fp-errors

Stop throwing everywhere - handle errors as values using Either and TaskEither for cleaner, more predictable code

Dépôt: phoroth/AGENTIC
Dernière activité de la source: 30 juillet 2026 à 18:53
Langue détectée de SKILL.md: anglais
Étoiles: 3
Forks: 0

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TaskEither

// What this function signature promises:
function getUser(id: string): User

// What it actually does:
function getUser(id: string): User {
  if (!id) throw new Error('ID required')
  const user = db.find(id)
  if (!user) throw new Error('User not found')
  return user
}

// The caller has no idea this can fail
const user = getUser(id) // Might explode!

// MESSY: try/catch everywhere
function processOrder(orderId: string) {
  let order
  try {
    order = getOrder(orderId)
  } catch (e) {
    console.error('Failed to get order')
    return null
  }

  let user
  try {
    user = getUser(order.userId)
  } catch (e) {
    console.error('Failed to get user')
    return null
  }

  let payment
  try {
    payment = chargeCard(user.cardId, order.total)
  } catch (e) {
    console.error('Payment failed')
    return null
  }

  return { order, user, payment }
}

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

// Now TypeScript KNOWS this can fail
function getUser(id: string): E.Either<string, User> {
  if (!id) return E.left('ID required')
  const user = db.find(id)
  if (!user) return E.left('User not found')
  return E.right(user)
}

// The caller is forced to handle both cases
const result = getUser(id)
// result is Either<string, User> - error OR success, never both

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

// Creating values
const success = E.right(42)           // Right(42)
const failure = E.left('Oops')        // Left('Oops')

// Checking what you have
if (E.isRight(result)) {
  console.log(result.right) // The success value
} else {
  console.log(result.left)  // The error
}

// Better: pattern match with fold
const message = pipe(
  result,
  E.fold(
    (error) => `Failed: ${error}`,
    (value) => `Got: ${value}`
  )
)

// Wrap any throwing function with tryCatch
const parseJSON = (json: string): E.Either<Error, unknown> =>
  E.tryCatch(
    () => JSON.parse(json),
    (e) => (e instanceof Error ? e : new Error(String(e)))
  )

parseJSON('{"valid": true}')  // Right({ valid: true })
parseJSON('not json')          // Left(SyntaxError: ...)

// For functions you'll reuse, use tryCatchK
const safeParseJSON = E.tryCatchK(
  JSON.parse,
  (e) => (e instanceof Error ? e : new Error(String(e)))
)

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

// Transform the success value
const doubled = pipe(
  E.right(21),
  E.map(n => n * 2)
) // Right(42)

// Transform the error
const betterError = pipe(
  E.left('bad'),
  E.mapLeft(e => `Error: ${e}`)
) // Left('Error: bad')

// Provide a default for errors
const value = pipe(
  E.left('failed'),
  E.getOrElse(() => 0)
) // 0

// Convert nullable to Either
const fromNullable = E.fromNullable('not found')
fromNullable(user)  // Right(user) if exists, Left('not found') if null/undefined

// MESSY: Each step can fail, nested try/catch everywhere
function processUserOrder(userId: string, productId: string): Result | null {
  let user
  try {
    user = getUser(userId)
  } catch (e) {
    logError('User fetch failed', e)
    return null
  }

  if (!user.isActive) {
    logError('User not active')
    return null
  }

  let product
  try {
    product = getProduct(productId)
  } catch (e) {
    logError('Product fetch failed', e)
    return null
  }

  if (product.stock < 1) {
    logError('Out of stock')
    return null
  }

  let order
  try {
    order = createOrder(user, product)
  } catch (e) {
    logError('Order creation failed', e)
    return null
  }

  return order
}

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

// Each function returns Either<Error, T>
const getUser = (id: string): E.Either<string, User> => { ... }
const getProduct = (id: string): E.Either<string, Product> => { ... }
const createOrder = (user: User, product: Product): E.Either<string, Order> => { ... }

// Chain them together - first error stops the chain
const processUserOrder = (userId: string, productId: string): E.Either<string, Order> =>
  pipe(
    getUser(userId),
    E.filterOrElse(
      user => user.isActive,
      () => 'User not active'
    ),
    E.chain(user =>
      pipe(
        getProduct(productId),
        E.filterOrElse(
          product => product.stock >= 1,
          () => 'Out of stock'
        ),
        E.chain(product => createOrder(user, product))
      )
    )
  )

