fp-async

name: fp-async description: "Practical Async Patterns with fp-ts workflow skill. Use this skill when the user needs Practical async patterns using TaskEither - clean pipelines instead of try/catch hell, with real API examples and the operator should preserve the upstream workflow, copied support files, and provenance before merging or handing off." version: "0.0.1" category: data-ai tags: ["fp-ts", "typescript", "async", "error-handling", "practical", "promises", "api", "fetch"] complexity: advanced risk: caution tools: ["codex-cli", "claude-code", "cursor", "gemini-cli", "opencode"] source: community author: "kadu" date_added: "2026-04-15" date_updated: "2026-04-25"

Practical Async Patterns with fp-ts

Overview

This public intake copy packages plugins/antigravity-awesome-skills-claude/skills/fp-async from https://github.com/sickn33/antigravity-awesome-skills into the native Omni Skills editorial shape without hiding its origin.

Use it when the operator needs the upstream workflow, support files, and repository context to stay intact while the public validator and private enhancer continue their normal downstream flow.

This intake keeps the copied upstream files intact and uses the external_source block in metadata.json plus ORIGIN.md as the provenance anchor for review.

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.

Imported source sections that did not map cleanly to the public headings are still preserved below or in the support files. Notable imported sections: 1. Wrapping Promises Safely, 2. Chaining Async Operations, 3. Parallel vs Sequential Execution, 4. Error Recovery Patterns, 6. Handling Results, Before/After Summary.

When to Use This Skill

Use this section as the trigger filter. It should make the activation boundary explicit before the operator loads files, runs commands, or opens a pull request.

• 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.
• Use when the request clearly matches the imported source intent: Practical async patterns using TaskEither - clean pipelines instead of try/catch hell, with real API examples.
• Use when the operator should preserve upstream workflow detail instead of rewriting the process from scratch.
• Use when provenance needs to stay visible in the answer, PR, or review packet.

Operating Table

Workflow

This workflow is intentionally editorial and operational at the same time. It keeps the imported source useful to the operator while still satisfying the public intake standards that feed the downstream enhancer flow.

1. Confirm the user goal, the scope of the imported workflow, and whether this skill is still the right router for the task.
2. Read the overview and provenance files before loading any copied upstream support files.
3. Load only the references, examples, prompts, or scripts that materially change the outcome for the current request.
4. Execute the upstream workflow while keeping provenance and source boundaries explicit in the working notes.
5. Validate the result against the upstream expectations and the evidence you can point to in the copied files.
6. Escalate or hand off to a related skill when the work moves out of this imported workflow's center of gravity.
7. Before merge or closure, record what was used, what changed, and what the reviewer still needs to verify.

Imported Workflow Notes

Imported: 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}`)
)

Examples

Example 1: Ask for the upstream workflow directly

Use @fp-async to handle <task>. Start from the copied upstream workflow, load only the files that change the outcome, and keep provenance visible in the answer.

Explanation: This is the safest starting point when the operator needs the imported workflow, but not the entire repository.

Example 2: Ask for a provenance-grounded review

Review @fp-async against metadata.json and ORIGIN.md, then explain which copied upstream files you would load first and why.

Explanation: Use this before review or troubleshooting when you need a precise, auditable explanation of origin and file selection.

Example 3: Narrow the copied support files before execution

Use @fp-async for <task>. Load only the copied references, examples, or scripts that change the outcome, and name the files explicitly before proceeding.

Explanation: This keeps the skill aligned with progressive disclosure instead of loading the whole copied package by default.

Example 4: Build a reviewer packet

Review @fp-async using the copied upstream files plus provenance, then summarize any gaps before merge.

Explanation: This is useful when the PR is waiting for human review and you want a repeatable audit packet.

Imported Usage Notes

Imported: 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))
  )

Best Practices

Treat the generated public skill as a reviewable packaging layer around the upstream repository. The goal is to keep provenance explicit and load only the copied source material that materially improves execution.

• Keep the imported skill grounded in the upstream repository; do not invent steps that the source material cannot support.
• Prefer the smallest useful set of support files so the workflow stays auditable and fast to review.
• Keep provenance, source commit, and imported file paths visible in notes and PR descriptions.
• Point directly at the copied upstream files that justify the workflow instead of relying on generic review boilerplate.
• Treat generated examples as scaffolding; adapt them to the concrete task before execution.
• Route to a stronger native skill when architecture, debugging, design, or security concerns become dominant.

Troubleshooting

Problem: The operator skipped the imported context and answered too generically

Symptoms: The result ignores the upstream workflow in plugins/antigravity-awesome-skills-claude/skills/fp-async, fails to mention provenance, or does not use any copied source files at all. Solution: Re-open metadata.json, ORIGIN.md, and the most relevant copied upstream files. Check the external_source block first, then restate the provenance before continuing.

Problem: The imported workflow feels incomplete during review

Symptoms: Reviewers can see the generated SKILL.md, but they cannot quickly tell which references, examples, or scripts matter for the current task. Solution: Point at the exact copied references, examples, scripts, or assets that justify the path you took. If the gap is still real, record it in the PR instead of hiding it.

Problem: The task drifted into a different specialization

Symptoms: The imported skill starts in the right place, but the work turns into debugging, architecture, design, security, or release orchestration that a native skill handles better. Solution: Use the related skills section to hand off deliberately. Keep the imported provenance visible so the next skill inherits the right context instead of starting blind.

Related Skills

• @00-andruia-consultant - Use when the work is better handled by that native specialization after this imported skill establishes context.
• @00-andruia-consultant-v2 - Use when the work is better handled by that native specialization after this imported skill establishes context.
• @10-andruia-skill-smith - Use when the work is better handled by that native specialization after this imported skill establishes context.
• @10-andruia-skill-smith-v2 - Use when the work is better handled by that native specialization after this imported skill establishes context.

Additional Resources

Use this support matrix and the linked files below as the operator packet for this imported skill. They should reflect real copied source material, not generic scaffolding.

Imported Reference Notes

Imported: 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')
)

Imported: Quick Reference Card

Imported: 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
    }))
  )

Imported: 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 }))
      )
    )
  )

Imported: 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

Imported: 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)
})

Imported: 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))
  )

Imported: 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.