A practical, jargon-free guide to fp-ts functional programming - the 80/20 approach that gets results without the academic overhead. Use when writing TypeScript with fp-ts library.
Scanned 5/30/2026
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name: fp-ts-pragmatic
description: A practical, jargon-free guide to fp-ts functional programming - the 80/20 approach that gets results without the academic overhead. Use when writing TypeScript with fp-ts library.
risk: safe
source: https://github.com/whatiskadudoing/fp-ts-skills
---
# Pragmatic Functional Programming
**Read this first.** This guide cuts through the academic jargon and shows you what actually matters. No category theory. No abstract nonsense. Just patterns that make your code better.
## When to Use This Skill
- When starting with fp-ts and need practical guidance
- When writing TypeScript code that handles nullable values, errors, or async operations
- When you want cleaner, more maintainable functional code without the academic overhead
- When refactoring imperative code to functional style
## The Golden Rule
> **If functional programming makes your code harder to read, don't use it.**
FP is a tool, not a religion. Use it when it helps. Skip it when it doesn't.
---
## The 80/20 of FP
These five patterns give you most of the benefits. Master these before exploring anything else.
### 1. Pipe: Chain Operations Clearly
Instead of nesting function calls or creating intermediate variables, chain operations in reading order.
```typescript
import { pipe } from 'fp-ts/function'
// Before: Hard to read (inside-out)
const result = format(validate(parse(input)))
// Before: Too many variables
const parsed = parse(input)
const validated = validate(parsed)
const result = format(validated)
// After: Clear, linear flow
const result = pipe(
input,
parse,
validate,
format
)
```
**When to use pipe:**
- 3+ transformations on the same data
- You find yourself naming throwaway variables
- Logic reads better top-to-bottom
**When to skip pipe:**
- Just 1-2 operations (direct call is fine)
- The operations don't naturally chain
### 2. Option: Handle Missing Values Without null Checks
Stop writing `if (x !== null && x !== undefined)` everywhere.
```typescript
import * as O from 'fp-ts/Option'
import { pipe } from 'fp-ts/function'
// Before: Defensive null checking
function getUserCity(user: User | null): string {
if (user === null) return 'Unknown'
if (user.address === null) return 'Unknown'
if (user.address.city === null) return 'Unknown'
return user.address.city
}
// After: Chain through potential missing values
const getUserCity = (user: User | null): string =>
pipe(
O.fromNullable(user),
O.flatMap(u => O.fromNullable(u.address)),
O.flatMap(a => O.fromNullable(a.city)),
O.getOrElse(() => 'Unknown')
)
```
**Plain language translation:**
- `O.fromNullable(x)` = "wrap this value, treating null/undefined as 'nothing'"
- `O.flatMap(fn)` = "if we have something, apply this function"
- `O.getOrElse(() => default)` = "unwrap, or use this default if nothing"
### 3. Either: Make Errors Explicit
Stop throwing exceptions for expected failures. Return errors as values.
```typescript
import * as E from 'fp-ts/Either'
import { pipe } from 'fp-ts/function'
// Before: Hidden failure mode
function parseAge(input: string): number {
const age = parseInt(input, 10)
if (isNaN(age)) throw new Error('Invalid age')
if (age < 0) throw new Error('Age cannot be negative')
return age
}
// After: Errors are visible in the type
function parseAge(input: string): E.Either<string, number> {
const age = parseInt(input, 10)
if (isNaN(age)) return E.left('Invalid age')
if (age < 0) return E.left('Age cannot be negative')
return E.right(age)
}
// Using it
const result = parseAge(userInput)
if (E.isRight(result)) {
console.log(`Age is ${result.right}`)
} else {
console.log(`Error: ${result.left}`)
}
```
**Plain language translation:**
- `E.right(value)` = "success with this value"
- `E.left(error)` = "failure with this error"
- `E.isRight(x)` = "did it succeed?"
### 4. Map: Transform Without Unpacking
Transform values inside containers without extracting them first.