// Or use Do notation for cleaner access to intermediate values
const processUserOrder = (userId: string, productId: string): E.Either<string, Order> =>
  pipe(
    E.Do,
    E.bind('user', () => getUser(userId)),
    E.filterOrElse(
      ({ user }) => user.isActive,
      () => 'User not active'
    ),
    E.bind('product', () => getProduct(productId)),
    E.filterOrElse(
      ({ product }) => product.stock >= 1,
      () => 'Out of stock'
    ),
    E.chain(({ user, product }) => createOrder(user, product))
  )

type ValidationError = { type: 'validation'; message: string }
type DbError = { type: 'db'; message: string }

const validateInput = (id: string): E.Either<ValidationError, string> => { ... }
const fetchFromDb = (id: string): E.Either<DbError, User> => { ... }

// chainW (W = "wider") automatically unions the error types
const process = (id: string): E.Either<ValidationError | DbError, User> =>
  pipe(
    validateInput(id),
    E.chainW(validId => fetchFromDb(validId))
  )

// MESSY: Manual error accumulation
function validateForm(form: FormData): { valid: boolean; errors: string[] } {
  const errors: string[] = []

  if (!form.email) {
    errors.push('Email required')
  } else if (!form.email.includes('@')) {
    errors.push('Invalid email')
  }

  if (!form.password) {
    errors.push('Password required')
  } else if (form.password.length < 8) {
    errors.push('Password too short')
  }

  if (!form.age) {
    errors.push('Age required')
  } else if (form.age < 18) {
    errors.push('Must be 18+')
  }

  return { valid: errors.length === 0, errors }
}

import * as E from 'fp-ts/Either'
import * as NEA from 'fp-ts/NonEmptyArray'
import { sequenceS } from 'fp-ts/Apply'
import { pipe } from 'fp-ts/function'

// Errors as a NonEmptyArray (always at least one)
type Errors = NEA.NonEmptyArray<string>

// Create the applicative that accumulates errors
const validation = E.getApplicativeValidation(NEA.getSemigroup<string>())

// Validators that return Either<Errors, T>
const validateEmail = (email: string): E.Either<Errors, string> =>
  !email ? E.left(NEA.of('Email required'))
  : !email.includes('@') ? E.left(NEA.of('Invalid email'))
  : E.right(email)

const validatePassword = (password: string): E.Either<Errors, string> =>
  !password ? E.left(NEA.of('Password required'))
  : password.length < 8 ? E.left(NEA.of('Password too short'))
  : E.right(password)

const validateAge = (age: number | undefined): E.Either<Errors, number> =>
  age === undefined ? E.left(NEA.of('Age required'))
  : age < 18 ? E.left(NEA.of('Must be 18+'))
  : E.right(age)

// Combine all validations - collects ALL errors
const validateForm = (form: FormData) =>
  sequenceS(validation)({
    email: validateEmail(form.email),
    password: validatePassword(form.password),
    age: validateAge(form.age)
  })

// Usage
validateForm({ email: '', password: '123', age: 15 })
// Left(['Email required', 'Password too short', 'Must be 18+'])

validateForm({ email: 'a@b.com', password: 'longpassword', age: 25 })
// Right({ email: 'a@b.com', password: 'longpassword', age: 25 })

interface FieldError {
  field: string
  message: string
}

type FormErrors = NEA.NonEmptyArray<FieldError>

const fieldError = (field: string, message: string): FormErrors =>
  NEA.of({ field, message })

const formValidation = E.getApplicativeValidation(NEA.getSemigroup<FieldError>())

// Now errors know which field they belong to
const validateEmail = (email: string): E.Either<FormErrors, string> =>
  !email ? E.left(fieldError('email', 'Required'))
  : !email.includes('@') ? E.left(fieldError('email', 'Invalid format'))
  : E.right(email)

// Easy to display in UI
const getFieldError = (errors: FormErrors, field: string): string | undefined =>
  errors.find(e => e.field === field)?.message

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

// Wrap any async operation
const fetchUser = (id: string): TE.TaskEither<Error, User> =>
  TE.tryCatch(
    () => fetch(`/api/users/${id}`).then(r => r.json()),
    (e) => (e instanceof Error ? e : new Error(String(e)))
  )

