Cookbooks
Swift
Chapter 5. Good Coding Practices
Handling errors

Swift:
Handling errors

How to:

Swift uses error handling with do, try, and catch blocks. Let’s take a look:

enum FileError: Error {
    case fileDoesNotExist
    case noPermission
}

func readFile(atPath path: String) throws -> String {
    // Pretend we have some logic here to check if a file exists and if we have permission to read it
    let fileExists = false
    let havePermission = true

    if !fileExists {
        throw FileError.fileDoesNotExist
    }

    if !havePermission {
        throw FileError.noPermission
    }

    return "File content goes here"
}

do {
    let fileContent = try readFile(atPath: "/path/to/file")
    print(fileContent)
} catch FileError.fileDoesNotExist {
    print("Whoops! File not found.")
} catch FileError.noPermission {
    print("Ah! No permission to read the file.")
} catch {
    print("An unknown error occurred.")
}

Sample Output:

Whoops! File not found.

Deep Dive

Error handling wasn’t always as swish as it is now. In Objective-C, you’d deal with pointers to NSError objects, which felt clunky. Now, we have a more elegant system with Swift enums and the Error protocol.

Swift’s throw lets us signal something’s gone wonky. do blocks act like error-aware realms, try prefix calls the risky business, and catch handles things if they go south.

Optionals are an alternative for situations that aren’t quite “error” status but might still have “no result”. They’re a bit like Schrödinger’s variables—they’ve got a value or they don’t.

For real depth, check out Result types, which are snazzy hybrids between regular-return and error patterns.

See Also

Logging