Logging, assert/precondition/fatalError

Diagnostics toolbox

Lightweight logging

let user = ["name": "Ana", "role": "admin"]
print("User:", user)            // user-friendly
debugPrint("User:", user)       // includes type details when available

let point = (x: 3, y: 5)
print("Point:", point)
debugPrint("Point:", point)

assert (debug-only)

// Even numbers only (demo)
func half(of x: Int) -> Int {
    assert(x % 2 == 0, "Expected even number, got \(x)")
    return x / 2
}
print(half(of: 8))   // OK in Debug/Release
// In Debug, failing the assert would stop and print the message.
// In Release, assert checks are disabled by default.

precondition (release-checked)

// Safe divide with required precondition
func safeDivide(_ a: Int, by b: Int) -> Int {
    precondition(b != 0, "Divider must be nonzero")
    return a / b
}
print(safeDivide(10, by: 2))    // 5
// If called with 0, precondition fails in Debug and Release.

fatalError (stop now)

enum FileKind { case text, binary }

func open(kind: FileKind) {
    switch kind {
    case .text:
        print("Open text mode")
    case .binary:
        print("Open binary mode")
    // If we ever add a new case and forget to handle it, we could use:
    // default: fatalError("Unsupported file kind")
    }
}
open(kind: .text)

When to use which

Release-checked diagnostic

Recap