// Chain async operations - just like Either
const getUserPosts = (userId: string): TE.TaskEither<Error, Post[]> =>
  pipe(
    fetchUser(userId),
    TE.chain(user => fetchPosts(user.id))
  )

// Execute when ready
const result = await getUserPosts('123')() // Returns Either<Error, Post[]>

// MESSY: try/catch mixed with promise chains
async function loadDashboard(userId: string) {
  try {
    const user = await fetchUser(userId)
    if (!user) throw new Error('User not found')

    let posts, notifications, settings
    try {
      [posts, notifications, settings] = await Promise.all([
        fetchPosts(user.id),
        fetchNotifications(user.id),
        fetchSettings(user.id)
      ])
    } catch (e) {
      // Which one failed? Who knows!
      console.error('Failed to load data', e)
      return null
    }

    return { user, posts, notifications, settings }
  } catch (e) {
    console.error('Failed to load user', e)
    return null
  }
}

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

const loadDashboard = (userId: string) =>
  pipe(
    fetchUser(userId),
    TE.chain(user =>
      pipe(
        // Parallel fetch with sequenceS
        sequenceS(TE.ApplyPar)({
          posts: fetchPosts(user.id),
          notifications: fetchNotifications(user.id),
          settings: fetchSettings(user.id)
        }),
        TE.map(data => ({ user, ...data }))
      )
    )
  )

// Execute and handle both cases
pipe(
  loadDashboard('123'),
  TE.fold(
    (error) => T.of(renderError(error)),
    (data) => T.of(renderDashboard(data))
  )
)()

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

const retry = <E, A>(
  task: TE.TaskEither<E, A>,
  attempts: number,
  delayMs: number
): TE.TaskEither<E, A> =>
  pipe(
    task,
    TE.orElse((error) =>
      attempts > 1
        ? pipe(
            T.delay(delayMs)(T.of(undefined)),
            T.chain(() => retry(task, attempts - 1, delayMs * 2))
          )
        : TE.left(error)
    )
  )

// Retry up to 3 times with exponential backoff
const fetchWithRetry = retry(fetchUser('123'), 3, 1000)

// Try cache first, fall back to API
const getUserData = (id: string) =>
  pipe(
    fetchFromCache(id),
    TE.orElse(() => fetchFromApi(id)),
    TE.orElse(() => TE.right(defaultUser)) // Last resort default
  )

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

// Direct conversion
const user = users.find(u => u.id === id) // User | undefined
const result = E.fromNullable('User not found')(user)

// From Option
const maybeUser: O.Option<User> = O.fromNullable(user)
const eitherUser = pipe(
  maybeUser,
  E.fromOption(() => 'User not found')
)

// Wrap at the boundary
const safeParse = <T>(schema: ZodSchema<T>) => (data: unknown): E.Either<ZodError, T> =>
  E.tryCatch(
    () => schema.parse(data),
    (e) => e as ZodError
  )

// Use throughout your code
const parseUser = safeParse(UserSchema)
const result = parseUser(rawData) // Either<ZodError, User>

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

// Wrap external async functions
const fetchJson = <T>(url: string): TE.TaskEither<Error, T> =>
  TE.tryCatch(
    () => fetch(url).then(r => r.json()),
    (e) => new Error(`Fetch failed: ${e}`)
  )

// Wrap axios, prisma, any async library
const getUserFromDb = (id: string): TE.TaskEither<DbError, User> =>
  TE.tryCatch(
    () => prisma.user.findUniqueOrThrow({ where: { id } }),
    (e) => ({ code: 'DB_ERROR', cause: e })
  )

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

const myTaskEither: TE.TaskEither<Error, User> = fetchUser('123')

// Option 1: Get the Either (preserves both cases)
const either: E.Either<Error, User> = await myTaskEither()

