Pure functions

Learn pure functions and side effects.

Pure functions

A pure function returns the same output for the same input and has no side effects. It does not mutate global state or perform I/O. It is easy to test and cache.

Example: fun double(n: Int): Int { return n * 2 }. This is pure; for the same n it always returns the same result. In contrast, fun now(): Long { return System.currentTimeMillis() } is impure; each call returns a different value. Purity is essential in functional programming.

Side effects and determinism

A function's side effect is changing external state or performing I/O. Deterministic means the same input always yields the same output. Pure functions satisfy both.

Example: fun square(n: Int): Int { return n * n } is both side-effect free and deterministic. In contrast, fun log(msg: String) { println(msg) } has a side effect (it performs I/O). Side-effect free functions are safe in parallel execution; they need no locks.

Exercise Write a pure square function

Define a pure function square(n: Int): Int whose body returns n * n. In main call println(square(5)) and println(square(7)).

Show example solution

fun square(n: Int): Int {
    return n * n
}

fun main() {
    println(square(5))
    println(square(7))
}

Test yourself

Complete the pure function body

fun double(n: Int): Int {
    ____
}

Show answer

fun double(n: Int): Int {
    return n * 2
}

Which of these is a pure function?
- fun print(s: String) { println(s) }
- fun double(n: Int) = n * 2
- fun time() = System.currentTimeMillis()
- fun read() = readLine()
Show answer

fun double(n: Int) = n * 2 — double(n) is side-effect free and deterministic.
Match the pure-function parts

double(n: Int)

Pure function
System.currentTimeMillis()

Side effect
return n * 2

Deterministic body
println(msg)

I/O side effect

Next lesson: Sequence

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