Swift Fundamentals
Learn how Swift stores values, infers types, and uses constants to make intent clear.
Before you begin
Learning stage
A program works with values. Swift associates each stored value with a name and a type. Declare a constant with let when the value should not change, and declare a variable with var when reassignment is part of the design.
Swift is type-safe: a value must match the type expected at the place where it is used. The compiler can often infer a type from an initial value, but the type still exists even when it is not written explicitly.
Learning stage
The values below demonstrate constants, variables, inference, and an explicit annotation.
let displayName = "Maya" // inferred as String
var completedLessons = 3 // inferred as Int
let progress: Double = 0.25 // explicit annotation
let notificationsEnabled = true
completedLessons += 1
print("\(displayName) completed \(completedLessons) lessons")
print("Progress: \(progress * 100)%")Learning stage
These concepts appear throughout production iOS code. Focus on the requirement each technique communicates.
Learning stage
Notice these mistakes early so the compiler and type system can help instead of becoming obstacles.
Learning stage
Work through these activities in a small Xcode project. No embedded playground is required on this site.
Activity 1
For a profile screen, decide whether username, edit count, account creation date, and unsaved biography text should use let or var. Explain each choice.
Hint: Ask whether reassignment is part of the value's responsibility.
Activity 2
Write declarations initialized with 42, 3.5, false, and "Swift". Predict each inferred type before checking it in Xcode.
Hint: Swift normally infers Int for integer literals and Double for floating-point literals.
Learning stage
Answer each question before revealing the explanation.
QUESTION 1
let
Use let whenever a stored value should remain stable after initialization.
QUESTION 2
No. Swift infers a concrete type from the expression and context.
Type inference reduces repeated syntax while preserving compile-time type checking.
Learning stage
Explain the idea in terms of its purpose, behavior, and tradeoffs rather than reciting syntax.
Short answer: let creates a constant binding and var creates a variable binding that can be reassigned.
Both declarations have a fixed type after Swift determines it. A let binding cannot be assigned a new value after initialization, while a var binding can receive compatible values later. Preferring let communicates intent and prevents unintended mutation.
Possible follow-ups
Learning stage
Names, types, and mutability form the base of every Swift program. Make stable values constants and let inference remove syntax only when intent remains clear.