterOrElse(
    (user) => user.age >= 18,
    (user) => new Error(`User ${user.name} is underage`)
  )
)

---

3. Chaining Async Operations

Sequential chaining with chain/flatMap

interface User { id: string; name: string; teamId: string }
interface Team { id: string; name: string; orgId: string }
interface Org { id: string; name: string }

const fetchUser = (id: string): TE.TaskEither<Error, User> =>
  TE.tryCatch(() => api.getUser(id), toError)

const fetchTeam = (teamId: string): TE.TaskEither<Error, Team> =>
  TE.tryCatch(() => api.getTeam(teamId), toError)

const fetchOrg = (orgId: string): TE.TaskEither<Error, Org> =>
  TE.tryCatch(() => api.getOrg(orgId), toError)

// Chain operations sequentially
const getUserOrg = (userId: string): TE.TaskEither<Error, Org> =>
  pipe(
    fetchUser(userId),
    TE.chain((user) => fetchTeam(user.teamId)),
    TE.chain((team) => fetchOrg(team.orgId))
  )

// flatMap is an alias for chain
const getUserOrgAlt = (userId: string): TE.TaskEither<Error, Org> =>
  pipe(
    fetchUser(userId),
    TE.flatMap((user) => fetchTeam(user.teamId)),
    TE.flatMap((team) => fetchOrg(team.orgId))
  )

Chaining with intermediate values

// Use bind to accumulate values
const getFullContext = (userId: string) =>
  pipe(
    TE.Do,
    TE.bind('user', () => fetchUser(userId)),
    TE.bind('team', ({ user }) => fetchTeam(user.teamId)),
    TE.bind('org', ({ team }) => fetchOrg(team.orgId)),
    TE.map(({ user, team, org }) => ({
      userName: user.name,
      teamName: team.name,
      orgName: org.name
    }))
  )

---

4. Parallel vs Sequential Execution

Parallel execution with sequenceArray

import * as A from 'fp-ts/Array'

const userIds = ['1', '2', '3', '4', '5']

// Parallel: all requests start immediately
// Fails fast: returns first error encountered
const fetchAllUsersParallel = pipe(
  userIds.map(fetchUser),
  TE.sequenceArray  // TaskEither<Error, readonly User[]>
)

// Sequential: one at a time (use when order matters or rate limiting)
const fetchAllUsersSequential = pipe(
  userIds,
  A.traverse(TE.ApplicativeSeq)(fetchUser)
)

Parallel with traverseArray

// More idiomatic: traverse combines map + sequence
const fetchAllUsers = (ids: string[]): TE.TaskEither<Error, readonly User[]> =>
  pipe(ids, TE.traverseArray(fetchUser))

// With index
const fetchWithIndex = pipe(
  userIds,
  TE.traverseArrayWithIndex((index, id) =>
    pipe(
      fetchUser(id),
      TE.map(user => ({ ...user, index }))
    )
  )
)

Collecting all errors vs fail fast

import * as These from 'fp-ts/These'
import * as TH from 'fp-ts/TaskThese'

// For collecting all errors, consider TaskThese
// Or use validation with sequenceT

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

// Parallel execution, collects results
const parallel = sequenceT(TE.ApplyPar)(
  fetchUser('1'),
  fetchTeam('team-1'),
  fetchOrg('org-1')
) // TaskEither<Error, [User, Team, Org]>

// Sequential execution
const sequential = sequenceT(TE.ApplySeq)(
  fetchUser('1'),
  fetchTeam('team-1'),
  fetchOrg('org-1')
)

Concurrent with limit

// For controlled concurrency, batch your operations
const batchSize = 3

const fetchInBatches = (ids: string[]): TE.TaskEither<Error, User[]> => {
  const batches = chunk(ids, batchSize)

  return pipe(
    batches,
    A.traverse(TE.ApplicativeSeq)((batch) =>
      pipe(batch, TE.traverseArray(fetchUser))
    ),
    TE.map(A.flatten)
  )
}

