# Fp Taskeither Ref

> ALWAYS use this when the user mentions FP Taskeither REF, asks to build, debug, review, document, automate, test, configure, migrate, or make decisions in this domain, or the task clearly depends on FP Taskeither REF; scope: Quick reference for TaskEither. Apply the bundled workflow, references, scripts, Senior Master standard, and Codex strict review gate before final output.

- Skill: `anhvu1107/fp-taskeither-ref` (Agent Skill, multi-file: 6 files)
- Install (CLI): `npx skillmds@latest add anhvu1107/fp-taskeither-ref`
- Raw SKILL.md: https://api.skillmd.com/api/skills/anhvu1107/fp-taskeither-ref/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: AI & ML
- Author: anhvu1107 (https://skillmd.com/u/anhvu1107)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/anhvu1107/fp-taskeither-ref

---


# TaskEither Quick Reference

## Selective Reading Rule

Start with:

- `references/senior-master-standard.md`
- `references/usage-routing.md`
- `references/quality-checklist.md`

Then load only the inherited docs, scripts, assets, or examples that match the user's actual task.

TaskEither = async operation that can fail. Like `Promise<Either<E, A>>`.

## When to Use
- You need a quick fp-ts reference for async operations that can fail.
- The task involves API calls, Promise wrapping, or composing asynchronous error-handling pipelines.
- You want a concise cheat sheet for `TaskEither` operators and patterns.

## Create

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

TE.right(value)          // Async success
TE.left(error)           // Async failure
TE.tryCatch(asyncFn, toError)  // Promise → TaskEither
TE.fromEither(either)    // Either → TaskEither
```

## Transform

```typescript
TE.map(fn)               // Transform success value
TE.mapLeft(fn)           // Transform error
TE.flatMap(fn)           // Chain (fn returns TaskEither)
TE.orElse(fn)            // Recover from error
```

## Execute

```typescript
// TaskEither is lazy - must call () to run
const result = await myTaskEither()  // Either<E, A>

// Or pattern match
await pipe(
  myTaskEither,
  TE.match(
    (err) => console.error(err),
    (val) => console.log(val)
  )
)()
```

## Common Patterns

```typescript
import { pipe } from 'fp-ts/function'
import * as TE from 'fp-ts/TaskEither'

// Wrap fetch
const fetchUser = (id: string) => TE.tryCatch(
  () => fetch(`/api/users/${id}`).then(r => r.json()),
  (e) => ({ type: 'NETWORK_ERROR', message: String(e) })
)

// Chain async calls
pipe(
  fetchUser('123'),
  TE.flatMap(user => fetchPosts(user.id)),
  TE.map(posts => posts.length)
)

// Parallel calls
import { sequenceT } from 'fp-ts/Apply'
sequenceT(TE.ApplyPar)(
  fetchUser('1'),
  fetchPosts('1'),
  fetchComments('1')
)

// With recovery
pipe(
  fetchUser('123'),
  TE.orElse(() => TE.right(defaultUser)),
  TE.getOrElse(() => defaultUser)
)
```

## vs async/await

```typescript
// ❌ async/await - errors hidden
async function getUser(id: string) {
  try {
    const res = await fetch(`/api/users/${id}`)
    return await res.json()
  } catch (e) {
    return null  // Error info lost
  }
}

// ✅ TaskEither - errors typed and composable
const getUser = (id: string) => pipe(
  TE.tryCatch(() => fetch(`/api/users/${id}`), toNetworkError),
  TE.flatMap(res => TE.tryCatch(() => res.json(), toParseError))
)
```

Use TaskEither when you need **typed errors** for async operations.

## 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.