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Functional Programming Fundamentals

This skill covers the foundational concepts of functional programming. Master these before diving into fp-ts types like Option, Either, and Task. These concepts apply universally across functional programming languages and libraries.

Why Functional Programming?

Functional programming provides:

• Predictability : Pure functions always produce the same output for the same input
• Testability : No mocking of global state or dependencies
• Composability : Small functions combine into complex behavior
• Maintainability : Isolated pieces are easier to understand and change
• Parallelization : Pure functions can safely run concurrently

---

1. Pure Functions and Referential Transparency

What is a Pure Function?

A pure function has two properties:

1. Same input, same output : Given the same arguments, it always returns the same result
2. No side effects : It doesn't modify external state, perform I/O, or cause observable changes

Why It Matters

Pure functions are:

• Cacheable : Results can be memoized since they're deterministic
• Portable : No hidden dependencies on external state
• Testable : No setup/teardown of global state needed
• Parallelizable : Safe to run concurrently without race conditions
• Reasoned about locally : You can understand them without knowing the entire system

Examples

// PURE: Same input always gives same output
const add = (x: number, y: number): number => x + y

const greet = (name: string): string => `Hello, ${name}!`

const head = <T>(arr: readonly T[]): T | undefined => arr[0]

// IMPURE: Depends on external state
let counter = 0
const incrementCounter = (): number => {
  counter += 1  // Mutates external state
  return counter
}

// IMPURE: Different output for same input
const randomNumber = (): number => Math.random()

// IMPURE: Performs I/O
const logMessage = (msg: string): void => {
  console.log(msg)  // Side effect: writes to console
}

// IMPURE: Depends on mutable external state
const config = { apiUrl: 'https://api.example.com' }
const getApiUrl = (): string => config.apiUrl  // Could return different values

Referential Transparency

An expression is referentially transparent if it can be replaced with its value without changing program behavior.

// Referentially transparent
const double = (x: number): number => x * 2
const result = double(5) + double(5)
// Can be replaced with: 10 + 10 = 20

// NOT referentially transparent
let count = 0
const incrementAndGet = (): number => {
  count += 1
  return count
}
const result2 = incrementAndGet() + incrementAndGet()
// Cannot replace with values: first call returns 1, second returns 2
// Result is 3, not 1 + 1 = 2

Making Impure Functions Pure

// IMPURE: Modifies input
const addItem = (arr: string[], item: string): void => {
  arr.push(item)  // Mutates the original array
}

// PURE: Returns new array
const addItem = (arr: readonly string[], item: string): readonly string[] =>
  [...arr, item]

// IMPURE: Depends on Date
const getAge = (birthYear: number): number =>
  new Date().getFullYear() - birthYear

// PURE: Accept date as parameter (dependency injection)
const getAge = (birthYear: number, currentYear: number): number =>
  currentYear - birthYear

// IMPURE: Reads from database
const getUser = async (id: string): Promise<User> =>
  await database.find(id)

// Functional approach: Return a description of the effect
// (This is the concept behind Task and IO in fp-ts)
const getUser = (id: string): Task<User> =>
  () => database.find(id)

Common Mistakes

// MISTAKE 1: Mutating arguments
const sortArray = (arr: number[]): number[] => {
  return arr.sort((a, b) => a - b)  // sort() mutates the original!
}

// FIX: Create a copy first
const sortArray = (arr: readonly number[]): number[] =>
  [...arr].sort((a, b) => a - b)

// MISTAKE 2: Using Date, Math.random, or other non-deterministic APIs
const generateId = (): string =>
  `id-${Date.now()}-${Math.random()}`

// FIX: Accept dependencies as parameters
const generateId = (timestamp: number, random: number): string =>
  `id-${timestamp}-${random}`

// MISTAKE 3: Hidden dependencies via closure
const multiplier = 2
const double = (x: number): number => x * multiplier  // Captures external variable

// FIX: Make dependencies explicit
const multiply = (factor: number) => (x: number): number => x * factor
const double = multiply(2)

---

2. Currying and Partial Application

What is Currying?

