fp-ts TaskEither Async Patterns

TaskEither combines the laziness of Task with the error handling of Either, providing a powerful abstraction for async operations that can fail.

Core Concepts

import * as TE from 'fp-ts/TaskEither'
import * as T from 'fp-ts/Task'
import * as E from 'fp-ts/Either'
import { pipe, flow } from 'fp-ts/function'

TaskEither<E, A> is equivalent to () => Promise<Either<E, A>>

• E = Error type (left)
• A = Success type (right)
• Lazy: nothing executes until you call the function
• Composable: chain operations without try/catch

---

1. Converting Promises to TaskEither

Using tryCatch

The primary way to lift Promises into TaskEither:

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

// Basic tryCatch pattern
const fetchUser = (id: string): TE.TaskEither<Error, User> =>
  TE.tryCatch(
    () => fetch(`/api/users/${id}`).then(res => res.json()),
    (reason) => new Error(String(reason))
  )

// With typed errors
interface ApiError {
  code: string
  message: string
  status: number
}

const fetchUserTyped = (id: string): TE.TaskEither<ApiError, User> =>
  TE.tryCatch(
    () => fetch(`/api/users/${id}`).then(res => res.json()),
    (reason): ApiError => ({
      code: 'FETCH_ERROR',
      message: reason instanceof Error ? reason.message : 'Unknown error',
      status: 500
    })
  )

From existing Either

// Lift an Either into TaskEither
const fromEither: TE.TaskEither<Error, number> = TE.fromEither(E.right(42))

// From a nullable value
const fromNullable = TE.fromNullable(new Error('Value was null'))
const result = fromNullable(maybeValue) // TaskEither<Error, NonNullable<T>>

// From an Option
import * as O from 'fp-ts/Option'
const fromOption = TE.fromOption(() => new Error('None value'))
const optionResult = fromOption(O.some(42)) // TaskEither<Error, number>

Creating TaskEither values directly

// Success value
const success = TE.right<Error, n...