Practical Error Handling with fp-ts
This skill teaches you how to handle errors without try/catch spaghetti. No academic jargon - just practical patterns for real problems.
The core idea: Errors are just data. Instead of throwing them into the void and hoping someone catches them, return them as values that TypeScript can track.
1. Stop Throwing Everywhere
The Problem with Exceptions
Exceptions are invisible in your types. They break the contract between functions.
typescript
// 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!
You end up with code like this:
typescript
// 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 }
}
The Solution: Return Errors as Values
typescript
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
2. The Result Pattern (Either)
Either<E, A> is simple: it holds either an error (E) or a value (A).
Left = error caseRight = success case (think "right" as in "correct")
typescript
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}`
)
)
Converting Throwing Code to Either
typescript
// 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)))
)
Common Either Operations
typescript
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
3. Chaining Operations That Might Fail
The real power comes from chaining. Each step can fail, but you write it as a clean pipeline.
Before: Nested Try/Catch Hell
typescript
// 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
}
After: Clean Chain with Either
typescript
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))
)
Different Error Types? Use chainW
typescript
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))
)
4. Collecting Multiple Errors
Sometimes you want ALL errors, not just the first one. Form validation is the classic example.
Before: Collecting Errors Manually
typescript
// 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 }
}
After: Validation with Error Accumulation
typescript
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 })
Field-Level Errors for Forms
typescript
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
5. Async Operations (TaskEither)
For async operations that can fail, use TaskEither. It's like Either but for promises.
TaskEither<E, A> = a function that returns Promise<Either<E, A>>- Lazy: nothing runs until you execute it
typescript
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[]>
Before: Promise Chain with Error Handling
typescript
// 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
}
}
After: Clean TaskEither Pipeline
typescript
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 }))
)
)
)