Currying transforms a function that takes multiple arguments into a sequence of functions that each take a single argument.

// Uncurried: takes all arguments at once
const add = (x: number, y: number, z: number): number => x + y + z
add(1, 2, 3)  // 6

// Curried: takes one argument at a time
const addCurried = (x: number) => (y: number) => (z: number): number => x + y + z
addCurried(1)(2)(3)  // 6

Why It Matters

Currying enables:

• Partial application : Create specialized functions by supplying some arguments
• Composition : Single-argument functions compose cleanly
• Reusability : Create families of related functions from one definition
• Deferred computation : Supply arguments as they become available

Currying in Practice

// Curried function definition
const multiply = (x: number) => (y: number): number => x * y

// Create specialized functions through partial application
const double = multiply(2)
const triple = multiply(3)

double(5)  // 10
triple(5)  // 15

// Curried comparison helpers
const greaterThan = (threshold: number) => (value: number): boolean =>
  value > threshold

const isAdult = greaterThan(17)
const isExpensive = greaterThan(100)

isAdult(25)      // true
isExpensive(50)  // false

Currying for Array Methods

// Curried filter predicate
const filterBy = <T>(predicate: (item: T) => boolean) =>
  (items: readonly T[]): T[] => items.filter(predicate)

// Create reusable filters
const keepPositive = filterBy((n: number) => n > 0)
const keepEven = filterBy((n: number) => n % 2 === 0)

keepPositive([-1, 2, -3, 4])  // [2, 4]
keepEven([1, 2, 3, 4, 5, 6])  // [2, 4, 6]

// Curried map transformer
const mapWith = <A, B>(fn: (a: A) => B) =>
  (items: readonly A[]): B[] => items.map(fn)

const doubleAll = mapWith((n: number) => n * 2)
const stringify = mapWith(String)

doubleAll([1, 2, 3])  // [2, 4, 6]
stringify([1, 2, 3])  // ['1', '2', '3']

Partial Application vs Currying

// Partial application: fixing some arguments of a function
const greet = (greeting: string, name: string): string =>
  `${greeting}, ${name}!`

// Using bind for partial application (uncurried)
const sayHello = greet.bind(null, 'Hello')
sayHello('Alice')  // "Hello, Alice!"

// Curried version is naturally partially applicable
const greetCurried = (greeting: string) => (name: string): string =>
  `${greeting}, ${name}!`

const sayHello = greetCurried('Hello')
const sayGoodbye = greetCurried('Goodbye')

sayHello('Alice')    // "Hello, Alice!"
sayGoodbye('Bob')    // "Goodbye, Bob!"

Argument Order Matters

// BAD: Data first prevents easy partial application
const map = <A, B>(arr: A[], fn: (a: A) => B): B[] => arr.map(fn)

// Must use anonymous function to specialize
const doubleAll = (arr: number[]) => map(arr, n => n * 2)

// GOOD: Data last enables clean partial application
const map = <A, B>(fn: (a: A) => B) => (arr: readonly A[]): B[] => arr.map(fn)

// Specialize by supplying the function
const doubleAll = map((n: number) => n * 2)
const stringify = map(String)

// This is why fp-ts uses data-last curried functions
import * as A from 'fp-ts/Array'
import { pipe } from 'fp-ts/function'

pipe(
  [1, 2, 3],
  A.map(n => n * 2),      // A.map takes fn first, returns function expecting array
  A.filter(n => n > 2)     // Same pattern
)

Common Mistakes

// MISTAKE 1: Inconsistent argument order
const process = (config: Config) => (data: Data) => (options: Options) => ...
const handle = (data: Data) => (config: Config) => ...  // Inconsistent!

// FIX: Establish convention - typically: config -> options -> data
const process = (config: Config) => (options: Options) => (data: Data) => ...
const handle = (config: Config) => (data: Data) => ...