// Option 2: Throw on error (for legacy code)
const toThrowingPromise = <E, A>(te: TE.TaskEither<E, A>): Promise<A> =>
  te().then(E.fold(
    (error) => Promise.reject(error),
    (value) => Promise.resolve(value)
  ))

const user = await toThrowingPromise(fetchUser('123')) // Throws if Left

// Option 3: Default on error
const user = await pipe(
  fetchUser('123'),
  TE.getOrElse(() => T.of(defaultUser))
)()

interface ParsedInput {
  id: number
  name: string
  tags: string[]
}

const parseInput = (raw: unknown): E.Either<string, ParsedInput> =>
  pipe(
    E.Do,
    E.bind('obj', () =>
      typeof raw === 'object' && raw !== null
        ? E.right(raw as Record<string, unknown>)
        : E.left('Input must be an object')
    ),
    E.bind('id', ({ obj }) =>
      typeof obj.id === 'number'
        ? E.right(obj.id)
        : E.left('id must be a number')
    ),
    E.bind('name', ({ obj }) =>
      typeof obj.name === 'string' && obj.name.length > 0
        ? E.right(obj.name)
        : E.left('name must be a non-empty string')
    ),
    E.bind('tags', ({ obj }) =>
      Array.isArray(obj.tags) && obj.tags.every(t => typeof t === 'string')
        ? E.right(obj.tags as string[])
        : E.left('tags must be an array of strings')
    ),
    E.map(({ id, name, tags }) => ({ id, name, tags }))
  )

// Usage
parseInput({ id: 1, name: 'test', tags: ['a', 'b'] })
// Right({ id: 1, name: 'test', tags: ['a', 'b'] })

parseInput({ id: 'wrong', name: '', tags: null })
// Left('id must be a number')

interface ApiError {
  code: string
  message: string
  status?: number
}

const createApiError = (message: string, code = 'UNKNOWN', status?: number): ApiError =>
  ({ code, message, status })

const fetchWithErrorHandling = <T>(url: string): TE.TaskEither<ApiError, T> =>
  pipe(
    TE.tryCatch(
      () => fetch(url),
      () => createApiError('Network error', 'NETWORK')
    ),
    TE.chain(response =>
      response.ok
        ? TE.tryCatch(
            () => response.json() as Promise<T>,
            () => createApiError('Invalid JSON', 'PARSE')
          )
        : TE.left(createApiError(
            `HTTP ${response.status}`,
            response.status === 404 ? 'NOT_FOUND' : 'HTTP_ERROR',
            response.status
          ))
    )
  )

// Usage with pattern matching on error codes
const handleUserFetch = (userId: string) =>
  pipe(
    fetchWithErrorHandling<User>(`/api/users/${userId}`),
    TE.fold(
      (error) => {
        switch (error.code) {
          case 'NOT_FOUND': return T.of(showNotFoundPage())
          case 'NETWORK': return T.of(showOfflineMessage())
          default: return T.of(showGenericError(error.message))
        }
      },
      (user) => T.of(showUserProfile(user))
    )
  )

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

interface ProcessResult<T> {
  successes: T[]
  failures: Array<{ item: unknown; error: string }>
}

// Process all, collect successes and failures separately
const processAllCollectErrors = <T, R>(
  items: T[],
  process: (item: T) => E.Either<string, R>
): ProcessResult<R> => {
  const results = items.map((item, index) =>
    pipe(
      process(item),
      E.mapLeft(error => ({ item, error, index }))
    )
  )

  return {
    successes: pipe(results, A.filterMap(E.toOption)),
    failures: pipe(
      results,
      A.filterMap(r => E.isLeft(r) ? O.some(r.left) : O.none)
    )
  }
}

// Usage
const parseNumbers = (inputs: string[]) =>
  processAllCollectErrors(inputs, input => {
    const n = parseInt(input, 10)
    return isNaN(n) ? E.left(`Invalid number: ${input}`) : E.right(n)
  })

parseNumbers(['1', 'abc', '3', 'def'])
// {
//   successes: [1, 3],
//   failures: [
//     { item: 'abc', error: 'Invalid number: abc', index: 1 },
//     { item: 'def', error: 'Invalid number: def', index: 3 }
//   ]
// }

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

interface BulkResult<T> {
  succeeded: T[]
  failed: Array<{ id: string; error: string }>
}

const bulkProcess = <T>(
  ids: string[],
  process: (id: string) => TE.TaskEither<string, T>
): T.Task<BulkResult<T>> =>
  pipe(
    ids,
    A.map(id =>
      pipe(
        process(id),
        TE.fold(
          (error) => T.of({ type: 'failed' as const, id, error }),
          (result) => T.of({ type: 'succeeded' as const, result })
        )
      )
    ),
    T.sequenceArray,
    T.map(results => ({
      succeeded: results
        .filter((r): r is { type: 'succeeded'; result: T } => r.type === 'succeeded')
        .map(r => r.result),
      failed: results
        .filter((r): r is { type: 'failed'; id: string; error: string } => r.type === 'failed')
        .map(({ id, error }) => ({ id, error }))
    }))
  )