---

5. Error Recovery with orElse

Basic error recovery

// Try primary, fall back to secondary
const fetchWithFallback = pipe(
  fetchFromPrimaryApi(id),
  TE.orElse((primaryError) =>
    pipe(
      fetchFromBackupApi(id),
      TE.mapLeft((backupError) => ({
        primary: primaryError,
        backup: backupError
      }))
    )
  )
)

// Recover to a default value
const fetchWithDefault = pipe(
  fetchUser(id),
  TE.orElse(() => TE.right(defaultUser))
)

// orElseW when recovery has different error type
const fetchWithTypedFallback = pipe(
  fetchFromApi(id),           // TaskEither<ApiError, User>
  TE.orElseW((apiError) =>    // orElseW allows different error type
    fetchFromCache(id)         // TaskEither<CacheError, User>
  )
) // TaskEither<CacheError, User>

Retry patterns

const retry = <E, A>(
  te: TE.TaskEither<E, A>,
  retries: number,
  delay: number
): TE.TaskEither<E, A> =>
  pipe(
    te,
    TE.orElse((error) =>
      retries > 0
        ? pipe(
            T.delay(delay)(T.of(undefined)),
            T.chain(() => retry(te, retries - 1, delay * 2))
          )
        : TE.left(error)
    )
  )

// Usage
const fetchWithRetry = retry(fetchUser('123'), 3, 1000)

Conditional recovery

// Only recover from specific errors
const recoverFromNotFound = pipe(
  fetchUser(id),
  TE.orElse((error) =>
    error.code === 'NOT_FOUND'
      ? TE.right(createDefaultUser(id))
      : TE.left(error)  // re-throw other errors
  )
)

// Alt: try alternatives in order
import { alt } from 'fp-ts/TaskEither'

const fetchFromAnywhere = pipe(
  fetchFromCache(id),
  TE.alt(() => fetchFromApi(id)),
  TE.alt(() => fetchFromBackup(id))
)

---

6. Pattern Matching Async Results

Using fold/match

// fold executes the TaskEither and handles both cases
const handleResult = pipe(
  fetchUser('123'),
  TE.fold(
    (error) => T.of(`Error: ${error.message}`),
    (user) => T.of(`Welcome, ${user.name}`)
  )
) // Task<string> - no longer has error channel

// match is an alias for fold
const handleWithMatch = pipe(
  fetchUser('123'),
  TE.match(
    (error) => ({ success: false, error }),
    (user) => ({ success: true, data: user })
  )
)

// matchW when handlers return different types
const handleWithMatchW = pipe(
  fetchUser('123'),
  TE.matchW(
    (error) => ({ type: 'error' as const, error }),
    (user) => ({ type: 'success' as const, user })
  )
)

Getting the underlying Either

// Execute and get the Either
const getEither = async () => {
  const either = await fetchUser('123')()

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

// Using getOrElse for default
const getWithDefault = pipe(
  fetchUser('123'),
  TE.getOrElse((error) => T.of(defaultUser))
) // Task<User>

// getOrElseW when default has different type
const getOrNull = pipe(
  fetchUser('123'),
  TE.getOrElseW(() => T.of(null))
) // Task<User | null>

---

7. Do Notation for Complex Workflows

Building complex operations

interface OrderContext {
  user: User
  cart: Cart
  payment: PaymentMethod
  shipping: ShippingAddress
}

const processOrder = (
  userId: string,
  cartId: string
): TE.TaskEither<OrderError, OrderConfirmation> =>
  pipe(
    TE.Do,
    // Bind values sequentially
    TE.bind('user', () => fetchUser(userId)),
    TE.bind('cart', () => fetchCart(cartId)),

    // Validate intermediate results
    TE.filterOrElse(
      ({ cart }) => cart.items.length > 0,
      () => ({ code: 'EMPTY_CART', message: 'Cart is empty' })
    ),

    // Continue building context
    TE.bind('payment', ({ user }) => getDefaultPayment(user.id)),
    TE.bind('shipping', ({ user }) => getDefaultShipping(user.id)),

