Scope

Scope is the region of code where a name can be used. In Swift, every pair of curly braces creates a new scope.

Real-world analogy: scopes are nested rooms. From an inner room you can see into the rooms around it, but people outside can't see in.

Example

let taxRate = 0.2          // top-level: visible below

func total(for price: Double) -> Double {
    let tax = price * taxRate    // inner code can see outer names
    return price + tax
}

print(total(for: 100))

if true {
    let message = "inside"
    print(message)
}

120.0
inside

• taxRate is declared outside the function, so the function can read it.
• tax exists only inside total. After the function returns, it's gone.
• message exists only inside the if braces.

Trying to use a name outside its scope is a compile error:

if true {
    let message = "inside"
}
print(message)

error: cannot find 'message' in scope

Shadowing

An inner scope can declare a new name that hides an outer one with the same name. The if let nickname shorthand you've seen is an example — inside the braces, nickname is the unwrapped value, and outside it's still the optional.

Keep scope small

Declare values as close as possible to where they're used, and prefer passing values into functions as parameters over reading many outside variables. Small scopes make code easier to follow and change.