Intro to Functional Programming

What is Functional Programming?

Functional programming is a paradigm centered around the composition of mathematical functions. It emphasizes immutability and pure functions to create predictable and testable code. Unlike imperative programming which focuses on "how" to solve problems, functional programming focuses on "what" needs to be solved.

Key Principles

Immutability
Pure Functions
High Order Functions
Recursion over loops

// Imperative
let sum = 0;
for (let i = 0; i < numbers.length; i++) {
    sum += numbers[i];
}

// Functional
const sum = numbers.reduce((acc, num) => acc + num, 0);

Why Use Functional Programming?

Predictable Results

Pure functions always return the same output for the same input, making debugging and testing much simpler. With no side effects, you can reason about code more easily.

Easier Parallelism

Since immutable data is shared instead of modified, functional code is naturally better suited for multithreaded environments and distributed systems.

Composeable

Functions can be combined and nested in powerful ways to build complex systems from simple pieces. This composability leads to elegant, reusable code.

Better Tooling

Modern tools like Elm and ReasonML have excellent IDE integrations and powerful type systems that help catch bugs at compile time.

Core Concepts

Immutability

In functional programming, data isn't mutated after it's created. Instead of changing a variable's value, you create a new data structure with the desired change.

// Immutable
const updatedList = [...list, newItem]

// Mutable
list.push(newItem)

Pure Functions

Functions are pure if they don't have side effects and return the same result for the same inputs. This makes testing easier and code more predictable.

const square = (x) => Math.pow(x, 2)

square(3) // Always returns 9

Recursion vs. Loops

Functional programming favors recursion over traditional loops. This aligns better with immutable data principles.

const factorial = (n) => {
  if (n === 1) return 1
  return n * factorial(n - 1)
}

Functional vs. Object-Oriented

Concept	Functional	OO
Core Unit	Function	Class
Data Mutation	Immutable by design	Mmutable state
Execution Flow	Declarative	Imperative

Getting Started with Functional Concepts

1. Start Small

Begin by identifying pure functions in your existing code. Look for functions that calculate values but don't touch external state.

2. Leverage Immutables

Use libraries like Immutable.js or language features to ensure data structures aren't modified in-place.

3. Embrace Higher-Order

Learn to use functions that take or return functions. This enables powerful abstractions like map, filter, and reduce.