Immutability and Pure Functions

Immutability and pure functions are two foundational principles of functional programming. Together, they eliminate many common sources of bugs and make programs easier to reason about, test, and parallelise.

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Pure Functions

A pure function has two properties:

1. Deterministic: Given the same inputs, it always produces the same output.
2. No side effects: It does not modify any external state, perform I/O, or change its input arguments.

Examples of Pure Functions

-- Pure: always returns the same result for the same input
add :: Int -> Int -> Int
add x y = x + y

-- Pure: no side effects
square :: Int -> Int
square x = x * x

# Pure
def add(x, y):
    return x + y

# Pure
def square(x):
    return x * x

Examples of Impure Functions

# Impure: modifies external state (side effect)
counter = 0
def increment():
    global counter
    counter += 1
    return counter

# Impure: depends on external state (not deterministic)
import random
def random_number():
    return random.randint(1, 10)

# Impure: performs I/O (side effect)
def greet(name):
    print(f"Hello, {name}!")
    return name