Definition:
Functional Programming is a programming paradigm that treats computation as the evaluation of mathematical functions, emphasizing immutability, pure functions, and avoiding side effects.
Key Principles:
- Pure Functions - Same input always produces same output
- Immutability - Data cannot be changed after creation
- No Side Effects - Functions don’t modify external state
- Higher-Order Functions - Functions can take other functions as arguments or return them
Popular Functional Languages: Haskell, Lisp, Clojure, F#, Erlang, Scala
Example:
JavaScript - Pure vs Impure Function
// IMPURE - Modifies external state
let counter = 0;
function increment() {
counter++; // Side effect
return counter;
}
// PURE - No side effects, same input = same output
function add(a, b) {
return a + b;
}Python - Functional Approach
# Functional style with map, filter, reduce
numbers = [1, 2, 3, 4, 5]
# Using higher-order functions
squared = list(map(lambda x: x**2, numbers))
evens = list(filter(lambda x: x % 2 == 0, numbers))
from functools import reduce
sum_all = reduce(lambda x, y: x + y, numbers)
print(f"Original: {numbers}")
print(f"Squared: {squared}")
print(f"Evens: {evens}")
print(f"Sum: {sum_all}")