```typescript
import * as O from 'fp-ts/Option'
import * as E from 'fp-ts/Either'
import * as A from 'fp-ts/Array'
import { pipe } from 'fp-ts/function'
// Transform inside Option
const maybeUser: O.Option<User> = O.some({ name: 'Alice', age: 30 })
const maybeName: O.Option<string> = pipe(
maybeUser,
O.map(user => user.name)
)
// Transform inside Either
const result: E.Either<Error, number> = E.right(5)
const doubled: E.Either<Error, number> = pipe(
result,
E.map(n => n * 2)
)
// Transform arrays (same concept!)
const numbers = [1, 2, 3]
const doubled = pipe(
numbers,
A.map(n => n * 2)
)
```
### 5. FlatMap: Chain Operations That Might Fail
When each step might fail, chain them together.
```typescript
import * as E from 'fp-ts/Either'
import { pipe } from 'fp-ts/function'
const parseJSON = (s: string): E.Either<string, unknown> =>
E.tryCatch(() => JSON.parse(s), () => 'Invalid JSON')
const extractEmail = (data: unknown): E.Either<string, string> => {
if (typeof data === 'object' && data !== null && 'email' in data) {
return E.right((data as { email: string }).email)
}
return E.left('No email field')
}
const validateEmail = (email: string): E.Either<string, string> =>
email.includes('@') ? E.right(email) : E.left('Invalid email format')
// Chain all steps - if any fails, the whole thing fails
const getValidEmail = (input: string): E.Either<string, string> =>
pipe(
parseJSON(input),
E.flatMap(extractEmail),
E.flatMap(validateEmail)
)
// Success path: Right('user@example.com')
// Any failure: Left('specific error message')
```
**Plain language:** `flatMap` means "if this succeeded, try the next thing"
---
## When NOT to Use FP
Functional programming is not always the answer. Here's when to keep it simple.
### Simple Null Checks
```typescript
// Just use optional chaining - it's built into the language
const city = user?.address?.city ?? 'Unknown'
// DON'T overcomplicate it
const city = pipe(
O.fromNullable(user),
O.flatMap(u => O.fromNullable(u.address)),
O.flatMap(a => O.fromNullable(a.city)),
O.getOrElse(() => 'Unknown')
)
```
### Simple Loops
```typescript
// A for loop is fine when you need early exit or complex logic
function findFirst(items: Item[], predicate: (i: Item) => boolean): Item | null {
for (const item of items) {
if (predicate(item)) return item
}
return null
}
// DON'T force FP when it doesn't help
const result = pipe(
items,
A.findFirst(predicate),
O.toNullable
)
```
### Performance-Critical Code
```typescript
// For hot paths, imperative is faster (no intermediate arrays)
function sumLarge(numbers: number[]): number {
let sum = 0
for (let i = 0; i < numbers.length; i++) {
sum += numbers[i]
}
return sum
}
// fp-ts creates intermediate structures
const sum = pipe(numbers, A.reduce(0, (acc, n) => acc + n))
```
### When Your Team Doesn't Know FP
If you're the only one who can read the code, it's not good code.