// MISTAKE 2: Over-currying simple functions
const add = (a: number) => (b: number) => (c: number) => (d: number) =>
  a + b + c + d

add(1)(2)(3)(4)  // Tedious to call

// FIX: Curry only to the level needed for your use case
const add = (a: number, b: number, c: number, d: number) => a + b + c + d
// Or group related parameters
const addPairs = (a: number, b: number) => (c: number, d: number) =>
  a + b + c + d

// MISTAKE 3: Forgetting to curry when using with pipe
const formatPrice = (currency: string, amount: number): string =>
  `${currency}${amount.toFixed(2)}`

pipe(
  100,
  formatPrice('$', ???)  // Can't use in pipe - needs both args!
)

// FIX: Curry with data last
const formatPrice = (currency: string) => (amount: number): string =>
  `${currency}${amount.toFixed(2)}`

pipe(
  100,
  formatPrice('$')  // Works!
)  // "$100.00"

---

3. Function Composition

What is Function Composition?

Function composition combines two or more functions to create a new function. The output of one function becomes the input of the next.

// Mathematical composition: (f . g)(x) = f(g(x))
const compose = <A, B, C>(f: (b: B) => C, g: (a: A) => B) =>
  (x: A): C => f(g(x))

const addOne = (x: number): number => x + 1
const double = (x: number): number => x * 2

const addOneThenDouble = compose(double, addOne)
addOneThenDouble(5)  // double(addOne(5)) = double(6) = 12

Why It Matters

Composition enables:

• Building complex behavior from simple pieces : Small, focused functions combine into powerful transformations
• Reusability : Composed functions can themselves be composed further
• Readability : Each step has a clear, single purpose
• Testing : Test small pieces in isolation, composition is mathematically guaranteed

Pipe vs Flow (Left-to-Right Composition)

Traditional mathematical composition reads right-to-left, which can be confusing. pipe and flow provide left-to-right composition.

import { pipe, flow } from 'fp-ts/function'

const addOne = (x: number): number => x + 1
const double = (x: number): number => x * 2
const toString = (x: number): string => `Value: ${x}`

// pipe: Execute immediately with a value
const result = pipe(
  5,
  addOne,     // 6
  double,     // 12
  toString    // "Value: 12"
)

// flow: Create a reusable function
const transform = flow(
  addOne,
  double,
  toString
)

transform(5)   // "Value: 12"
transform(10)  // "Value: 22"

Composition with Multiple Types

// Functions don't need matching types, just compatible connections
const parseNumber = (s: string): number => parseInt(s, 10)
const isEven = (n: number): boolean => n % 2 === 0
const toYesNo = (b: boolean): string => b ? 'Yes' : 'No'

const isInputEven = flow(
  parseNumber,  // string -> number
  isEven,       // number -> boolean
  toYesNo       // boolean -> string
)

isInputEven('4')  // "Yes"
isInputEven('7')  // "No"

Building Pipelines

import { pipe, flow } from 'fp-ts/function'

interface User {
  name: string
  email: string
  age: number
}

// Small, focused functions
const getName = (user: User): string => user.name
const toUpperCase = (s: string): string => s.toUpperCase()
const addGreeting = (name: string): string => `Hello, ${name}!`
const addExcitement = (s: string): string => `${s}!!!`

// Compose into a pipeline
const greetUser = flow(
  getName,
  toUpperCase,
  addGreeting,
  addExcitement
)

greetUser({ name: 'alice', email: 'alice@example.com', age: 30 })
// "Hello, ALICE!!!!"

// Or use pipe for one-off transformations
const greeting = pipe(
  { name: 'bob', email: 'bob@example.com', age: 25 },
  getName,
  toUpperCase,
  addGreeting
)
// "Hello, BOB!"

