Pure Functions

Pure Functions

In this lesson — part of Functional Foundations — you'll learn pure functions in Swift and why it matters in real work.

Why it matters

Functional style — pure functions and immutability — makes code easier to test and reason about.

Key ideas

• Pure functions
• Immutability
• No shared mutable state
• Predictable outputs

In practice

Here's how it looks in idiomatic Swift:

func square(_ x: Int) -> Int {
    return x * x   // no side effects, depends only on input
}
print(square(5))   // 25

Swift note: A pure function in Swift reads no external mutable state and produces no side effects; marking parameters let (the default) reinforces immutability.

Try it yourself

Exercise: In Swift, rewrite a function that mutates a global so it's pure.

Recap

You now understand pure functions and can apply it in Swift. Mark this lesson complete and continue to the next one.