Values, variables, and types | Swift

Most Swift code starts by naming values. A value is a piece of data. A constant is a name that cannot be reassigned. A variable is a name that can be reassigned.

Swift makes this distinction explicit with let and var.

Prefer let by default

Use let when a name should not change:

let username = "colin"
let maxAttempts = 3

Use var only when the value must change:

var attempts = 0
attempts += 1

This habit matters. Immutable values are easier to reason about because readers know the name will keep the same value after it is created.

Type inference

Swift can usually infer a type from the value:

let title = "Inbox"    // String
let unreadCount = 12   // Int
let isPinned = false   // Bool

Inference does not mean Swift is dynamically typed. The type is still fixed at compile time. Swift infers it once, then checks future uses against that type.

var count = 0
count = 1      // OK
count = "one"  // Error

Explicit types

Add a type annotation when it improves clarity or when Swift needs help:

let price: Double = 19
let tags: [String] = []

The first example tells Swift that 19 should be treated as a Double, not an Int. The second example gives an empty array a specific element type.

Names should carry meaning

Swift code is often read through types and names. Prefer names that explain the role of a value:

let retryDelaySeconds = 2
let selectedArticleID = "a-204"

Avoid encoding type names into variable names unless it clarifies a boundary. articleTitle is better than titleString in most code.

What to carry forward

let creates a constant
var creates a variable
prefer let unless reassignment is needed
Swift infers types but still checks them at compile time
use explicit type annotations when they make intent clearer

Next, you will work with Swift’s most common primitive types.