Associativity of Composition

Composition is associative: (f . g) . h = f . (g . h)

const f = (x: number): number => x + 1
const g = (x: number): number => x * 2
const h = (x: number): number => x - 3

// These are equivalent:
const way1 = flow(flow(h, g), f)  // (f . g) . h
const way2 = flow(h, flow(g, f))  // f . (g . h)
const way3 = flow(h, g, f)        // Direct composition

way1(10)  // 15
way2(10)  // 15
way3(10)  // 15

Common Mistakes

// MISTAKE 1: Composing functions with incompatible types
const toString = (n: number): string => String(n)
const double = (n: number): number => n * 2

const bad = flow(toString, double)  // Type error! double expects number, gets string

// MISTAKE 2: Trying to compose multi-argument functions
const add = (a: number, b: number): number => a + b
const double = (n: number): number => n * 2

const bad = flow(add, double)  // Won't work - add takes 2 args

// FIX: Curry first
const add = (a: number) => (b: number): number => a + b
const addFiveThenDouble = flow(add(5), double)
addFiveThenDouble(3)  // 16

// MISTAKE 3: Long unreadable pipelines
const result = pipe(
  data,
  fn1, fn2, fn3, fn4, fn5, fn6, fn7, fn8, fn9, fn10  // What does this do?
)

// FIX: Group related transformations and name them
const parseInput = flow(fn1, fn2, fn3)
const validateData = flow(fn4, fn5)
const formatOutput = flow(fn6, fn7, fn8, fn9, fn10)

const process = flow(parseInput, validateData, formatOutput)

---

4. Pointfree Style

What is Pointfree Style?

Pointfree (or tacit) programming defines functions without explicitly mentioning their arguments. The "points" are the arguments.

// Pointed: explicitly names the argument
const double = (x: number): number => x * 2

// Pointfree: no explicit argument
const double = multiply(2)  // Assuming multiply is curried

Why It Matters

Pointfree style:

• Reduces noise : Focuses on transformations, not data shuffling
• Encourages composition : Works naturally with flow/pipe
• Reveals patterns : Makes the structure of computation visible
• Prevents mistakes : No variable names to misspell or shadow

Pointfree in Practice

import { pipe, flow } from 'fp-ts/function'
import * as A from 'fp-ts/Array'

// Pointed style
const getActiveUserNames = (users: User[]): string[] =>
  users
    .filter(user => user.isActive)
    .map(user => user.name)

// Pointfree style
const isActive = (user: User): boolean => user.isActive
const getName = (user: User): string => user.name

const getActiveUserNames = flow(
  A.filter(isActive),
  A.map(getName)
)

// Both work the same:
getActiveUserNames(users)

Building Pointfree Functions

// Utility functions enable pointfree style
const prop = <T, K extends keyof T>(key: K) => (obj: T): T[K] => obj[key]
const equals = <T>(target: T) => (value: T): boolean => value === target
const gt = (threshold: number) => (value: number): boolean => value > threshold

interface Product {
  name: string
  price: number
  category: string
}

// Pointed
const getExpensiveElectronics = (products: Product[]): Product[] =>
  products.filter(p => p.category === 'electronics' && p.price > 100)

// Pointfree with helpers
const isElectronics = flow(prop<Product, 'category'>('category'), equals('electronics'))
const isExpensive = flow(prop<Product, 'price'>('price'), gt(100))
const both = <T>(f: (t: T) => boolean, g: (t: T) => boolean) =>
  (t: T): boolean => f(t) && g(t)

const getExpensiveElectronics = A.filter(both(isElectronics, isExpensive))

When Pointfree Helps

// GOOD: Simple transformations
const doubleAll = A.map((n: number) => n * 2)  // Semi-pointfree
const keepPositive = A.filter((n: number) => n > 0)

// BETTER: With named predicates
const double = (n: number): number => n * 2
const isPositive = (n: number): boolean => n > 0

const doubleAll = A.map(double)      // Fully pointfree
const keepPositive = A.filter(isPositive)

