fp-either-ref

Either Quick Reference

Either = success or failure. Right(value) or Left(error).

When to Use

• You need a quick fp-ts reference for typed synchronous error handling.
• The task involves validation, fallible operations, or converting throwing code to Either.
• You want a compact cheat sheet rather than a long tutorial.

Create

import * as E from 'fp-ts/Either'

E.right(value)           // Success
E.left(error)            // Failure
E.fromNullable(err)(x)   // null → Left(err), else Right(x)
E.tryCatch(fn, toError)  // try/catch → Either

Transform

E.map(fn)                // Transform Right value
E.mapLeft(fn)            // Transform Left error
E.flatMap(fn)            // Chain (fn returns Either)
E.filterOrElse(pred, toErr) // Right → Left if pred fails

Extract

E.getOrElse(err => default)  // Get Right or default
E.match(onLeft, onRight)     // Pattern match
E.toUnion(either)            // E | A (loses type info)

Common Patterns

import { pipe } from 'fp-ts/function'
import * as E from 'fp-ts/Either'

// Validation
const validateEmail = (s: string): E.Either<string, string> =>
  s.includes('@') ? E.right(s) : E.left('Invalid email')

// Chain validations (stops at first error)
pipe(
  E.right({ email: 'test@example.com', age: 25 }),
  E.flatMap(d => pipe(validateEmail(d.email), E.