    // Calculate derived values
    TE.bind('total', ({ cart }) => TE.right(calculateTotal(cart))),

    // Validate before final operation
    TE.filterOrElse(
      ({ payment, total }) => payment.limit >= total,
      ({ total }) => ({ code: 'LIMIT_EXCEEDED', message: `Order total ${total} exceeds limit` })
    ),

    // Final operation
    TE.chain(({ user, cart, payment, shipping, total }) =>
      createOrder({ user, cart, payment, shipping, total })
    )
  )

Parallel fetching within Do

const getOrderDetails = (orderId: string) =>
  pipe(
    TE.Do,
    TE.bind('order', () => fetchOrder(orderId)),

    // Parallel fetch based on order data
    TE.bind('details', ({ order }) =>
      sequenceT(TE.ApplyPar)(
        fetchUser(order.userId),
        fetchProducts(order.productIds),
        fetchShipping(order.shippingId)
      )
    ),

    TE.map(({ order, details: [user, products, shipping] }) => ({
      order,
      user,
      products,
      shipping
    }))
  )

Using apS for simpler additions

// apS is like bind but doesn't depend on previous values
const enrichUser = (userId: string) =>
  pipe(
    fetchUser(userId),
    TE.bindTo('user'),  // Wrap in { user: ... }
    TE.apS('config', fetchAppConfig()),  // Add independent value
    TE.apS('features', fetchFeatureFlags()),
    TE.bind('preferences', ({ user }) => fetchPreferences(user.id))  // Dependent
  )

---

8. Real-World API Call Patterns

Typed API client

interface ApiConfig {
  baseUrl: string
  timeout: number
}

interface ApiError {
  code: string
  message: string
  status: number
  details?: unknown
}

const createApiClient = (config: ApiConfig) => {
  const request = <T>(
    method: string,
    path: string,
    body?: unknown
  ): TE.TaskEither<ApiError, T> =>
    TE.tryCatch(
      async () => {
        const response = await fetch(`${config.baseUrl}${path}`, {
          method,
          headers: { 'Content-Type': 'application/json' },
          body: body ? JSON.stringify(body) : undefined,
          signal: AbortSignal.timeout(config.timeout)
        })

        if (!response.ok) {
          const error = await response.json().catch(() => ({}))
          throw { status: response.status, ...error }
        }

        return response.json()
      },
      (error): ApiError => ({
        code: 'API_ERROR',
        message: error instanceof Error ? error.message : 'Request failed',
        status: (error as any)?.status ?? 500,
        details: error
      })
    )

  return {
    get: <T>(path: string) => request<T>('GET', path),
    post: <T>(path: string, body: unknown) => request<T>('POST', path, body),
    put: <T>(path: string, body: unknown) => request<T>('PUT', path, body),
    delete: <T>(path: string) => request<T>('DELETE', path)
  }
}

// Usage
const api = createApiClient({ baseUrl: '/api', timeout: 5000 })

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

Request with validation

import * as t from 'io-ts'
import { PathReporter } from 'io-ts/PathReporter'

const UserCodec = t.type({
  id: t.string,
  name: t.string,
  email: t.string
})

type User = t.TypeOf<typeof UserCodec>

const fetchAndValidate = <A>(
  codec: t.Type<A>,
  url: string
): TE.TaskEither<Error, A> =>
  pipe(
    TE.tryCatch(
      () => fetch(url).then(r => r.json()),
      (e) => new Error(`Fetch failed: ${e}`)
    ),
    TE.chainEitherK((data) =>
      pipe(
        codec.decode(data),
        E.mapLeft((errors) =>
          new Error(`Validation failed: ${PathReporter.report(E.left(errors)).join(', ')}`)
        )
      )
    )
  )

const getValidatedUser = (id: string) =>
  fetchAndValidate(UserCodec, `/api/users/${id}`)