// Compose them
const processNumbers = flow(
  keepPositive,
  doubleAll
)

When to Avoid Pointfree

// AVOID: When it obscures meaning
const mystery = flow(
  A.map(flow(prop('x'), add(1))),
  A.filter(flow(prop('y'), gt(5))),
  A.reduce(0, flow(([acc, item]) => acc + item.z))
)

// PREFER: Clear variable names when logic is complex
const incrementX = (item: Item): Item => ({ ...item, x: item.x + 1 })
const hasLargeY = (item: Item): boolean => item.y > 5
const sumZ = (items: Item[]): number =>
  items.reduce((acc, item) => acc + item.z, 0)

const process = flow(
  A.map(incrementX),
  A.filter(hasLargeY),
  sumZ
)

// AVOID: Pointfree gymnastics
const weird = flip(compose(flip(map), filter))(isEven)(double)

// PREFER: Readable code
const doubleEvens = flow(
  A.filter(isEven),
  A.map(double)
)

Common Mistakes

// MISTAKE 1: Forcing pointfree when it's unclear
const process = flow(
  A.map(flow(
    juxt([prop('a'), prop('b')]),
    apply(add)
  ))
)

// FIX: Use pointed style when clearer
const process = A.map(({ a, b }) => a + b)

// MISTAKE 2: Missing type annotations in pointfree
const double = A.map(n => n * 2)  // n is 'unknown' without context

// FIX: Add type annotation
const double = A.map((n: number) => n * 2)

// MISTAKE 3: Mixing pointed and pointfree awkwardly
const process = (arr: number[]) => flow(
  A.filter(n => n > 0),
  A.map(n => n * 2)
)(arr)

// FIX: Choose one style
// Pointfree:
const process = flow(
  A.filter((n: number) => n > 0),
  A.map(n => n * 2)
)

// Or pointed:
const process = (arr: number[]): number[] =>
  pipe(arr, A.filter(n => n > 0), A.map(n => n * 2))

---

5. First-Class Functions

What are First-Class Functions?

In JavaScript/TypeScript, functions are first-class citizens. They can be:

• Assigned to variables
• Passed as arguments
• Returned from other functions
• Stored in data structures

Why It Matters

First-class functions enable:

• Higher-order functions : Functions that take or return functions
• Callbacks : Passing behavior as data
• Closures : Functions that capture their environment
• Functional composition : Building programs from function combinations

Avoiding Wrapper Functions

A common anti-pattern is wrapping functions unnecessarily.

// BAD: Unnecessary wrapper function
const numbers = [1, 2, 3, 4, 5]

// Don't do this:
numbers.map(x => double(x))
numbers.filter(x => isPositive(x))
numbers.forEach(x => console.log(x))

// GOOD: Pass the function directly
numbers.map(double)
numbers.filter(isPositive)
numbers.forEach(console.log)

// The wrapper adds nothing but noise

When Wrappers ARE Needed

// NEEDED: When you need to modify arguments
const users = [{ name: 'Alice', age: 30 }, { name: 'Bob', age: 25 }]

// getName expects a User, but we want just the index
users.map((user, index) => `${index}: ${user.name}`)  // Wrapper needed

// NEEDED: When you need to ignore arguments
const fetchAll = (urls: string[]) =>
  urls.map(url => fetch(url))  // fetch has many optional params

// If we did urls.map(fetch), TypeScript might complain about
// the (value, index, array) signature of map's callback

// NEEDED: When changing arity
const add = (a: number, b: number): number => a + b
[1, 2, 3].map(n => add(n, 10))  // Must wrap since map passes 3 args

// BETTER: Curry the function
const add = (b: number) => (a: number): number => a + b
[1, 2, 3].map(add(10))  // [11, 12, 13]

Higher-Order Functions

// Function that returns a function
const multiply = (x: number) => (y: number): number => x * y
const double = multiply(2)
const triple = multiply(3)

