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.

Detailed Guide

Read [the detailed guide](references/detailed-guide.md) before executing this skill. It retains the complete procedure and reference material. Treat its safety, prerequisites, and validation requirements as mandatory. For focused work, load the relevant sections; for end-to-end work, read the guide completely.

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.

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

---

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))
  )

---

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.