# Fp Async

> Practical async patterns using TaskEither - clean pipelines instead of try/catch hell, with real API examples

- Skill: `ranbot-ai/fp-async-2` (Agent Skill)
- Install (CLI): `npx skillmds@latest add ranbot-ai/fp-async-2`
- Raw SKILL.md: https://api.skillmd.com/api/skills/ranbot-ai/fp-async-2/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Integrations & APIs
- Author: ranbot-ai (https://skillmd.com/u/ranbot-ai)
- Updated: 2026-09-21
- Page: https://skillmd.com/skills/ranbot-ai/fp-async-2

---



# 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.

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

---

## 5. Real API Examples

### Complete Fetch Wrapper

```typescript
// 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)

```typescript
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
    use