Error Handling: Throwing Functions, Do-Catch & Try Options

In Plain English

Swift uses explicit error propagation. Learn the Error protocol, throwing functions (throws), throwing errors (throw), capturing errors with do...catch, converting errors to optionals (try?), and disabling error propagation (try!).

The Mental Model

"A throwing function is like a vending machine with a coin return slot: if you request an item that is sold out (throw), the transaction halts and the machine ejects an explicit error token into your hands (catch)."

Deep Dive: How It Works

The Error Protocol: Types represent errors by conforming to the empty Error protocol (typically an enum).

Throwing Functions: func perform() throws -> Result marks a function that can propagate errors.

The try Keyword: Must prefix every call to a throwing function.

try?: Converts throwing calls into Optionals (returns nil on error, unwrapped value on success).

try!: Asserts that an error will never happen; crashes on error.

do...catch Blocks: Captures errors and provides an implicit error variable inside the catch block.

Syntax Blueprint

enum AuthError: Error { case invalidPassword }

do {
    try authenticate()
} catch {
    print("Caught: \(error)")
}

Throw errors with throw, call with try inside do blocks, and handle failures in catch.

Core Rules to Remember

Explicit Propagation: Every point where an error can be thrown is clearly marked with try at the call site.

Pattern Matching Catch: catch AuthError.invalidPassword handles specific domain error cases directly.

Implicit error Variable: catch blocks without explicit variable names automatically bind the thrown error to error.

Common Beginner Traps & How to Fix Them

Mistake:Calling a throwing function without writing try (e.g. let res = loadData()).

Why it happens: Swift compiler requires explicit try, try?, or try! on all throwing calls.

How to fix: Prefix call with try inside a do-catch block, or use try?.

Pro Tip: Use try? when you only care whether an operation succeeded or failed and don't need the specific error details.

Live Interactive Example

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Custom Error Handling with Do-Catch and try?

Swift 5.10 / LLVM

Output Console

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How it works: The 100 withdrawal threw insufficientFunds caught by the specific catch block, and 10 succeeded with try?.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Safe Division Error Handling

Define an enum MathError: Error with case .divisionByZero.
Write a function divide(_ a: Double, _ b: Double) throws -> Double.
If b == 0, throw MathError.divisionByZero. Otherwise return a / b.
In top-level code, call divide(10.0, 0.0) inside a do-catch block and print "Caught error: Division by zero".

Sandbox Output

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