fp-ts Do Notation Guide

Do notation is fp-ts's answer to callback hell. It provides a way to write sequential, imperative-looking code while maintaining functional purity and type safety.

The Problem: Callback Hell in Functional Code

Without Do notation, chaining dependent operations leads to deeply nested code:

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

// BAD: Nested chain hell
const processOrder = (orderId: string) =>
  pipe(
    fetchOrder(orderId),
    TE.chain((order) =>
      pipe(
        fetchUser(order.userId),