Logging, assert/precondition/fatalError
Log state with print/debugPrint , and choose between assert (debug-only), precondition (checked in release), and fatalError (crash) for invariants.
Logging, assert/precondition/fatalError is a free Swift Academy lesson on CoddyKit. This is lesson 2 of 2. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Swift Academy learning path, and your progress syncs across the web and the CoddyKit app. The Swift Academy course includes 2 lessons in total.
Diagnostics toolbox
Use simple logging to see state; enforce assumptions with assert, precondition, and fatalError.
- assert: debug-only checks
- precondition: checked in release too
- fatalError: stop immediately
Lightweight logging
print is readable; debugPrint shows more detail (useful while debugging).
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)
assert stops the program in Debug when the condition is false; it is removed in optimized Release builds.
// 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)
precondition validates critical requirements in Debug and Release. Use it for programmer errors that must never happen.
// 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)
fatalError stops execution immediately and reports a message. Use it for unreachable code paths or unimplemented functionality during development.
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
Guidelines:
- print/debugPrint for quick logs (remove or gate behind flags later).
- assert for debug-only developer checks.
- precondition when the program must not continue in any build if a requirement fails.
- fatalError for unreachable/unimplemented paths.
Release-checked diagnostic
Quick check: Which diagnostic is still checked in Release builds?
Recap
Recap: Log with print/debugPrint, guard invariants with assert (debug), precondition (release too), and use fatalError for truly unreachable states.
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Frequently Asked Questions
Is the “Logging, assert/precondition/fatalError” lesson free?
Yes — any 3 lessons of the Swift Academy learning path, including “Logging, assert/precondition/fatalError”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Swift Academy course includes 2 lessons in total.
What will I learn in “Logging, assert/precondition/fatalError”?
Log state with print/debugPrint , and choose between assert (debug-only), precondition (checked in release), and fatalError (crash) for invariants. 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, so you can start here or from the beginning and move at your own pace. This is lesson 2 of 2.
How long does the “Logging, assert/precondition/fatalError” 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
- XCTest basics (targets, fixtures)
- Logging, assert/precondition/fatalError