fp-either-ref

name	fp-either-ref
description	Quick reference for Either type. Use when user needs error handling, validation, or operations that can fail with typed errors.
type	skill
created	2026-02-27T00:00:00.000Z
domain	communication-content
category	documentation
risk	unknown
source	community
tags	["skill","communication-content","documentation","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 *  E  


 validateEmail = (: ): E.<, > =>
  s.() ? E.(s) : E.()


(
  E.({ : , :  }),
  E.( ((d.), E.( d))),
  E.( d. >=  ? E.(d) : E.())
)


  = () => E.(
   .(s),
   
)