// Function that takes a function
const applyTwice = <T>(fn: (t: T) => T) => (value: T): T =>
  fn(fn(value))

const addOne = (n: number): number => n + 1
const addTwo = applyTwice(addOne)
addTwo(5)  // 7

// Function that does both
const compose = <A, B, C>(f: (b: B) => C) => (g: (a: A) => B) => (x: A): C =>
  f(g(x))

const increment = (n: number): number => n + 1
const toString = (n: number): string => String(n)

const incrementThenStringify = compose(toString)(increment)
incrementThenStringify(5)  // "6"

Method References

// Be careful with method references and 'this'

// PROBLEM: Losing 'this' context
const obj = {
  value: 42,
  getValue() {
    return this.value
  }
}

const getValue = obj.getValue
getValue()  // undefined or error - 'this' is lost

// SOLUTION 1: Bind the method
const getValue = obj.getValue.bind(obj)
getValue()  // 42

// SOLUTION 2: Arrow function wrapper
const getValue = () => obj.getValue()
getValue()  // 42

// SOLUTION 3: Use static functions (FP approach)
const getValue = (obj: { value: number }): number => obj.value
getValue({ value: 42 })  // 42

Common Mistakes

// MISTAKE 1: Unnecessary wrappers
const names = users.map(user => getName(user))
// FIX:
const names = users.map(getName)

// MISTAKE 2: Wrapping to match types incorrectly
const numbers = ['1', '2', '3'].map(s => parseInt(s))  // Seems fine...
// Actually problematic: parseInt takes (string, radix)
// map passes (value, index, array)
// parseInt('2', 1) is NaN!

// FIX: Explicit wrapper when semantics differ
const numbers = ['1', '2', '3'].map(s => parseInt(s, 10))
// Or use Number:
const numbers = ['1', '2', '3'].map(Number)

// MISTAKE 3: Not leveraging first-class functions
const process = (items: Item[], transform: (item: Item) => Item) => {
  const result = []
  for (const item of items) {
    result.push(transform(item))
  }
  return result
}
// FIX: Just use map
const process = (items: Item[], transform: (item: Item) => Item) =>
  items.map(transform)

// Or even simpler - don't wrap map at all:
items.map(transform)

// MISTAKE 4: Immediately invoking returned functions
const createAdder = (x: number) => (y: number) => x + y

// Wrong - this calls the inner function immediately with undefined
const add5 = createAdder(5)()  // NaN

// Right - store the function for later use
const add5 = createAdder(5)
add5(3)  // 8

---

Practical Exercises

Exercise 1: Pure Functions

Identify what makes these functions impure and rewrite them as pure functions.

// 1a. Makes HTTP call
const fetchUser = async (id: string) => {
  const response = await fetch(`/api/users/${id}`)
  return response.json()
}

// 1b. Uses current time
const isExpired = (expiryDate: Date) => {
  return expiryDate < new Date()
}

// 1c. Mutates input
const addToCart = (cart: CartItem[], item: CartItem) => {
  cart.push(item)
  return cart
}

// 1d. Depends on global config
const config = { taxRate: 0.08 }
const calculateTotal = (subtotal: number) => {
  return subtotal * (1 + config.taxRate)
}



// 1a. Return a description of the effect (Task pattern)
type FetchUser = (id: string) => () => Promise<User>
const fetchUser: FetchUser = (id) => () =>
  fetch(`/api/users/${id}`).then(r => r.json())

// 1b. Accept current time as parameter
const isExpired = (expiryDate: Date, now: Date): boolean =>
  expiryDate < now

// 1c. Return new array instead of mutating
const addToCart = (cart: readonly CartItem[], item: CartItem): CartItem[] =>
  [...cart, item]

// 1d. Accept config as parameter
const calculateTotal = (taxRate: number) => (subtotal: number): number =>
  subtotal * (1 + taxRate)

// Or group them
interface TaxConfig { taxRate: number }
const calculateTotal = (config: TaxConfig, subtotal: number): number =>
  subtotal * (1 + config.taxRate)

Exercise 2: Currying and Partial Application

Convert these functions to curried form and create specialized versions.

