fp-taskeither-ref

TaskEither Quick Reference

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

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

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

// 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

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)