---

9. Database Operation Patterns

Repository pattern

interface Repository<E, T, ID> {
  findById: (id: ID) => TE.TaskEither<E, T>
  findAll: () => TE.TaskEither<E, readonly T[]>
  save: (entity: T) => TE.TaskEither<E, T>
  delete: (id: ID) => TE.TaskEither<E, void>
}

interface DbError {
  code: 'NOT_FOUND' | 'DUPLICATE' | 'CONSTRAINT' | 'CONNECTION'
  message: string
  cause?: unknown
}

const createUserRepository = (db: Database): Repository<DbError, User, string> => ({
  findById: (id) =>
    pipe(
      TE.tryCatch(
        () => db.query('SELECT * FROM users WHERE id = ?', [id]),
        (e): DbError => ({ code: 'CONNECTION', message: String(e), cause: e })
      ),
      TE.chain((rows) =>
        rows.length === 0
          ? TE.left({ code: 'NOT_FOUND', message: `User ${id} not found` })
          : TE.right(rows[0] as User)
      )
    ),

  findAll: () =>
    TE.tryCatch(
      () => db.query('SELECT * FROM users'),
      (e): DbError => ({ code: 'CONNECTION', message: String(e), cause: e })
    ),

  save: (user) =>
    TE.tryCatch(
      () => db.query(
        'INSERT INTO users (id, name, email) VALUES (?, ?, ?) ON CONFLICT (id) DO UPDATE SET name = ?, email = ?',
        [user.id, user.name, user.email, user.name, user.email]
      ).then(() => user),
      (e): DbError => {
        if (String(e).includes('UNIQUE constraint')) {
          return { code: 'DUPLICATE', message: 'Email already exists', cause: e }
        }
        return { code: 'CONNECTION', message: String(e), cause: e }
      }
    ),

  delete: (id) =>
    TE.tryCatch(
      () => db.query('DELETE FROM users WHERE id = ?', [id]).then(() => undefined),
      (e): DbError => ({ code: 'CONNECTION', message: String(e), cause: e })
    )
})

Transaction handling

interface Transaction {
  query: (sql: string, params?: unknown[]) => Promise<unknown>
  commit: () => Promise<void>
  rollback: () => Promise<void>
}

const withTransaction = <E, A>(
  db: Database,
  operation: (tx: Transaction) => TE.TaskEither<E, A>
): TE.TaskEither<E | DbError, A> =>
  pipe(
    TE.tryCatch(
      () => db.beginTransaction(),
      (e): DbError => ({ code: 'CONNECTION', message: 'Failed to start transaction', cause: e })
    ),
    TE.chain((tx) =>
      pipe(
        operation(tx),
        TE.chainFirst(() =>
          TE.tryCatch(
            () => tx.commit(),
            (e): DbError => ({ code: 'CONNECTION', message: 'Commit failed', cause: e })
          )
        ),
        TE.orElse((error) =>
          pipe(
            TE.tryCatch(() => tx.rollback(), () => error),
            TE.chain(() => TE.left(error))
          )
        )
      )
    )
  )

// Usage
const transferFunds = (fromId: string, toId: string, amount: number) =>
  withTransaction(db, (tx) =>
    pipe(
      TE.Do,
      TE.bind('from', () => getAccount(tx, fromId)),
      TE.bind('to', () => getAccount(tx, toId)),
      TE.filterOrElse(
        ({ from }) => from.balance >= amount,
        () => ({ code: 'INSUFFICIENT_FUNDS', message: 'Not enough balance' })
      ),
      TE.chain(({ from, to }) =>
        sequenceT(TE.ApplySeq)(
          updateBalance(tx, fromId, from.balance - amount),
          updateBalance(tx, toId, to.balance + amount)
        )
      ),
      TE.map(() => ({ success: true, amount }))
    )
  )

---

10. Task vs TaskEither: When to Use Which

Use Task when:

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

// 1. Operation cannot fail
const delay = (ms: number): T.Task<void> =>
  () => new Promise(resolve => setTimeout(resolve, ms))