```typescript
// If your team knows this pattern
async function getUser(id: string): Promise<User | null> {
try {
const response = await fetch(`/api/users/${id}`)
if (!response.ok) return null
return await response.json()
} catch {
return null
}
}
// Don't force this on them
const getUser = (id: string): TE.TaskEither<Error, User> =>
pipe(
TE.tryCatch(() => fetch(`/api/users/${id}`), E.toError),
TE.flatMap(r => r.ok ? TE.right(r) : TE.left(new Error('Not found'))),
TE.flatMap(r => TE.tryCatch(() => r.json(), E.toError))
)
```
---
## Quick Wins: Easy Changes That Improve Code Today
### 1. Replace Nested Ternaries with pipe + fold
```typescript
// Before: Nested ternary nightmare
const message = user === null
? 'No user'
: user.isAdmin
? `Admin: ${user.name}`
: `User: ${user.name}`
// After: Clear case handling
const message = pipe(
O.fromNullable(user),
O.fold(
() => 'No user',
(u) => u.isAdmin ? `Admin: ${u.name}` : `User: ${u.name}`
)
)
```
### 2. Replace try-catch with tryCatch
```typescript
// Before: try-catch everywhere
let config
try {
config = JSON.parse(rawConfig)
} catch {
config = defaultConfig
}
// After: One-liner
const config = pipe(
E.tryCatch(() => JSON.parse(rawConfig), () => 'parse error'),
E.getOrElse(() => defaultConfig)
)
```
### 3. Replace undefined Returns with Option
```typescript
// Before: Caller might forget to check
function findUser(id: string): User | undefined {
return users.find(u => u.id === id)
}
// After: Type forces caller to handle missing case
function findUser(id: string): O.Option<User> {
return O.fromNullable(users.find(u => u.id === id))
}
```
### 4. Replace Error Strings with Typed Errors
```typescript
// Before: Just strings
function validate(data: unknown): E.Either<string, User> {
// ...
return E.left('validation failed')
}
// After: Structured errors
type ValidationError = {
field: string
message: string
}
function validate(data: unknown): E.Either<ValidationError, User> {
// ...
return E.left({ field: 'email', message: 'Invalid format' })
}
```
### 5. Use const Assertions for Error Types
```typescript
// Create specific error types without classes
const NotFound = (id: string) => ({ _tag: 'NotFound' as const, id })
const Unauthorized = { _tag: 'Unauthorized' as const }
const ValidationFailed = (errors: string[]) =>
({ _tag: 'ValidationFailed' as const, errors })
type AppError =
| ReturnType<typeof NotFound>
| typeof Unauthorized
| ReturnType<typeof ValidationFailed>
// Now you can pattern match
const handleError = (error: AppError): string => {
switch (error._tag) {
case 'NotFound': return `Item ${error.id} not found`
case 'Unauthorized': return 'Please log in'
case 'ValidationFailed': return error.errors.join(', ')
}
}
```
---
## Common Refactors: Before and After
### Callback Hell to Pipe
```typescript
// Before
fetchUser(id, (user) => {
if (!user) return handleNoUser()
fetchPosts(user.id, (posts) => {
if (!posts) return handleNoPosts()
fetchComments(posts[0].id, (comments) => {
render(user, posts, comments)
})
})
})
// After (with TaskEither for async)
import * as TE from 'fp-ts/TaskEither'
const loadData = (id: string) =>
pipe(
fetchUser(id),
TE.flatMap(user => pipe(
fetchPosts(user.id),
TE.map(posts => ({ user, posts }))
)),
TE.flatMap(({ user, posts }) => pipe(
fetchComments(posts[0].id),
TE.map(comments => ({ user, posts, comments }))
))
)
// Execute
const result = await loadData('123')()
pipe(
result,
E.fold(handleError, ({ user, posts, comments }) => render(user, posts, comments))
)
```
### Multiple null Checks to Option Chain
```typescript
// Before
function getManagerEmail(employee: Employee): string | null {
if (!employee.department) return null
if (!employee.department.manager) return null
if (!employee.department.manager.email) return null
return employee.department.manager.email
}
// After
const getManagerEmail = (employee: Employee): O.Option<string> =>
pipe(
O.fromNullable(employee.department),
O.flatMap(d => O.fromNullable(d.manager)),
O.flatMap(m => O.fromNullable(m.email))
)
// Use it
pipe(
getManagerEmail(employee),
O.fold(
() => sendToDefault(),
(email) => sendTo(email)
)
)
```
### Validation with Multiple Checks
```typescript
// Before: Throws on first error
function validateUser(data: unknown): User {
