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Swift Academy · Lesson

Pure Functions and Side-Effect-Free Design

Writing functions that always return the same output for the same input.

Pure Functions and Side-Effect-Free Design is a free Swift Academy lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Swift Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Welcome

Pure functions always produce the same output for the same input and cause no side effects. They are the building block of functional programming in Swift.

Definition of a Pure Function

A function is pure if: • Same input always yields same output • No observable side effects (no mutations, I/O, global state) ```swift func add(_ a: Int, _ b: Int) -> Int { a + b } // pure ```

Impure Function Example

```swift var total = 0 func addToTotal(_ n: Int) { total += n } // impure — mutates global ``` Calling this twice with `5` produces different results if called in different contexts.

Referential Transparency

A pure function call can be replaced by its result without changing program behaviour: ```swift let result = add(2, 3) // same as writing 5 ```

Benefits of Pure Functions

• Easy to test — no setup/teardown • Safe to parallelise — no shared state • Composable — chain without worrying about side effects • Cacheable — memoisation is safe

Swift Value Types Enable Purity

Because struct methods receive copies, they naturally tend toward purity: ```swift struct Vector { var x, y: Double } func scaled(_ v: Vector, by f: Double) -> Vector { Vector(x:v.x*f, y:v.y*f) } ```

Isolating Side Effects

Push side effects to the boundary of your system: ```swift // Pure core logic: func processOrders(_ orders: [Order]) -> Invoice { ... } // Impure at the edge: func run() { let inv = processOrders(load()); save(inv) } ```

Pure Functions and Testing

```swift func test() { let result = add(2, 3) assert(result == 5) // no mocks needed! } ``` Pure functions need no mock objects — just call and assert.

Real-World Example

```swift func discountedPrice(base: Double, discount: Double) -> Double { base * (1 - discount) } let final = discountedPrice(base: 100, discount: 0.2) // 80.0 ```

Composing Pure Functions

```swift func double(_ n: Int) -> Int { n * 2 } func addOne(_ n: Int) -> Int { n + 1 } let transform: (Int) -> Int = { addOne(double($0)) } print(transform(5)) // 11 ```

Quick Check

Which property makes a function easy to test without mocks?

Recap

Key takeaways: • Pure: same input → same output, no side effects • Referential transparency enables substitution • Push side effects to system boundaries • Pure functions are trivially testable and parallelisable Next: higher-order functions.

Frequently asked questions

Is the “Pure Functions and Side-Effect-Free Design” lesson free?

Yes — the full text of “Pure Functions and Side-Effect-Free Design” is free to read here on the web, and the Swift Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Swift Academy course, upgrade to CoddyKit PRO.

What will I learn in “Pure Functions and Side-Effect-Free Design”?

Writing functions that always return the same output for the same input. You practise Swift Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Swift Academy?

No prior experience is required. Swift Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Pure Functions and Side-Effect-Free Design” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Swift Academy lesson?

Yes. Every Swift Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Pure Functions and Side-Effect-Free Design
  2. Higher-Order Functions: map, flatMap, compactMap
  3. Function Composition and Pipelines
  4. Lazy Sequences for Efficiency
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