// 2a. Convert to curried form
const formatDate = (locale: string, options: Intl.DateTimeFormatOptions, date: Date): string =>
  date.toLocaleDateString(locale, options)

// Create: formatUSDate, formatShortDate

// 2b. Convert to curried form
const clamp = (min: number, max: number, value: number): number =>
  Math.max(min, Math.min(max, value))

// Create: clampPercentage (0-100), clampByte (0-255)

// 2c. Convert to curried form with good argument order
const hasProperty = (obj: object, key: string): boolean =>
  key in obj

// Should work with: users.filter(hasProperty(???))



// 2a.
const formatDate = (locale: string) =>
  (options: Intl.DateTimeFormatOptions) =>
    (date: Date): string =>
      date.toLocaleDateString(locale, options)

const formatUSDate = formatDate('en-US')
const formatShortDate = formatUSDate({ month: 'short', day: 'numeric' })

formatShortDate(new Date())  // "Jan 29"

// 2b.
const clamp = (min: number) => (max: number) => (value: number): number =>
  Math.max(min, Math.min(max, value))

const clampPercentage = clamp(0)(100)
const clampByte = clamp(0)(255)

clampPercentage(150)  // 100
clampByte(-10)        // 0

// 2c. Reorder arguments: key first (config), object last (data)
const hasProperty = (key: string) => (obj: object): boolean =>
  key in obj

const hasEmail = hasProperty('email')
const hasId = hasProperty('id')

users.filter(hasEmail)  // Users with email property

Exercise 3: Function Composition

Build these pipelines using flow and pipe.

import { pipe, flow } from 'fp-ts/function'
import * as A from 'fp-ts/Array'

interface Product {
  name: string
  price: number
  category: string
  inStock: boolean
}

// 3a. Create a pipeline that:
// - Filters to products in stock
// - Filters to products under $50
// - Maps to product names
// - Sorts alphabetically

// 3b. Create a reusable pipeline that:
// - Takes a string
// - Trims whitespace
// - Converts to lowercase
// - Replaces spaces with hyphens
// - Prefixes with "slug-"

// 3c. Create a number processing pipeline that:
// - Filters out negative numbers
// - Doubles each number
// - Sums all numbers
// - Returns "Total: X"



// 3a.
const getAffordableInStockNames = (products: Product[]): string[] =>
  pipe(
    products,
    A.filter(p => p.inStock),
    A.filter(p => p.price < 50),
    A.map(p => p.name),
    A.sort((a, b) => a.localeCompare(b))
  )

// Or as a reusable function:
const getAffordableInStockNames = flow(
  A.filter((p: Product) => p.inStock),
  A.filter(p => p.price < 50),
  A.map(p => p.name),
  A.sort<string>((a, b) => a.localeCompare(b))
)

// 3b.
const slugify = flow(
  (s: string) => s.trim(),
  s => s.toLowerCase(),
  s => s.replace(/\s+/g, '-'),
  s => `slug-${s}`
)

slugify('  Hello World  ')  // "slug-hello-world"

// 3c.
const isPositive = (n: number): boolean => n >= 0
const double = (n: number): number => n * 2
const sum = (numbers: number[]): number =>
  numbers.reduce((acc, n) => acc + n, 0)
const formatTotal = (n: number): string => `Total: ${n}`

const processNumbers = flow(
  A.filter(isPositive),
  A.map(double),
  sum,
  formatTotal
)

processNumbers([-1, 2, 3, -4, 5])  // "Total: 20"

Exercise 4: Pointfree Style

Refactor these functions to pointfree style where appropriate.