if (!data || typeof data !== 'object') throw new Error('Must be object')
const obj = data as Record<string, unknown>
if (typeof obj.email !== 'string') throw new Error('Email required')
if (!obj.email.includes('@')) throw new Error('Invalid email')
if (typeof obj.age !== 'number') throw new Error('Age required')
if (obj.age < 0) throw new Error('Age must be positive')
return obj as User
}
// After: Returns first error, type-safe
const validateUser = (data: unknown): E.Either<string, User> =>
pipe(
E.Do,
E.bind('obj', () =>
typeof data === 'object' && data !== null
? E.right(data as Record<string, unknown>)
: E.left('Must be object')
),
E.bind('email', ({ obj }) =>
typeof obj.email === 'string' && obj.email.includes('@')
? E.right(obj.email)
: E.left('Valid email required')
),
E.bind('age', ({ obj }) =>
typeof obj.age === 'number' && obj.age >= 0
? E.right(obj.age)
: E.left('Valid age required')
),
E.map(({ email, age }) => ({ email, age }))
)
```
### Promise Chain to TaskEither
```typescript
// Before
async function processOrder(orderId: string): Promise<Receipt> {
const order = await fetchOrder(orderId)
if (!order) throw new Error('Order not found')
const validated = await validateOrder(order)
if (!validated.success) throw new Error(validated.error)
const payment = await processPayment(validated.order)
if (!payment.success) throw new Error('Payment failed')
return generateReceipt(payment)
}
// After
const processOrder = (orderId: string): TE.TaskEither<string, Receipt> =>
pipe(
fetchOrderTE(orderId),
TE.flatMap(order =>
order ? TE.right(order) : TE.left('Order not found')
),
TE.flatMap(validateOrderTE),
TE.flatMap(processPaymentTE),
TE.map(generateReceipt)
)
```
---
## The Readability Rule
Before using any FP pattern, ask: **"Would a junior developer understand this?"**
### Too Clever (Avoid)
```typescript
const result = pipe(
data,
A.filter(flow(prop('status'), equals('active'))),
A.map(flow(prop('value'), multiply(2))),
A.reduce(monoid.concat, monoid.empty),
O.fromPredicate(gt(threshold))
)
```
### Just Right (Prefer)
```typescript
const activeItems = data.filter(item => item.status === 'active')
const doubledValues = activeItems.map(item => item.value * 2)
const total = doubledValues.reduce((sum, val) => sum + val, 0)
const result = total > threshold ? O.some(total) : O.none
```
### The Middle Ground (Often Best)
```typescript
const result = pipe(
data,
A.filter(item => item.status === 'active'),
A.map(item => item.value * 2),
A.reduce(0, (sum, val) => sum + val),
total => total > threshold ? O.some(total) : O.none
)
```
---
## Cheat Sheet
| What you want | Plain language | fp-ts |
|--------------|----------------|-------|
| Handle null/undefined | "Wrap this nullable" | `O.fromNullable(x)` |
| Default for missing | "Use this if nothing" | `O.getOrElse(() => default)` |
| Transform if present | "If something, change it" | `O.map(fn)` |
| Chain nullable operations | "If something, try this" | `O.flatMap(fn)` |
| Return success | "Worked, here's the value" | `E.right(value)` |
| Return failure | "Failed, here's why" | `E.left(error)` |
| Wrap throwing function | "Try this, catch errors" | `E.tryCatch(fn, onError)` |
| Handle both cases | "Do this for error, that for success" | `E.fold(onLeft, onRight)` |
| Chain operations | "Then do this, then that" | `pipe(x, fn1, fn2, fn3)` |
---
## When to Level Up
Once comfortable with these patterns, explore:
1. **TaskEither** - Async operations that can fail (replaces Promise + try/catch)
2. **Validation** - Collect ALL errors instead of stopping at first
3. **Reader** - Dependency injection without classes
4. **Do notation** - Cleaner syntax for multiple bindings
But don't rush. The basics here will handle 80% of real-world scenarios. Get comfortable with these before adding more tools to your belt.
---
## Summary
1. **Use pipe** for 3+ operations
2. **Use Option** for nullable chains
3. **Use Either** for operations that can fail
4. **Use map** to transform wrapped values
5. **Use flatMap** to chain operations that might fail
6. **Skip FP** when it hurts readability
7. **Keep it simple** - if your team can't read it, it's not good code
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