// Usage
const deleteUsers = (userIds: string[]) =>
  bulkProcess(userIds, id =>
    pipe(
      deleteUser(id),
      TE.mapLeft(e => e.message)
    )
  )

// All operations run, you get a report of what worked and what didn't

Pattern	Use When	Example
`E.right(value)`	Creating a success	`E.right(42)`
`E.left(error)`	Creating a failure	`E.left('not found')`
`E.tryCatch(fn, onError)`	Wrapping throwing code	`E.tryCatch(() => JSON.parse(s), toError)`
`E.fromNullable(error)`	Converting nullable	`E.fromNullable('missing')(maybeValue)`
`E.map(fn)`	Transform success	`pipe(result, E.map(x => x * 2))`
`E.mapLeft(fn)`	Transform error	`pipe(result, E.mapLeft(addContext))`
`E.chain(fn)`	Chain operations	`pipe(getA(), E.chain(a => getB(a.id)))`
`E.chainW(fn)`	Chain with different error type	`pipe(validate(), E.chainW(save))`
`E.fold(onError, onSuccess)`	Handle both cases	`E.fold(showError, showData)`
`E.getOrElse(onError)`	Extract with default	`E.getOrElse(() => 0)`
`E.filterOrElse(pred, onFalse)`	Validate with error	`E.filterOrElse(x => x > 0, () => 'must be positive')`
`sequenceS(validation)({...})`	Collect all errors	Form validation

fp-errors

Options d'installation

Vérifiez les fichiers source

Practical Error Handling with fp-ts

When to Use

Practical Error Handling with fp-ts

When to Use

1. Stop Throwing Everywhere

The Problem with Exceptions

The Solution: Return Errors as Values

2. The Result Pattern (Either)

Converting Throwing Code to Either

Common Either Operations

3. Chaining Operations That Might Fail

Before: Nested Try/Catch Hell

After: Clean Chain with Either

Different Error Types? Use chainW

4. Collecting Multiple Errors

Before: Collecting Errors Manually

After: Validation with Error Accumulation

Field-Level Errors for Forms

5. Async Operations (TaskEither)

Before: Promise Chain with Error Handling

After: Clean TaskEither Pipeline

Retry Failed Operations

Fallback to Alternative

6. Converting Between Patterns

From Nullable to Either

From Throwing Function to Either

From Promise to TaskEither

Back to Promise (Escape Hatch)

Real Scenarios

Parse User Input Safely

API Call with Full Error Handling

Process List Where Some Items Might Fail

Bulk Operations with Partial Success

Quick Reference

TaskEither Equivalents

Summary

Limitations

name	fp-errors
description	Stop throwing everywhere - handle errors as values using Either and TaskEither for cleaner, more predictable code
risk	unknown
source	community
version	1.0.0
author	kadu
tags	["fp-ts","error-handling","either","task-either","typescript","validation","practical"]

fp-errors

Informations de source

Options d'installation

Vérifiez les fichiers source

Plus depuis ce dépôt

Plus depuis ce dépôt

Practical Error Handling with fp-ts

When to Use

Practical Error Handling with fp-ts

When to Use

1. Stop Throwing Everywhere

The Problem with Exceptions

The Solution: Return Errors as Values

2. The Result Pattern (Either)

Converting Throwing Code to Either

Common Either Operations

3. Chaining Operations That Might Fail

Before: Nested Try/Catch Hell

After: Clean Chain with Either

Different Error Types? Use chainW

4. Collecting Multiple Errors

Before: Collecting Errors Manually

After: Validation with Error Accumulation

Field-Level Errors for Forms

5. Async Operations (TaskEither)

Before: Promise Chain with Error Handling

After: Clean TaskEither Pipeline

Retry Failed Operations

Fallback to Alternative

6. Converting Between Patterns

From Nullable to Either

From Throwing Function to Either

From Promise to TaskEither

Back to Promise (Escape Hatch)

Real Scenarios

Parse User Input Safely

API Call with Full Error Handling

Process List Where Some Items Might Fail

Bulk Operations with Partial Success

Quick Reference

TaskEither Equivalents

Summary

Limitations