// 4a. Refactor to pointfree
const getAdultNames = (users: User[]): string[] =>
  users
    .filter(user => user.age >= 18)
    .map(user => user.name)

// 4b. Refactor to pointfree
const sumPrices = (products: Product[]): number =>
  products.reduce((total, product) => total + product.price, 0)

// 4c. Decide: Should this be pointfree or not? Why?
const formatUserForDisplay = (user: User): string =>
  `${user.name} (${user.email}) - ${user.isActive ? 'Active' : 'Inactive'}`



// 4a. Good candidate for pointfree
const isAdult = (user: User): boolean => user.age >= 18
const getName = (user: User): string => user.name

const getAdultNames = flow(
  A.filter(isAdult),
  A.map(getName)
)

// 4b. Partially pointfree - the reducer is clearer pointed
const prop = <T, K extends keyof T>(key: K) => (obj: T): T[K] => obj[key]
const sum = (numbers: number[]): number =>
  numbers.reduce((a, b) => a + b, 0)

const sumPrices = flow(
  A.map(prop<Product, 'price'>('price')),
  sum
)

// Or keep it simple:
const getPrice = (p: Product): number => p.price
const sumPrices = flow(A.map(getPrice), sum)

// 4c. Keep pointed - the string template is clearer with explicit access
const formatUserForDisplay = (user: User): string =>
  `${user.name} (${user.email}) - ${user.isActive ? 'Active' : 'Inactive'}`

// Pointfree version would be overly complex:
const formatUserForDisplay = flow(
  user => [user.name, user.email, user.isActive] as const,
  ([name, email, isActive]) =>
    `${name} (${email}) - ${isActive ? 'Active' : 'Inactive'}`
)
// This is worse - no benefit, harder to read

Exercise 5: First-Class Functions

Fix these unnecessary wrappers and improve the code.

// 5a. Remove unnecessary wrappers
const results = items
  .map(item => processItem(item))
  .filter(result => isValid(result))
  .forEach(result => logResult(result))

// 5b. Fix the parseInt issue
const numbers = ['1', '2', '3', '10', '11'].map(parseInt)
// Current result: [1, NaN, NaN, 3, 4] - Why? Fix it.

// 5c. Create a safe version of this
const handlers = {
  onClick: (e: Event) => handleClick(e),
  onSubmit: (e: Event) => handleSubmit(e),
  onHover: (e: Event) => handleHover(e),
}
// Too repetitive - can we do better?



// 5a. Pass functions directly
const results = items
  .map(processItem)
  .filter(isValid)
  .forEach(logResult)

// 5b. parseInt receives (value, index, array) from map
// parseInt('2', 1) interprets '2' as base-1 (invalid) = NaN
// parseInt('3', 2) interprets '3' as base-2 (invalid) = NaN
// parseInt('10', 3) interprets '10' as base-3 = 3
// parseInt('11', 4) interprets '11' as base-4 = 5

// Fix: Use wrapper with explicit radix
const numbers = ['1', '2', '3', '10', '11'].map(s => parseInt(s, 10))
// Or use Number (doesn't have radix parameter)
const numbers = ['1', '2', '3', '10', '11'].map(Number)
// Result: [1, 2, 3, 10, 11]

// 5c. If handlers match the expected signature, just use them directly
const handlers = {
  onClick: handleClick,
  onSubmit: handleSubmit,
  onHover: handleHover,
}

// Or if you need to create them dynamically:
const createHandlers = <T extends Record<string, (e: Event) => void>>(
  handlerMap: T
): T => handlerMap

const handlers = createHandlers({
  onClick: handleClick,
  onSubmit: handleSubmit,
  onHover: handleHover,
})

---

Summary

Next Steps

With these fundamentals mastered, you're ready for:

1. fp-ts Option and Either : Functional error handling
2. fp-ts Pipe and Flow : Advanced composition patterns
3. fp-ts Task and TaskEither : Async functional programming
4. Monads and Functors : The algebraic structures behind fp-ts

Remember: FP is about building complex behavior from simple, composable pieces. Start small, practice composition, and gradually adopt more advanced patterns as they prove useful in your code.

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