// 2. Errors are handled elsewhere
const logMessage = (msg: string): T.Task<void> =>
  () => console.log(msg) as unknown as Promise<void>

// 3. You want to ignore errors
const fetchOrDefault = (url: string, defaultValue: Data): T.Task<Data> =>
  pipe(
    TE.tryCatch(() => fetch(url).then(r => r.json()), E.toError),
    TE.getOrElse(() => T.of(defaultValue))
  )

// 4. Fire and forget
const trackAnalytics = (event: Event): T.Task<void> =>
  () => analytics.track(event).catch(() => {}) // Errors swallowed

Use TaskEither when:

// 1. Operation can fail and you need to handle the error
const fetchUser = (id: string): TE.TaskEither<Error, User> =>
  TE.tryCatch(() => api.getUser(id), E.toError)

// 2. You need typed errors for different failure modes
type AuthError =
  | { type: 'INVALID_CREDENTIALS' }
  | { type: 'EXPIRED_TOKEN' }
  | { type: 'NETWORK_ERROR'; cause: Error }

const authenticate = (token: string): TE.TaskEither<AuthError, User> => { /* ... */ }

// 3. Error recovery is part of business logic
const getConfig = (): TE.TaskEither<ConfigError, Config> =>
  pipe(
    fetchRemoteConfig(),
    TE.orElse(() => loadLocalConfig()),
    TE.orElse(() => TE.right(defaultConfig))
  )

// 4. Composing multiple fallible operations
const processOrder = (orderId: string): TE.TaskEither<OrderError, Receipt> =>
  pipe(
    validateOrder(orderId),
    TE.chain(chargePayment),
    TE.chain(fulfillOrder),
    TE.chain(sendConfirmation)
  )

Converting between them

// Task to TaskEither (infallible to fallible)
const taskToTE = <A>(task: T.Task<A>): TE.TaskEither<never, A> =>
  pipe(task, T.map(E.right))

// TaskEither to Task (handle/ignore error)
const teToTask = <E, A>(te: TE.TaskEither<E, A>, defaultValue: A): T.Task<A> =>
  TE.getOrElse(() => T.of(defaultValue))(te)

// TaskEither to Task (throw on error - escape hatch)
const teToTaskThrow = <E, A>(te: TE.TaskEither<E, A>): T.Task<A> =>
  pipe(
    te,
    TE.getOrElse((e) => () => Promise.reject(e))
  )

---

Quick Reference

Operation	Function	Description
Create success	TE.right(value)	Wrap value in Right
Create failure	TE.left(error)	Wrap error in Left
From Promise	TE.tryCatch(promise, onError)	Convert Promise to TE
Transform value	TE.map(f)	Apply f to success value
Transform error	TE.mapLeft(f)	Apply f to error value
Chain operations	TE.chain(f) / TE.flatMap(f)	Sequence dependent operations
Recover from error	TE.orElse(f)	Try alternative on error
Handle both cases	TE.fold(onError, onSuccess)	Pattern match result
Parallel array	TE.traverseArray(f)	Map + sequence in parallel
Sequential array	A.traverse(TE.ApplicativeSeq)(f)	Map + sequence in order
Filter with error	TE.filterOrElse(pred, onFalse)	Validate with error
Get or default	TE.getOrElse(onError)	Extract value with fallback

---

Common Patterns Summary

// 1. Fetch with error handling
const fetch = TE.tryCatch(() => api.get(url), toError)

// 2. Chain dependent calls
pipe(getA(), TE.chain(a => getB(a.id)), TE.chain(b => getC(b.id)))

// 3. Parallel independent calls
sequenceT(TE.ApplyPar)(getA(), getB(), getC())

// 4. Build context with Do
pipe(TE.Do, TE.bind('a', () => getA()), TE.bind('b', ({a}) => getB(a)))

// 5. Recover from errors
pipe(primary(), TE.orElse(() => fallback()))

// 6. Execute and handle result
pipe(operation(), TE.fold(handleError, handleSuccess))()